The Science of Aerosols

[AbruptSLR]

As discussions of the influence of aerosols is rather scattered though out this forum, I thought that it would be helpful to open a thread focused on this important anthropogenic forcing mechanism, and I open this thread with the following about contrails:

The linked article (with an open access pdf) discusses how global warming will increase the potential formation of contrails in some parts of the world and decrease it in other parts.

Irvine, E. A. and Shine, K. P.: Ice supersaturation and the potential for contrail formation in a changing climate, Earth Syst. Dynam., 6, 555-568, doi:10.5194/esd-6-555-2015, 2015.

http://www.earth-syst-dynam.net/6/555/2015/esd-6-555-2015.html

Abstract. Ice supersaturation (ISS) in the upper troposphere and lower stratosphere is important for the formation of cirrus clouds and long-lived contrails. Cold ISS (CISS) regions (taken here to be ice-supersaturated regions with temperature below 233 K) are most relevant for contrail formation. We analyse projected changes to the 250 hPa distribution and frequency of CISS regions over the 21st century using data from the Representative Concentration Pathway 8.5 simulations for a selection of Coupled Model Intercomparison Project Phase 5 models. The models show a global-mean, annual-mean decrease in CISS frequency by about one-third, from 11 to 7% by the end of the 21st century, relative to the present-day period 1979–2005. Changes are analysed in further detail for three subregions where air traffic is already high and increasing (Northern Hemisphere mid-latitudes) or expected to increase (tropics and Northern Hemisphere polar regions). The largest change is seen in the tropics, where a reduction of around 9 percentage points in CISS frequency by the end of the century is driven by the strong warming of the upper troposphere. In the Northern Hemisphere mid-latitudes the multi-model-mean change is an increase in CISS frequency of 1 percentage point; however the sign of the change is dependent not only on the model but also on latitude and season. In the Northern Hemisphere polar regions there is an increase in CISS frequency of 5 percentage points in the annual mean. These results suggest that, over the 21st century, climate change may have large impacts on the potential for contrail formation; actual changes to contrail cover will also depend on changes to the volume of air traffic, aircraft technology and flight routing.

[AbruptSLR]

The linked reference (with an open access preprint) provides a discussion of a new framework for understanding climate sensitivity using adjustments that are responses to forcings that are not controlled by global mean warming.  This new approach offers the promise of reducing some of the uncertainties associated with the range of climate sensitivity and is particularly well suited for clarifying the uncertainties associated with aerosols:

Sherwood, S. C., S. Bony, O. Boucher, C. Bretherton, P. M. Forster, J. M. Gregory and B. Stevens, (2014), "Adjustments in the forcing-feedback framework for understanding climate change", Bull. Amer. Meteorol. Soc., doi: http://dx.doi.org/10.1175/BAMS-D-13-00167.1


http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-13-00167.1

For an open access pre-print:

http://web.science.unsw.edu.au/~stevensherwood/forcefeed_v2.2.1.pdf

Abstract: "The traditional forcing-feedback framework has provided an indispensable basis for discussing global climate changes. However, as analysis of model behavior has become more detailed, shortcomings and ambiguities in the framework have become more evident and physical effects unaccounted for by the traditional framework have become interesting. In particular, the new concept of adjustments, which are responses to forcings that are not mediated by the global mean temperature, has emerged. This concept, related to the older ones of climate efficacy and stratospheric adjustment, is a more physical way of capturing unique responses to specific forcings. We present a pedagogical review of the adjustment concept, why it is important, and how it can be used. The concept is particularly useful for aerosols, where it helps to organize what has become a complex array of forcing mechanisms.  It also helps clarify issues around cloud and hydrological response, transient vs. equilibrium climate change, and geoengineering."

Hopefully in AR6 the IPCC will include some of Sherwood et al 2014's thinking about adjustments to the forcing-feedback framework.  In this regards I make the following comments:

1.  The current anthropogenic aerosol allowances within the RCP scenario are dependent on the amount of economic activity assumed within the scenario.  Thus RCP 8.5 has a large amount of negative feedback from large anthropogenic aerosols.  As we now know that at least China will make a dramatic effort to cut aerosol emissions while maintaining a relatively high consumption-driven economic level; hopefully the IPCC will break-out the anthropogenic aerosol forcing component to allow scientists to reduce these large negative feedbacks from their forcing models; and,

2. Ocean acidification can act as a positive feedback by encouraging the reproduction of smaller plankton that do not sink down to remove carbon from the atmospheric cycle, and at high levels can kill many types of plankton.  Furthermore, nanoplankton (& dead plankton) produce less dimethyl sulfide, DSM, which is a natural aerosol that acts as a negative forcing agent.

[AbruptSLR]

The linked reference provides evidence that aerosols have recently played a large role in masking Arctic Amplification, thus as China cleans-up its aerosol emissions, we can expect Arctic Amplification to accelerate:

Mohammad Reza Najafi, Francis W. Zwiers and Nathan P. Gillett (February 2015), "Attribution of Arctic temperature change to greenhouse-gas and aerosol influences", Nature Climate Change, DOI:10.1038/NCLIMATE2524

http://www.nature.com/nclimate/journal/v5/n3/full/nclimate2524.html

Abstract: "The Arctic has warmed significantly more than global mean surface air temperature over recent decades, as expected from amplification mechanisms. Previous studies have attributed the observed Arctic warming to the combined effect of greenhouse gases and other anthropogenic influences4. However, given the sensitivity of the Arctic to external forcing and the intense interest in the effects of aerosols on its climate, it is important to examine and quantify the effects of individual groups of anthropogenic forcing agents. Here we quantify the separate contributions to observed Arctic land temperature change from greenhouse gases, other anthropogenic forcing agents (which are dominated by aerosols) and natural forcing agents. We show that although increases in greenhouse-gas concentrations have driven the observed warming over the past century, approximately 60% of the greenhouse-gas-induced warming has been offset by the combined response to other anthropogenic forcings, which is substantially greater than the fraction of global greenhouse-gas-induced warming that has been offset by these forcings. The climate models considered on average simulate the amplitude of response to anthropogenic forcings well, increasing confidence in their projections of profound future Arctic climate change."

[AbruptSLR]

The Sherwood (2015) presentation focuses on the influence of aerosols on the radiative forcing over the Southern Ocean, which were not previously recognized.
Steven Sherwood (2015) "Radiosonde trends and a (perhaps) unexpected aerosol forcing mechanism", Ringberg workshop:

http://www.mpimet.mpg.de/fileadmin/atmosphaere/WCRP_Grand_Challenge_Workshop/Ringberg_2015/Talks/Sherwood_24032015.pdf


The first image shows that the cloud cover over the Southern Ocean is particularly susceptible to sulfate-induced changes in cloud radiative effect from a doubling of CO₂.
The second image shows a 7%/decade increase in Southern Ocean cloud condensation nuclei (CNN) from 1990 to 2005.
The third image shows observed data from the Southern Ocean comparing aerosol optical depth, AOD, to rainfall rates, cloud top pressure, cloud top temperature and cloud fraction and concludes that at low values of CNN that cloud fraction is proportional to CCN.
The fourth image summarizes Shewood Ringberg (2015) findings that since 1979 the Southern Ocean cloud fraction has increased by 10% per decade largely due to an increase in wind strength associated with the ozone hole over Antarctic; which has resulted in a zonal mean trend decrease of annual mean reflected shortwave forcing of about 1 W/m² per decade.

Extract:
"- We should not assume aerosol effects can only be in the northern hemisphere.
- Possible that greenhouse forcing in SH-extratropics has been negated by aerosol (or sea-ice) increases for some time. Deserves further attention?
- Would help to explain both (a) sluggish recent warming and (b) weird SH-NH contrast since 1979.
- Ozone depletion is the most likely culprit for the wind increase—would make this a rapid adjustment to ozone forcing."

[AbruptSLR]

This post is a follow-up to my last post about Sherwood Ringberg (2015) findings about aerosols and the Southern Ocean.  McCoy et al. (2015) found that seasonal variations in plankton emitted dimethyl sulfide result in an estimated increase the local summertime mean reflected solar radiation in excess of 10 W m–2 over parts of the Southern Ocean, which is comparable to the annual mean increases expected from anthropogenic aerosols over heavily polluted regions of the Northern Hemisphere.  If future dimethyl sulfide emissions decrease say due to ocean acidification and/or a decrease in the size of Southern Ocean plankton due to ocean warming and/or freshening, then global warming would accelerate faster than currently expected:


Daniel T. McCoy, Susannah M. Burrows, Robert Wood, Daniel P. Grosvenor, Scott M. Elliott, Po-Lun Ma, Phillip J. Rasch and Dennis L. Hartmann (17 Jul 2015), "Natural aerosols explain seasonal and spatial patterns of Southern Ocean cloud albedo", Science Advances, Vol. 1, no. 6, e1500157, DOI: 10.1126/sciadv.1500157


http://advances.sciencemag.org/content/1/6/e1500157


Abstract: "Atmospheric aerosols, suspended solid and liquid particles, act as nucleation sites for cloud drop formation, affecting clouds and cloud properties—ultimately influencing the cloud dynamics, lifetime, water path, and areal extent that determine the reflectivity (albedo) of clouds. The concentration Nd of droplets in clouds that influences planetary albedo is sensitive to the availability of aerosol particles on which the droplets form. Natural aerosol concentrations affect not only cloud properties themselves but also modulate the sensitivity of clouds to changes in anthropogenic aerosols. It is shown that modeled natural aerosols, principally marine biogenic primary and secondary aerosol sources, explain more than half of the spatiotemporal variability in satellite-observed Nd. Enhanced Nd is spatially correlated with regions of high chlorophyll a, and the spatiotemporal variability in Nd is found to be driven primarily by high concentrations of sulfate aerosol at lower Southern Ocean latitudes (35o to 45oS) and by organic matter in sea spray aerosol at higher latitudes (45o to 55oS). Biogenic sources are estimated to increase the summertime mean reflected solar radiation in excess of 10 W m–2 over parts of the Southern Ocean, which is comparable to the annual mean increases expected from anthropogenic aerosols over heavily polluted regions of the Northern Hemisphere."


See also:

http://www.livescience.com/51598-marine-aerosols-clouds-climate-change.html

Extract: "Since the aerosols are difficult to distinguish when viewed from space, the researchers used models that tracked the compound dimethyl sulfide, which is released by phytoplankton and turns into a sulfate aerosol in the atmosphere. They also designed a model that included simulations of the process by which salty water known as "sea spray" is enriched with organic matter produced by phytoplankton (essentially, phytoplankton poop).
Not all aerosols attract water droplets, said Susannah Burrows, the other lead author of the study and a climate scientist at the Department of Energy's Pacific Northwest National Laboratory. Although most aerosols are carried up by the same atmospheric circulation patterns, their chemical and physical properties determine whether or not they become "cloud condensation nuclei," which are the points around which droplets form before they become cloud droplets.
Smaller aerosols may have a harder time attracting water droplets than do larger ones, Burrows said. Solubility also plays a role in determining how easily the aerosol will take up water vapor from the atmosphere. Sea salt is very soluble and "likes to suck up water vapor from the atmosphere, so organic particles are less effective cloud-condensation nuclei than salt," Burrows told Live Science.
The researchers found that they could predict the observed concentration of cloud droplets with their model. The results were "interesting in a climate sense, because the amount of sunlight that is being reflected by these clouds is to some extent determined by the number of cloud droplets," McCoy told Live Science.
The scientists calculated the amount of light reflected by the clouds and determined that "it ends up being a 60 percent increase in cloud droplets throughout the year, doubling in summer, when the phytoplankton are most active, translating to a 4-watt-per-meter-squared increase in reflected sunlight, and 10-watt-per-meter-squared increase during the summer," McCoy said."

[AbruptSLR]

The following two linked references point out that forests emit large quantities of volatile organic compounds (VOCs) to the atmosphere. Their condensable oxidation products can form secondary organic aerosol, a significant and ubiquitous component of atmospheric aerosol, which is known to affect the Earth’s radiation balance by scattering solar radiation and by acting as cloud condensation nuclei, and thus act as a negative feedback mechanism.

It is not pointed out in either of the two following linked references that as current estimates of "climate sensitivity" do not include this negative feedback; in order for Global Circulation Models, GCM's including this negative feedback to match historical records they will need to utilize higher effective "climate sensitivity" values; which should resulting in higher projections of global temperature increase, if plant growth/activity does not keep pace with the rate of future green house gas, GHC, emissions.

Mikael Ehn, Joel A. Thornton, Einhard Kleist, Mikko Sipilä, Heikki Junninen, Iida Pullinen, Monika Springer, Florian Rubach, Ralf Tillmann, Ben Lee, Felipe Lopez-Hilfiker, Stefanie Andres, Ismail-Hakki Acir, Matti Rissanen, Tuija Jokinen, Siegfried Schobesberger, Juha Kangasluoma, Jenni Kontkanen, Tuomo Nieminen, Theo Kurtén, Lasse B. Nielsen, Solvejg Jørgensen, Henrik G. Kjaergaard, Manjula Canagaratna, Miikka Dal Maso et al (2014), " A large source of low-volatility secondary organic aerosol", Nature, 506, 476–479, doi:10.1038/nature13032


http://www.nature.com/nature/journal/v506/n7489/full/nature13032.html

Also, see:
http://www.bbc.com/news/science-environment-26340038


Abstract: "Forests emit large quantities of volatile organic compounds (VOCs) to the atmosphere. Their condensable oxidation products can form secondary organic aerosol, a significant and ubiquitous component of atmospheric aerosol, which is known to affect the Earth’s radiation balance by scattering solar radiation and by acting as cloud condensation nuclei. The quantitative assessment of such climate effects remains hampered by a number of factors, including an incomplete understanding of how biogenic VOCs contribute to the formation of atmospheric secondary organic aerosol. The growth of newly formed particles from sizes of less than three nanometres up to the sizes of cloud condensation nuclei (about one hundred nanometres) in many continental ecosystems requires abundant, essentially non-volatile organic vapours, but the sources and compositions of such vapours remain unknown. Here we investigate the oxidation of VOCs, in particular the terpene α-pinene, under atmospherically relevant conditions in chamber experiments. We find that a direct pathway leads from several biogenic VOCs, such as monoterpenes, to the formation of large amounts of extremely low-volatility vapours. These vapours form at significant mass yield in the gas phase and condense irreversibly onto aerosol surfaces to produce secondary organic aerosol, helping to explain the discrepancy between the observed atmospheric burden of secondary organic aerosol and that reported by many model studies. We further demonstrate how these low-volatility vapours can enhance, or even dominate, the formation and growth of aerosol particles over forested regions, providing a missing link between biogenic VOCs and their conversion to aerosol particles. Our findings could help to improve assessments of biosphere–aerosol–climate feedback mechanisms, and the air quality and climate effects of biogenic emissions generally."

Also, see the link to the following related reference:

Paasonen, P., et. al. (2013), "Evidence for negative climate feedback: warming increases aerosol number concentrations,", Nature Geoscience, 6, Pages: 438–442, doi: 10.1038/NGEO1800

http://www.nature.com/ngeo/journal/v6/n6/full/ngeo1800.html

[AbruptSLR]

This post is a follow-up to my last two posts about how organic emissions from both forests, and from plankton in the Southern Ocean, are currently masking the climate sensitivity to GHG emissions.  The following linked reference cites new evidence that many trees also emit extremely low volatility organic compounds (ELVOC); which are suggested to promote aerosol particle formation and cloud condensation nuclei (CCN) production in the atmosphere, that are also currently masking the true magnitude of Equilibrium Climate Sensitivity, ECS, considering that forests are projected to decline rapidly with continued global warming:


Tuija Jokinen, Torsten Berndt, Risto Makkonen, Veli-Matti Kerminen, Heikki Junninen, Pauli Paasonen, Frank Stratmann, Hartmut Herrmann, Alex B. Guenther, Douglas R. Worsnop, Markku Kulmala, Mikael Ehn, and Mikko Sipilä (June 9, 2015), "Production of extremely low-volatile organic compounds from biogenic emissions: measured yields and atmospheric implications", PNAS, vol. 112 no. 23 7123-7128, doi: 10.1073/pnas.1423977112


http://www.pnas.org/content/112/23/7123

Significance: "Extremely low volatility organic compounds (ELVOC) are suggested to promote aerosol particle formation and cloud condensation nuclei (CCN) production in the atmosphere. We show that the capability of biogenic VOC (BVOC) to produce ELVOC depends strongly on their chemical structure and relative oxidant levels. BVOC with an endocyclic double bond, representative emissions from, e.g., boreal forests, efficiently produce ELVOC from ozonolysis. Compounds with exocyclic double bonds or acyclic compounds including isoprene, emission representative of the tropics, produce minor quantities of ELVOC, and the role of OH radical oxidation is relatively larger. Implementing these findings into a global modeling framework shows that detailed assessment of ELVOC production pathways is crucial for understanding biogenic secondary organic aerosol and atmospheric CCN formation."

Abstract: "Oxidation products of monoterpenes and isoprene have a major influence on the global secondary organic aerosol (SOA) burden and the production of atmospheric nanoparticles and cloud condensation nuclei (CCN). Here, we investigate the formation of extremely low volatility organic compounds (ELVOC) from O3 and OH radical oxidation of several monoterpenes and isoprene in a series of laboratory experiments. We show that ELVOC from all precursors are formed within the first minute after the initial attack of an oxidant. We demonstrate that under atmospherically relevant concentrations, species with an endocyclic double bond efficiently produce ELVOC from ozonolysis, whereas the yields from OH radical-initiated reactions are smaller. If the double bond is exocyclic or the compound itself is acyclic, ozonolysis produces less ELVOC and the role of the OH radical-initiated ELVOC formation is increased. Isoprene oxidation produces marginal quantities of ELVOC regardless of the oxidant. Implementing our laboratory findings into a global modeling framework shows that biogenic SOA formation in general, and ELVOC in particular, play crucial roles in atmospheric CCN production. Monoterpene oxidation products enhance atmospheric new particle formation and growth in most continental regions, thereby increasing CCN concentrations, especially at high values of cloud supersaturation. Isoprene-derived SOA tends to suppress atmospheric new particle formation, yet it assists the growth of sub-CCN-size primary particles to CCN. Taking into account compound specific monoterpene emissions has a moderate effect on the modeled global CCN budget."

Also see:
http://www.science20.com/news_articles/lowvolatility_organic_compounds_how_forests_can_effect_clouds_and_climate-155798

Extract: "According to a new global-scale projection, terrestrial vegetation emits several million tons of extremely low-volatility organic compounds (ELVOCs) per year to the atmosphere, which affect cloud seeds via formation of low-volatility vapors. These oxidation products of compounds such as monoterpenes results in an increase of condensing vapors that can further form cloud condensation nuclei over the continents and have an influence on the formation of clouds.

The results show how a number of natural compounds, which together account for around 70 percent of the biological hydrocarbon emissions, produce low-volatility products and how they can possibly effect the climate via aerosol particles.

Aerosol particles act as cloud droplets and thus reflect solar radiation back to space cooling down the planet. They play a crucial role in cloud formation and therefore also affect rainfall, cloud cover and climate in general. The tiny aerosol particles can originate dust, pollen or sea spray, and are emitted straight into the atmosphere or they can be formed from precursor gases. This gas-to-particle conversion is a complicated process and some parts of its first steps still remain unsolved by scientist.
This includes the role of oxidized volatile organic compounds, such as limonene and alpha-pinene, the typical scents of the citrus fruits and coniferous forests, in aerosol formation.

These compounds are first emitted by plants into the atmosphere and then oxidized by common oxidants, ozone or the OH-radicals. Whether these reactions produce condensing vapors that can condense onto the smallest particles or even molecules can have strong impacts on aerosol formation. As long as these processes are poorly understood, it is difficult to give accurate predictions of the future climate by modern climate models.

The discovery of the extremely low-volatility organic compounds (ELVOCs) was published last year in Nature, which observed highly oxygenated organics or ELVOC's that can explain the contribution of organic compounds in aerosol formation. The other discovery was made by the Leipzig-Helsinki team who found the mechanism leading to the rapid formation of these oxidized organic compounds. Auto-oxidation, which can spoil plastics or food, also plays an important role in the atmospheric aerosol formation. In the new paper, the team outlines how different biogenic compounds produce ELVOC's and how relevant these compounds are for the atmospheric processes. For the first time they could estimate the global effects of ELVOC in cloud condensation nuclei production.

In order to investigate the formation of the extremely low-volatility organic compounds the team has studied five abundant biogenic organic compounds with different chemical structures. The compounds all reacted with ozone and OH-radicals producing ELVOC. They found that the ELVOC formation happens very fast and that the chemical structure of the precursor gases determines how effectively they form ELVOC's.
"The structure of biogenic compounds that are emitted into the atmosphere influences how they are oxidized in the air and if they produce vapours that can condensate," says lead author Tuija Jokinen of the University of Helsinki, who carried out these studies in Leipzig at TROPOS. In the experiments, five biogenic organic gases (limonene, α-pinene, β-pinene, myrcene and isoprene) were passed through the laminar flow tube and then examined by a CI-APi-TOF mass spectrometer. "Our results show that the ozonolysis of monoterpenes like α-pinene or limonene leads to a greater efficiency of ELVOC production than a well-known oxidation via OH-radicals. On the other hand, β-pinene, myrcene and isoprene, produce much less ELVOC's, which are an important biogenic sources for the formation of particles in the atmosphere," emphasizes Dr. Torsten Berndt of TROPOS, who was involved in all three publications.

The results from the experiments were incorporated into a global atmospheric model to assess the impact of ELVOC on the particle formation and growth in the atmosphere. The international team used ECHAM5-HAM, an aerosol climate model, which was originally developed at the Max Planck Institute for Meteorology in Hamburg. According to the researchers, the extended model in the study is the first global aerosol model that combines the formation processes of secondary organic aerosol (SOA) with the ELVOC production from experiments."

[AbruptSLR]

The linked reference highlights one aspect of the complex influence of aerosols on climate sensitivity (in addition to my prior posts about: contrails, VOCs, SOAs, DSM, ELVOCs, Arctic Amplification, etc.; via their influence on warm convective clouds.  This research indicates that depending on aerosol concentrations they can either interact with warm convective clouds to result in either positive, or negative, feedback.  It is important that new state-of-the-art climate models like ACME, incorporate such findings in their forthcoming climate projections.

G. Dagan, I. Koren, and O. Altaratz (2015), "Competition between core and periphery-based processes in warm convective clouds – from invigoration to suppression", Atmos. Chem. Phys., 15, 2749–2760, doi:10.5194/acp-15-2749-2015

http://www.atmos-chem-phys.net/15/2749/2015/acp-15-2749-2015.pdf

Abstract. How do changes in the amount and properties of aerosol affect warm clouds? Recent studies suggest that they have opposing effects. Some suggest that an increase in aerosol loading leads to enhanced evaporation and therefore smaller clouds, whereas other studies suggest clouds’ invigoration.  In this study, using an axisymmetric bin-microphysics cloud model, we propose a theoretical scheme that analyzes the evolution of key processes in warm clouds, under different aerosol loading and environmental conditions, to explain this contradiction.
Such an analysis of the key processes reveals a robust reversal in the trend of the clouds’ response to an increase in aerosol loading. When aerosol conditions are shifted from superpristine to slightly polluted, the clouds formed are deeper and have larger water mass. Such a trend continues up to an optimal concentration (Nop) that allows the cloud to achieve a maximal water mass. Hence, for any concentration below Nop the cloud formed contains less mass and therefore can be considered as aerosol-limited, whereas for concentrations greater than Nop cloud periphery processes, such as enhanced entrainment and evaporation, take over leading to cloud suppression. We show that Nop is a function of the thermodynamic conditions (temperature and humidity profiles).  Thus, profiles that favor deeper clouds would dictate larger values of Nop, whereas for profiles of shallow convective clouds, Nop corresponds to the pristine range of the aerosol loading.  Such a view of a trend reversal, marked by the optimal concentration, Nop, helps one to bridge the gap between the contradictory results of numerical models and observations.  Satellite studies are biased in favor of larger clouds that are characterized by larger Nop values and therefore invigoration is observed. On the other hand, modeling studies of cloud fields are biased in favor of small, mostly trade-like convective clouds, which are characterized by low Nop values (in the pristine range) and, therefore, cloud suppression is mostly reported as a response to an increase in aerosol loading."

[AbruptSLR]

The linked reference indicates that not only can aerosol have complex effects on climate globally, they can also have pronounced local effects:

Jiwen Fan, Daniel Rosenfeld, Yan Yang, Chun Zhao, L. Ruby Leung and Zhanqing Li (July 2015), "Substantial contribution of anthropogenic air pollution to catastrophic floods in Southwest China", Geophysical Research Letters, DOI: 10.1002/2015GL064479


http://onlinelibrary.wiley.com/doi/10.1002/2015GL064479/full


Abstract: "Extreme weather events have become more frequent and are likely linked to increases in greenhouse gases and aerosols, which alter the Earth's radiative balance and cloud processes. On 8–9 July 2013, a catastrophic flood devastated the mountainous area to the northwest of the Sichuan Basin. Atmospheric simulations at a convection-permitting scale with aerosols and chemistry included show that heavy air pollution trapped in the basin significantly enhances the rainfall intensity over the mountainous areas through “aerosol-enhanced conditional instability.” That is, aerosols suppress convection by absorbing solar radiation and increasing atmospheric stability in the basin during daytime. This allows excess moist air to be transported to the mountainous areas and orographically lifted, generating strong convection and extremely heavy precipitation at night. We show that reducing pollution in the Sichuan Basin can effectively mitigate floods. It is suggested that coupling aerosol with meteorology can be crucial to improve weather forecast in polluted regions."


See also:

http://news.sciencemag.org/asiapacific/2015/07/catastrophic-chinese-floods-triggered-air-pollution

Extract: "Air pollution can affect precipitation in many ways. Sometimes, the aerosol particles in smoke can reduce or delay rain. Sometimes, they can make thunderstorms more intense. Their best understood interaction is in changing how water vapor condenses to form droplets in clouds. But Fan and her team have proposed a first: that pollution also changes some air circulation patterns that lead to rainclouds.
In the case of the Sichuan storms, they write in a paper published online before print in Geophysical Research Letters, soot in particular contributed to the catastrophic flooding. It prevented rainclouds from forming over the basin during the day, leading to more intense rainfall in the mountains that evening. “We were amazed at the scale of the effect the pollution had,” Fan says. “Effectively it redistributed the precipitation from the wide area of the basin into the mountains.”"

[jai mitchell]

Fantastic work ASLR!  THANKS!
 

[AbruptSLR]

Fantastic work ASLR!  THANKS!
 

Feel free to jump, the water is fine! 

[jai mitchell]

The linked reference provides a series of potential model runs of future aerosol emission reduction due to current legislative practice (minimum values) and technical feasible potential (maximum).  It shows that the potential for aerosol reductions from fossil fuel conversions to clean energy will potentially provide as much additional radiative forcing as is currently being experienced by the earth on a globally averaged scale.

it should be noted that this is a direct impact of aerosols only which will also produce significant warming and the water vapor and lapse rate feedbacks will reach a maximum potential after 10 years post emission reductions.  This additional feedback will be significant, possibly doubling or tripling the resultant forcing values.

http://www.atmos-chem-phys.net/15/5501/2015/acp-15-5501-2015.html

Atmos. Chem. Phys., 15, 5501-5519, 2015
www.atmos-chem-phys.net/15/5501/2015/
doi:10.5194/acp-15-5501-2015

J.-P. Pietikäinen1, K. Kupiainen2,3, Z. Klimont2, R. Makkonen4, H. Korhonen1, R. Karinkanta1, A.-P. Hyvärinen1, N. Karvosenoja3, A. Laaksonen1, H. Lihavainen1, and V.-M. Kerminen4
1Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
2International Institute for Applied Systems Analysis, Schlossplatz 1, 2361 Laxenburg, Austria
3Finnish Environment Institute SYKE, P.O. Box 140, 00251 Helsinki, Finland
4Department of Physics, University of Helsinki, P.O. Box 44, 00014 Helsinki, Finland

Abstract. The global aerosol–climate model ECHAM-HAMMOZ was used to investigate changes in the aerosol burden and aerosol radiative effects in the coming decades. Four different emissions scenarios were applied for 2030 (two of them applied also for 2020) and the results were compared against the reference year 2005. Two of the scenarios are based on current legislation reductions: one shows the maximum potential of reductions that can be achieved by technical measures, and the other is targeted to short-lived climate forcers (SLCFs). We have analyzed the results in terms of global means and additionally focused on eight subregions. Based on our results, aerosol burdens show an overall decreasing trend as they basically follow the changes in primary and precursor emissions. However, in some locations, such as India, the burdens could increase significantly. The declining emissions have an impact on the clear-sky direct aerosol effect (DRE), i.e. the cooling effect. The DRE could decrease globally 0.06–0.4 W m−2 by 2030 with some regional increases, for example, over India (up to 0.84 W m−2). The global changes in the DRE depend on the scenario and are smallest in the targeted SLCF simulation. The aerosol indirect radiative effect could decline 0.25–0.82 W m−2 by 2030. This decrease takes place mostly over the oceans, whereas the DRE changes are greatest over the continents. Our results show that targeted emission reduction measures can be a much better choice for the climate than overall high reductions globally. Our simulations also suggest that more than half of the near-future forcing change is due to the radiative effects associated with aerosol–cloud interactions.

[AbruptSLR]

Nice reference jai.

The following is a listing of some recent papers from the Center for Aerosol Impacts on Climate and the Environment

Microbial Control of Sea Spray Aerosol Composition: A Tale of Two Blooms
Xiaofei Wang, Camille M. Sultana, Jonathan Trueblood, Thomas C.J. Hill, Francesca Malfatti, Christopher Lee, Olga Laskina, Kathryn A. Moore, Charlotte M. Beall, Christina S. McCluskey, Gavin C. Cornwell, Yanyan Zhou, Joshua L. Cox, Matthew A. Pendergraft, Mitchell V. Santander, Timothy H. Bertram, Christopher D. Cappa, Farooq Azam, Paul J. DeMott, Vicki H. Grassian, Kimberly A. Prather, ACS Cent. Sci., 1(3), 124-131, 2015.

Chemistry’s Contributions to Our Understanding of Atmospheric Science and Climate
Vicki H. Grassian and Elizabeth A. Stone, J. Chem. Educ., 92(4), 595-597, 2015.

Chemistry and Related Properties of Freshly Emitted Sea Spray Aerosol
Patricia K. Quinn, Douglas B. Collins, Vicki H. Grassian, Kimberly A. Prather, Timothy S. Bates, Chem. Rev., 115(10), 4383-4399, 2015.

Atmospheric Processes and Their Controlling Influence on Cloud Condensation Nuclei Activity
Delphine K. Farmer, Christopher D. Cappa, Sonia M. Kreidenweis, Chem. Rev., 115(10), 4199-4217, 2015.

Humidity-Dependent Surface Tension Measurements of Individual Inorganic and Organic Submicrometre Liquid Particles
Holly Morris, Vicki H. Grassian, Alexei V. Tivanski, Chem. Sci., 6, 3242-3247, 2015


See also:
http://caice.ucsd.edu/

[jai mitchell]

Does anyone really know the impacts of aerosol emission reductions on future warming?


Has there been attempts to model lapse rate and associated water vapor feedback responses subsequent to a "negative pulse" of  anthropogenic aerosols?  To my knowledge these feedbacks are not considered in the calculation of aerosol negative forcings.  There are indications that aerosols have a significant effect on lapse rate due to the inherent cooling that is targeted to the upper troposphere. see: "Global indirect aerosol effects: a review", U. Lohhmann (2005)  This indicates that the lapse rate and water vapor feedbacks from aerosol reductions would be more that the response for an identical Carbon Dioxide pulse.

[AbruptSLR]

The linked reference discusses the calibration of the CSW _v2.0 algorithm to account for temperature lapse rate due to water vapor & aerosol effects:

A-Ra Cho, Youn-Young Choi and Myoung-Seok Suh (2015), "Improvements of a COMS Land Surface Temperature Retrieval Algorithm Based on the Temperature Lapse Rate and Water Vapor/Aerosol Effect", Remote Sens., 7(2), 1777-1797; doi:10.3390/rs70201777

http://www.mdpi.com/2072-4292/7/2/1777

Abstract: "The National Meteorological Satellite Center in Korea retrieves land surface temperature (LST) by applying the split-window LST algorithm (CSW_v1.0) to Communication, Ocean, and Meteorological Satellite (COMS) data. Considerable errors were detected under conditions of high water vapor content or temperature lapse rates during validation with Moderate Resolution Imaging Spectroradiometer (MODIS) LST because of the too simplified LST algorithm. In this study, six types of LST retrieval equations (CSW_v2.0) were developed to upgrade the CSW_v1.0. These methods were developed by classifying “dry,” “normal,” and “wet” cases for day and night and considering the relative sizes of brightness temperature difference (BTD) values. Similar to CSW_v1.0, the LST retrieved by CSW_v2.0 had a correlation coefficient of 0.99 with the prescribed LST and a slightly larger bias of −0.03 K from 0.00K; the root mean square error (RMSE) improved from 1.41 K to 1.39 K. In general, CSW_v2.0 improved the retrieval accuracy compared to CSW_v1.0, especially when the lapse rate was high (mid-day and dawn) and the water vapor content was high. The spatial distributions of LST retrieved by CSW_v2.0 were found to be similar to the MODIS LST independently of the season, day/night, and geographic locations. The validation using one year’s MODIS LST data showed that CSW_v2.0 improved the retrieval accuracy of LST in terms of correlations (from 0.988 to 0.989), bias (from −1.009 K to 0.292 K), and RMSEs (from 2.613 K to 2.237 K)."

[jai mitchell]

Thanks ASLR

interesting but it seems that the paper addresses extreme aerosol and lapse rate conditions to increase model sensitivity but does not address specifically the interactive effects of these two, nor does it look at the expected feedback response to aerosol reductions.  Again, it seems that the lapse rate/water vapor feedbacks will be larger for aerosols than for GHGs due to the cooling effect of aerosols in the upper troposphere (allowing for a reduction in the lapse rate feedback being induced by greenhouse gasses.  in absence of aerosols this lapse rate effect will rebound and then warming of the biosphere will further increase these feedbacks!

I feel like the conservative scientists thread is bleeding into this one. 

These interactive effects of aerosols are not included in the CMIP5 model runs, to my knowledge.  In fact they are, as yet, relatively undefined.

[jai mitchell]

The overall effect of the aerosol forcing is a cooling near the surface in the polluted regions of the Northern Hemisphere that stabilizes the lower atmosphere whereas the near surface changes in temperature are smaller in the tropics and the mid-latitudes of the Southern Hemisphere. The implications of these aerosol induced lapse rate changes on other climate feedbacks such as the water vapor feedback are, however, not quantified yet.

http://www.atmos-chem-phys.net/5/715/2005/acp-5-715-2005.pdf

Global indirect aerosol effects: a review
U. Lohmann1 and J. Feichter2

Atmos. Chem. Phys., 5, 715–737, 2005
www.atmos-chem-phys.org/acp/5/715/
SRef-ID: 1680-7324/acp/2005-5-715
European Geosciences Union


Abstract:

Aerosols affect the climate system by changing cloud characteristics in many ways. They act as cloud condensation and ice nuclei, they may inhibit freezing and they could have an influence on the hydrological cycle. While the cloud albedo enhancement (Twomey effect) of warm clouds received most attention so far and traditionally is the only indirect aerosol forcing considered in transient climate simulations, here we discuss the multitude of effects. Different
approaches how the climatic implications of these aerosol effects can be estimated globally as well as improvements that are needed in global climate models in order to better represent
indirect aerosol effects are discussed in this paper.

[jai mitchell]

This paper shows that updated models indicate that the global impact of natural Dimethyl Sulfide emissions are underrepressented by up to 20%.

67- Quantifying the impacts of an updated global dimethyl sulfide climatology on cloud microphysics and aerosol radiative forcing

Anoop S. Mahajan, Suvarna Fadnavis, Manu A. Thomas, Luca Pozzoli, Smrati Gupta, Sarah-Jeanne Royer, Alfonso Saiz-Lopez, Rafel Simó.

J. Geophys. Res. 120, 6, 2524-2536. DOI: 10.1002/2014JD022687

http://ac2.iqfr.csic.es/es/component/content/category/images/pdf/2008/acp-8-4855-2008.pdf

Abstract. One of the critical parameters in assessing the global impacts of dimethyl sulfide (DMS) on cloud properties and the radiation budget is the estimation of phytoplankton-induced ocean emissions, which are derived from prescribed, climatological surface seawater DMS concentrations. The most widely used global ocean DMS climatology was published 15 years ago and has recently been updated using a much larger database of observations. The updated climatology displays significant differences in terms of the global distribution and regional monthly averages of sea surface DMS. In this study, we use the ECHAM5-HAMMOZ aerosol-chemistry-climate general circulation model to quantify the influence of the updated DMS climatology in computed atmospheric properties, namely, the spatial and temporal distributions of atmospheric DMS concentration, sulfuric acid concentration, sulfate aerosols, number of activated aerosols, cloud droplet number concentration, and the aerosol radiative forcing at the top of the atmosphere. Significant differences are observed for all the modeled variables. Comparison with observations of atmospheric DMS and total sulfate also shows that in places with large DMS emissions, the updated climatology shows a better match with the observations. This highlights the importance of using the updated climatology for projecting future impacts of oceanic DMS emissions, especially considering that the relative importance of the natural sulfur fluxes is likely to increase due to legislation to “clean up” anthropogenic emissions. The largest estimated differences are in the Southern Ocean, Indian Ocean, and parts of the Pacific Ocean, where the climatologies differ in seasonal concentrations over large geographical areas. The model results also indicate that the former DMS climatology underestimated the effect of DMS on the globally averaged annual aerosol radiative forcing at the top of the atmosphere by about 20%.

[AbruptSLR]

This paper shows that updated models indicate that the global impact of natural Dimethyl Sulfide emissions are underrepressented by up to 20%.

67- Quantifying the impacts of an updated global dimethyl sulfide climatology on cloud microphysics and aerosol radiative forcing

Anoop S. Mahajan, Suvarna Fadnavis, Manu A. Thomas, Luca Pozzoli, Smrati Gupta, Sarah-Jeanne Royer, Alfonso Saiz-Lopez, Rafel Simó.

J. Geophys. Res. 120, 6, 2524-2536. DOI: 10.1002/2014JD022687

http://ac2.iqfr.csic.es/es/component/content/category/images/pdf/2008/acp-8-4855-2008.pdf

Abstract. One of the critical parameters in assessing the global impacts of dimethyl sulfide (DMS) on cloud properties and the radiation budget is the estimation of phytoplankton-induced ocean emissions, which are derived from prescribed, climatological surface seawater DMS concentrations. The most widely used global ocean DMS climatology was published 15 years ago and has recently been updated using a much larger database of observations. The updated climatology displays significant differences in terms of the global distribution and regional monthly averages of sea surface DMS. In this study, we use the ECHAM5-HAMMOZ aerosol-chemistry-climate general circulation model to quantify the influence of the updated DMS climatology in computed atmospheric properties, namely, the spatial and temporal distributions of atmospheric DMS concentration, sulfuric acid concentration, sulfate aerosols, number of activated aerosols, cloud droplet number concentration, and the aerosol radiative forcing at the top of the atmosphere. Significant differences are observed for all the modeled variables. Comparison with observations of atmospheric DMS and total sulfate also shows that in places with large DMS emissions, the updated climatology shows a better match with the observations. This highlights the importance of using the updated climatology for projecting future impacts of oceanic DMS emissions, especially considering that the relative importance of the natural sulfur fluxes is likely to increase due to legislation to “clean up” anthropogenic emissions. The largest estimated differences are in the Southern Ocean, Indian Ocean, and parts of the Pacific Ocean, where the climatologies differ in seasonal concentrations over large geographical areas. The model results also indicate that the former DMS climatology underestimated the effect of DMS on the globally averaged annual aerosol radiative forcing at the top of the atmosphere by about 20%.

This clearly raises the risk that if the ocean's plankton are disrupted (by both warming & acidification) that the effective ECS will prove to be higher than previously realized.

[jai mitchell]

In addition, boreal forest and amazon basin carbon cycle response (as well as indonesia) will further exacerbate the reduction of this potent naturally occurring aerosol in coming years.

[AbruptSLR]

The linked reference shows that the errors incurred due to ignoring temporal sampling of aerosols, and show they are of similar magnitude as (but smaller than) actual model errors (20–60 %). This indicates that our ignorance about the impact of aerosols are higher than previously appreciated:

Schutgens, N. A. J., Partridge, D. G., and Stier, P. (2015), "The importance of temporal collocation for the evaluation of aerosol models with observations", Atmos. Chem. Phys. Discuss., 15, 26191-26230, doi:10.5194/acpd-15-26191-2015.

http://www.atmos-chem-phys-discuss.net/15/26191/2015/acpd-15-26191-2015.html

Abstract: "It is often implicitly assumed that over suitably long periods the mean of observations and models should be comparable, even if they have different temporal sampling. We assess the errors incurred due to ignoring temporal sampling and show they are of similar magnitude as (but smaller than) actual model errors (20–60 %).

Using temporal sampling from remote sensing datasets (the satellite imager MODIS and the ground-based sun photometer network AERONET) and three different global aerosol models, we compare annual and monthly averages of full model data to sampled model data. Our results show that sampling errors as large as 100 % in AOT (Aerosol Optical Thickness), 0.4 in AE (Ångström Exponent) and 0.05 in SSA (Single Scattering Albedo) are possible. Even in daily averages, sampling errors can be significant. More-over these sampling errors are often correlated over long distances giving rise to artificial contrasts between pristine and polluted events and regions. Additionally, we provide evidence that suggests that models will underestimate these errors. To prevent sampling errors, model data should be temporally collocated to the observations before any analysis is made.

We also discuss how this work has consequences for in-situ measurements (e.g. aircraft campaigns or surface measurements) in model evaluation."

[AbruptSLR]

The linked Carbon Brief article discusses how skeptics misinterpreted findings by Ciuraru et al. (2015) about the influence of isoprene on aerosols:

http://www.carbonbrief.org/blog/2015/10/factcheck-aerosols-research-misinterpreted/

Ciuraru, R. et al. (2015) Unravelling New Processes at Interfaces: Photochemical Isoprene Production at the Sea Surface, Environmental Science & Technology, doi: 10.1021/acs.est.5b02388

[AbruptSLR]

Normally the findings of the linked reference about the negative feedback associated with biogenic secondary organic aerosol (BSOA), could be considered as good news; however, given the dire condition of the boreal forests; this research could indicate that climate sensitivity could increase faster than previously thought if the boreal forests continue to degrade rapidly:

Heikki Lihavainen, Eija Asmi, Veijo Aaltonen, Ulla Makkonen and Veli-Matti Kerminen (Published 8 October 2015), "Direct radiative feedback due to biogenic secondary organic aerosol estimated from boreal forest site observations" Environmental Research Letters, Volume 10, Number 10


http://iopscience.iop.org/article/10.1088/1748-9326/10/10/104005/meta

http://iopscience.iop.org/article/10.1088/1748-9326/10/10/104005/pdf

Abstract: "We used more than five years of continuous aerosol measurements to estimate the direct radiative feedback parameter associated with the formation of biogenic secondary organic aerosol (BSOA) at a remote continental site at the edge of the boreal forest zone in Northern Finland. Our upper-limit estimate for this feedback parameter during the summer period (ambient temperatures above 10 °C) was −97 ± 66 mW m−2 K−1 (mean ± STD) when using measurements of the aerosol optical depth (fAOD) and −63 ± 40 mW m−2 K−1 when using measurements of the 'dry' aerosol scattering coefficient at the ground level (fσ). Here STD represents the variability in f caused by the observed variability in the quantities used to derive the value of f. Compared with our measurement site, the magnitude of the direct radiative feedback associated with BSOA is expected to be larger in warmer continental regions with more abundant biogenic emissions, and even larger in regions where biogenic emissions are mixed with anthropogenic pollution."

[jai mitchell]

This work on boreal forest aerosols has significant impact on paleoclimate modeling, as the boreal forest network during the eemian maximum was approximately 300% of the current value.  It is clear that the aerosol uncertainty will produce another revision to the paleocolimate estimate of ECS. (higher)

[AbruptSLR]

The linked reference cites new research regarding condensation particle (CP) concentrations at the coastal Neumayer Antarctic station, which is associated with cloud cover formation.  The AR5 model projections have particularly poor skill levels with regards to projecting Antarctic cloud formation, and reading between the lines, this research supports the idea that DSM is important w.r.t. Antarctic cloud formation, as the other CP concentrations appear to be insufficient to account for the discrepancy between model projections & observations:

Weller, R., Schmidt, K., Teinilä, K., and Hillamo, R.: Natural new particle formation at the coastal Antarctic site Neumayer, Atmos. Chem. Phys., 15, 11399-11410, doi:10.5194/acp-15-11399-2015, 2015.

http://www.atmos-chem-phys.net/15/11399/2015/acp-15-11399-2015.html

Abstract. We measured condensation particle (CP) concentrations and particle size distributions at the coastal Antarctic station Neumayer (70°39´ S, 8°15´ W) during two summer campaigns (from 20 January to 26 March 2012 and 1 February to 30 April 2014) and during the polar night between 12 August and 27 September 2014 in the particle diameter (Dp) range from 2.94 to 60.4 nm (2012) and from 6.26 to 212.9 nm (2014). During both summer campaigns we identified all in all 44 new particle formation (NPF) events. From 10 NPF events, particle growth rates could be determined to be around 0.90 ± 0.46 nm h−1 (mean ± SD; range: 0.4–1.9 nm h−1). With the exception of one case, particle growth was generally restricted to the nucleation mode (Dp < 25 nm) and the duration of NPF events was typically around 6.0 ± 1.5 h (mean ± SD; range: 4–9 h). Thus, in the surrounding area of Neumayer, particles did not grow up to sizes required for acting as cloud condensation nuclei. NPF during summer usually occurred in the afternoon in coherence with local photochemistry. During winter, two NPF events could be detected, though showing no ascertainable particle growth. A simple estimation indicated that apart from sulfuric acid, the derived growth rates required other low volatile precursor vapours.

[AbruptSLR]

The linked reference indicates that satellite observations indicate that aerosols can darken the albedo of subtropical marine stratocumulus clouds:

Frida Bender, Anders Engström, and Johannes Karlsson (2015), "Factors controlling cloud albedo in marine subtropical stratocumulus regions in climate models and satellite observations", Journal of Climate, doi: http://dx.doi.org/10.1175/JCLI-D-15-0095.1

http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-15-0095.1

Abstract: "This study focuses on the radiative properties of five subtropical marine stratocumulus cloud regions, on monthly mean scale. Through examination of the relation between total albedo and cloud fraction, and its variability and relation to other parameters, some of the factors controlling the reflectivity, or albedo, of the clouds in these regions are investigated. It is found that the main part of the variability in albedo at a given cloud fraction can be related to temporal, rather than spatial variability, indicating spatial homogeneity in cloud radiative properties in the studied regions. This is seen most clearly in satellite observations, but also in an ensemble of climate models. Further comparison between satellite data and output from climate models shows that there is good agreement with respect to the role of liquid water path, the parameter that can be assumed to be the primary source of variability in cloud reflectivity for a given cloud fraction. On the other hand, the influence of aerosol loading on cloud albedo differs between models and observations. The cloud-albedo effect, or cloud brightening caused by aerosol through its coupling to cloud droplet number concentration and droplet size, is found not to dominate in the satellite observations on monthly mean scale, as it appears to do on this scale in the climate models. The disagreement between models and observations is particularly strong in regions with frequent occurrence of absorbing aerosols above clouds, where satellite data contrary to the climate models indicate a scene darkening with increasing aerosol loading."

[AbruptSLR]

The linked reference confirms many of the warns provided by jai in this thread, i. e. that even if CoP21 reduces anthropogenic forcing down to the RCP 6.0 scenario that future aerosol mitigation efforts will likely increase Polar Amplification and will have substantial local climate effect and will promote telecommunication of energy to more remote regions (like high-latitude regions):

Clifford Chuwah, Twan van Noije, Detlef P. van Vuuren, Philippe Le Sager and Wilco Hazeleger (2015), "Global and regional climate impacts of future aerosol mitigation in an RCP6.0-like scenario in EC-Earth", Climatic Change, pp 1-14, doi:10.1007/s10584-015-1525-9


http://rd.springer.com/article/10.1007%2Fs10584-015-1525-9


Abstract: "Future changes in aerosol concentrations will influence the climate system over the coming decades. In this study we evaluate the equilibrium climate response to aerosol reductions in different parts of the world in 2050, using the global climate model EC-Earth. The aerosol concentrations are based on a set of scenarios similar to RCP6.0, developed using the IMAGE integrated assessment model and exploring stringent and weaker air pollution control. Reductions in aerosol concentrations lead to an increase in downward surface solar radiation under all-sky conditions in various parts of the world, especially in Asia where the local brightening may reach about 10 Wm−2. The associated increase in surface temperature may be as high as 0.5 °C. This signal is dominated by the reduced cooling effect of sulphate which in some areas is partially compensated by the decreased warming effect of black carbon. According to our simulations, the mitigation of BC may lead to decreases in mean summer surface temperature of up to 1 °C in central parts of North America and up to 0.3 °C in northern India. Aerosol reductions could significantly affect the climate at high latitudes especially in the winter, where temperature increases of up to 1 °C are simulated. In the Northern Hemisphere, this strong surface temperature response might be related to changes in circulation patterns and precipitation at low latitudes, which can give rise to a wave train and induce changes in weather patterns at high latitudes. Our model does not include a parameterization of aerosol indirect effects so that responses could be stronger in reality. We conclude that different, but plausible, air pollution control policies can have substantial local climate effects and induce remote responses through dynamic teleconnections."

[jai mitchell]

Our model does not include a parameterization of aerosol indirect effects so that responses could be stronger in reality.

"could" = will

[AbruptSLR]

The linked open access reference presents field observations of secondary organic aerosols (BSOA) from a coniferous forest mountain region at Whistler, British Columbia.  This field work indicates laboratory measures of BSOA level may not adequately characterize the full extent of BSOA emissions from coniferous (boreal) forests; which indicates that true levels of BSOA may be masking the true magnitude of TCR & ECS, so that if the boreal forest degrade rapidly in the next few decades, the world could be subjected to higher climate sensitivity values than currently accounted for in climate models:

Lee, A. K. Y., Abbatt, J. P. D., Leaitch, W. R., Li, S.-M., Sjostedt, S. J., Wentzell, J. J. B., Liggio, J., and Macdonald, A. M. (2015), "Substantial secondary organic aerosol formation in a coniferous forest: observations of both day and night time chemistry", Atmos. Chem. Phys. Discuss., 15, 28005-28035, doi:10.5194/acpd-15-28005-2015.

http://www.atmos-chem-phys-discuss.net/15/28005/2015/acpd-15-28005-2015.html

Abstract: "Substantial biogenic secondary organic aerosol (BSOA) formation was investigated in a coniferous forest mountain region at Whistler, British Columbia. A largely biogenic aerosol growth episode was observed, providing a unique opportunity to investigate BSOA formation chemistry in a forested environment with limited influence from anthropogenic emissions. Positive matrix factorization of aerosol mass spectrometry (AMS) measurement identified two types of BSOA (BSOA-1 and BSOA-2), which were primarily generated by gas-phase oxidation of monoterpenes and perhaps sesquiterpenes. The temporal variations of BSOA-1 and BSOA-2 can be explained by gas-particle partitioning in response to ambient temperature and the relative importance of different oxidation mechanisms between day and night. While BSOA-1 will arise from gas-phase ozonolysis and nitrate radical chemistry at night, BSOA-2 is less volatile than BSOA-1 and consists of products formed via gas-phase oxidation by the OH radical and ozone during the day. Organic nitrates produced through nitrate radical chemistry can account for 22–33 % of BSOA-1 mass at night. The mass spectra of BSOA-1 and BSOA-2 have higher values of the mass fraction of m/z 91 (f91) compared to the background organic aerosol, and so f91 is used as an indicator of BSOA formation pathways. A comparison between laboratory studies in the literature and our field observations highlights the potential importance of gas-phase formation chemistry of BSOA-2 type materials that may not be captured in smog chamber experiments, perhaps due to the wall loss of gas-phase intermediate products."

[AbruptSLR]

The linked open access reference provides evidence that air pollution will likely extend the life of Arctic sea ice (from climate change) for about one extra decade.  On the other hand this implies that the true strength of Arctic Amplification is stronger than we are currently experiencing, which will become apparent as the air pollution is cleaned-up (furthermore this research may well err on the side of least drama):

Gagné, M.-È., N. P. Gillett, and J. C. Fyfe (2015), "Impact of aerosol emission controls on future Arctic sea ice cover", Geophys. Res. Lett., 42, doi:10.1002/2015GL065504

http://onlinelibrary.wiley.com/doi/10.1002/2015GL065504/pdf

Abstract: "We examine the response of Arctic sea ice to projected aerosol and aerosol precursor emission changes under the Representative Concentration Pathway (RCP) scenarios in simulations of the Canadian Earth System Model. The overall decrease in aerosol loading causes a warming, largest over the Arctic, which leads to an annual mean reduction in sea ice extent of approximately 1 million km2 over the 21st century in all RCP scenarios. This accounts for approximately 25% of the simulated reduction in sea ice extent in RCP 4.5, and 40% of the reduction in RCP 2.5. In RCP 4.5, the Arctic ocean is projected to become ice-free during summertime in 2045, but it does not become ice-free until 2057 in simulations with aerosol precursor emissions held fixed at 2000 values. Thus, while reductions in aerosol emissions have significant health and environmental benefits, their substantial contribution to projected Arctic climate change should not be overlooked."


See also:

http://www.climatecentral.org/news/pollution-arctic-sea-ice-19583

Extract: "The main driver for Arctic sea ice’s disappearing act is the rising ocean and air temperatures driven by human greenhouse gas emissions. But that isn’t the only factor affecting Arctic sea ice. Air pollution also plays a role and can actually slow down warming.
In the tug of war, aerosols don’t necessarily counter the impacts of climate change on sea ice (or the planet as a whole for that matter). But new research shows that air pollution could buy the planet a decade of ice in the Arctic."

[Bruce Steele]

ASLR, Same paper covered in Scientific American. I got a little amusement with the magazine comment in parenthesis .
 " Using a middle of the road emissions scenario ( which is a little optimistic given currently pledges)
as well as rising aerosols Gillet and his team shows that the Arctic is likely to see an ice-free summer around 2057."

http://www.scientificamerican.com/article/pollution-could-buy-an-extra-decade-of-arctic-sea-ice/

[AbruptSLR]

The linked research presents finding of unexpected high ultrafine aerosols concentrations above East Antarctic sea ice.  The ultrafine aerosols were found to be transported to the surface boundary layer from the free troposphere by cyclones, and that the aerosols help to explain differences between observed and simulated Southern Ocean cloud cover:

Humphries, R. S., Klekociuk, A. R., Schofield, R., Keywood, M., Ward, J., and Wilson, S. R. (2015), "Unexpectedly high ultrafine aerosol concentrations above East Antarctic sea-ice", Atmos. Chem. Phys. Discuss., 15, 29125-29170, doi:10.5194/acpd-15-29125-2015.

http://www.atmos-chem-phys-discuss.net/15/29125/2015/acpd-15-29125-2015.html

Abstract. The effect of aerosols on clouds and their radiative properties is one of the largest uncertainties in our understanding of radiative forcing. A recent study has concluded that better characterisation of pristine, natural aerosol processes leads to the largest reduction in these uncertainties. Antarctica, being far from anthropogenic activities, is an ideal location for the study of natural aerosol processes. Aerosol measurements in Antarctica are often limited to boundary layer air-masses at spatially sparse coastal and continental research stations, with only a handful of studies in the sea ice region. In this paper, the first observational study of sub-micron aerosols in the East Antarctic sea ice region is presented. Measurements were conducted aboard the ice-breaker Aurora Australis in spring 2012 and found that boundary layer condensation nuclei (CN3) concentrations exhibited a five-fold increase moving across the Polar Front, with mean Polar Cell concentrations of 1130 cm−3 – higher than any observed elsewhere in the Antarctic and Southern Ocean region. The absence of evidence for aerosol growth suggested that nucleation was unlikely to be local. Air parcel trajectories indicated significant influence from the free troposphere above the Antarctic continent, implicating this as the likely nucleation region for surface aerosol, a similar conclusion to previous Antarctic aerosol studies. The highest aerosol concentrations were found to correlate with low pressure systems, suggesting that the passage of cyclones provided an accelerated pathway, delivering air-masses quickly from the free-troposphere to the surface. After descent from the Antarctic free troposphere, trajectories suggest that sea ice boundary layer air-masses travelled equator-ward into the low albedo Southern Ocean region, transporting with them emissions and these aerosol nuclei where, after growth, may potentially impact on the region's radiative balance. The high aerosol concentrations and their transport pathways described here, could help reduce the discrepancy currently present between simulations and observations of cloud and aerosol over the Southern Ocean.

[AbruptSLR]

The linked (open access) reference provides satellite data that indicates that China's efforts to reduce sulfur dioxide emissions from its coal-fired power plants has not been as effective as previously expected, probably due to poor implementation of regulations:

Siwen Wang, Qiang Zhang, Randall V Martin, Sajeev Philip, Fei Liu, Meng Li, Xujia Jiang and Kebin He (2015), "Satellite measurements oversee China's sulfur dioxide emission reductions from coal-fired power plants", Environmental Research Letters, Volume 10, Number 11 , doi:10.1088/1748-9326/10/11/11401


http://iopscience.iop.org/article/10.1088/1748-9326/10/11/114015


Abstract: "To evaluate the real reductions in sulfur dioxide (SO2) emissions from coal-fired power plants in China, Ozone Monitoring Instrument (OMI) remote sensing SO2 columns were used to inversely model the SO2 emission burdens surrounding 26 isolated power plants before and after the effective operation of their flue gas desulfurization (FGD) facilities. An improved two-dimensional Gaussian fitting method was developed to estimate SO2 burdens under complex background conditions, by using the accurate local background columns and the customized fitting domains for each target source. The OMI-derived SO2 burdens before effective FGD operation were correlated well with the bottom-up emission estimates (R = 0.92), showing the reliability of the OMI-derived SO2 burdens as a linear indicator of the associated source strength. OMI observations indicated that the average lag time period between installation and effective operation of FGD facilities at these 26 power plants was around 2 years, and no FGD facilities have actually operated before the year 2008. The OMI estimated average SO2 removal equivalence (56.0%) was substantially lower than the official report (74.6%) for these 26 power plants. Therefore, it has been concluded that the real reductions of SO2 emissions in China associated with the FGD facilities at coal-fired power plants were considerably diminished in the context of the current weak supervision measures."

[AbruptSLR]

The linked (open access) reference discusses progress in modeling aerosol-cloud interactions, and concludes that it is important to include these better characterized interactions in future climate model projections:

Gettelman, A.: Putting the clouds back in aerosol–cloud interactions, Atmos. Chem. Phys., 15, 12397-12411, doi:10.5194/acp-15-12397-2015, 2015

http://www.atmos-chem-phys.net/15/12397/2015/acp-15-12397-2015.html

Abstract. Aerosol–cloud interactions (ACI) are the consequence of perturbed aerosols affecting cloud drop and crystal number, with corresponding microphysical and radiative effects. ACI are sensitive to both cloud microphysical processes (the "C" in ACI) and aerosol emissions and processes (the "A" in ACI). This work highlights the importance of cloud microphysical processes, using idealized and global tests of a cloud microphysics scheme used for global climate prediction. Uncertainties in key cloud microphysical processes examined with sensitivity tests cause uncertainties of nearly −30 to +60 % in ACI, similar to or stronger than uncertainties identified due to natural aerosol emissions (−30 to +30 %). The different dimensions and sensitivities of ACI to microphysical processes identified in previous work are analyzed in detail, showing that precipitation processes are critical for understanding ACI and that uncertain cloud lifetime effects are nearly one-third of simulated ACI. Buffering of different processes is important, as is the mixed phase and coupling of the microphysics to the condensation and turbulence schemes in the model.


Extract: " The overall conclusion is that getting better a representation of ACI is critical for reducing uncertainty in anthropogenic climate forcing: cloud microphysical development needs to go hand in hand with better constraints on aerosol emissions to properly constrain ACI and total forcing."

[AbruptSLR]

The linked (open access) reference indicates that secondary organic aerosols (SOAs) have production rates 4 times higher and sinks a factor of 3.7 more efficient than in the base model; which, corresponds to a direct radiative forcing at top of the atmosphere of: −0.35 W m−2

Hodzic, A., Kasibhatla, P. S., Jo, D. S., Cappa, C., Jimenez, J. L., Madronich, S., and Park, R. J. (2015), "Rethinking the global secondary organic aerosol (SOA) budget: stronger production, faster removal, shorter lifetime", Atmos. Chem. Phys. Discuss., 15, 32413-32468, doi:10.5194/acpd-15-32413-2015.

http://www.atmos-chem-phys-discuss.net/15/32413/2015/acpd-15-32413-2015.html

Abstract: "Recent laboratory studies suggest that secondary organic aerosol (SOA) formation rates are higher than assumed in current models. There is also evidence that SOA removal by dry and wet deposition occurs more efficiently than some current models suggest, and that photolysis and heterogeneous oxidation may be important (but currently ignored) SOA sinks. Here, we have updated the global GEOS-Chem model to include this new information on formation (i.e. wall-corrected yields and emissions of semi-volatile and intermediate volatility organic compounds) and on removal processes (photolysis and heterogeneous oxidation). We compare simulated SOA from various model configurations against ground, aircraft and satellite measurements to assess the extent to which these improved representations of SOA formation and removal processes are consistent with observed characteristics of the SOA distribution. The updated model presents a more dynamic picture of the lifecycle of atmospheric SOA, with production rates 4 times higher and sinks a factor of 3.7 more efficient than in the base model. In particular, the updated model predicts larger SOA concentrations in the boundary layer and lower concentrations in the upper troposphere, leading to better agreement with surface and aircraft measurements of organic aerosol compared to the base model. Our analysis thus suggests that the long-standing discrepancy in model predictions of the vertical SOA distribution can now be resolved, at least in part, by a stronger source and stronger sinks leading to a shorter lifetime. The predicted global SOA burden in the updated model is 0.95 Tg and the corresponding direct radiative forcing at top of the atmosphere is −0.35 W m−2, which is comparable to recent model estimates constrained by observations. The updated model predicts a population-weighed global mean surface SOA concentration that is a factor of 2 higher than in the base model, suggesting the need for a reanalysis of the contribution of SOA to PM pollution-related human health effects. The potential importance of our estimates highlights the need for more extensive field and laboratory studies focused on characterizing organic aerosol removal mechanisms and rates."

[AbruptSLR]

The linked reference confirms that current UN projections for future global warming underestimate the influence of the expected aerosol reductions anticipated in the RCP scenarios; which will require more strenuous efforts than the UN is currently contemplating if the world is to remain below the 2C "limit".

Westervelt, D. M., Horowitz, L. W., Naik, V., Golaz, J.-C., and Mauzerall, D. L. (2015), "Radiative forcing and climate response to projected 21st century aerosol decreases", Atmos. Chem. Phys., 15, 12681-12703, doi:10.5194/acp-15-12681-2015

http://www.atmos-chem-phys.net/15/12681/2015/acp-15-12681-2015.html

Abstract: "It is widely expected that global emissions of atmospheric aerosols and their precursors will decrease strongly throughout the remainder of the 21st century, due to emission reduction policies enacted to protect human health. For instance, global emissions of aerosols and their precursors are projected to decrease by as much as 80 % by the year 2100, according to the four Representative Concentration Pathway (RCP) scenarios. The removal of aerosols will cause unintended climate consequences, including an unmasking of global warming from long-lived greenhouse gases. We use the Geophysical Fluid Dynamics Laboratory Coupled Climate Model version 3 (GFDL CM3) to simulate future climate over the 21st century with and without the aerosol emission changes projected by each of the RCPs in order to isolate the radiative forcing and climate response resulting from the aerosol reductions. We find that the projected global radiative forcing and climate response due to aerosol decreases do not vary significantly across the four RCPs by 2100, although there is some mid-century variation, especially in cloud droplet effective radius, that closely follows the RCP emissions and energy consumption projections. Up to 1 W m−2 of radiative forcing may be unmasked globally from 2005 to 2100 due to reductions in aerosol and precursor emissions, leading to average global temperature increases up to 1 K and global precipitation rate increases up to 0.09 mm day−1. However, when using a version of CM3 with reduced present-day aerosol radiative forcing (−1.0 W m−2), the global temperature increase for RCP8.5 is about 0.5 K, with similar magnitude decreases in other climate response parameters as well. Regionally and locally, climate impacts can be much larger than the global mean, with a 2.1 K warming projected over China, Japan, and Korea due to the reduced aerosol emissions in RCP8.5, as well as nearly a 0.2 mm day−1 precipitation increase, a 7 g m−2 LWP decrease, and a 2 μm increase in cloud droplet effective radius. Future aerosol decreases could be responsible for 30–40 % of total climate warming (or 10–20 % with weaker aerosol forcing) by 2100 in East Asia, even under the high greenhouse gas emissions scenario (RCP8.5). The expected unmasking of global warming caused by aerosol reductions will require more aggressive greenhouse gas mitigation policies than anticipated in order to meet desired climate targets."

[AbruptSLR]

I am not such that I understand the linked open access reference, that seems to indicate that anthropogenic direct aerosol forcing is near zero, and apparently, per the attached associated image, that the direct anthropogenic direct aerosol forcing is concentrated near equatorial Africa, as neither of these findings make much sense to me:

Chung, Chul E., Chu, Jung-Eun, Lee, Yunha, van Noije, Twan, Jeoung, Hwayoung, Ha, Kyung-Ja, and Marks, Marguerite: Global direct aerosol radiative forcing, as constrained by comprehensive observations, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-30, in review, 2016.

http://www.atmos-chem-phys-discuss.net/acp-2016-30/

Abstract. Aerosols directly affect the radiative balance of the Earth through absorption and scattering of solar radiation. Although the contributions of absorption (heating) and scattering (cooling) of sunlight have proved difficult to quantify, the consensus is that anthropogenic aerosols cool the climate, partially offsetting the warming by rising greenhouse gas concentrations. Recent estimates of global direct aerosol radiative forcing are −0.35 ± 0.5 Wm−2, and these estimates depend either entirely or heavily on aerosol simulation. Here, we integrate a comprehensive suite of satellite and ground-based observations to constrain total AOD, its fine-mode fraction, the vertical distribution of aerosols and clouds, and the co-location of clouds and overlying aerosols. We find that fine-mode forcing is −0.46 Wm−2 (−0.54 ~ −0.39 Wm−2). Fine-mode aerosols include sea salt and dust aerosols, and we find that these natural aerosols pose a very large cooling (−0.44 ~ −0.26 Wm−2) when constrained by observations. When the contribution of these natural aerosols is subtracted from the fine-mode forcing, the net becomes −0.10 (−0.28 ~ +0.05) Wm−2. The net forcing arises from carbonaceous, sulfate and nitrate aerosols. Despite uncertainties in the anthropogenic fraction of these aerosols, this −0.28 ~ +0.05 Wm−2 range compels the direct aerosol forcing to be near zero.

[AbruptSLR]

The linked reference provides a summary of the latest IPCC thinking on anthropogenic aerosols & their impact on global climate:

Hua Zhang, Shuyun Zhao, Zhili Wang, Xiaoye Zhang & Lianchun Song (25 January 2016), "The updated effective radiative forcing of major anthropogenic aerosols and their effects on global climate at present and in the future", International Journal of Climatology, DOI: 10.1002/joc.4613

http://onlinelibrary.wiley.com/doi/10.1002/joc.4613/abstract

Abstract: "The effective radiative forcing (ERF), as newly defined in the Intergovernmental Panel on Climate Change's Fifth Assessment Report (IPCC AR5), of three anthropogenic aerosols [sulphate (SF), black carbon (BC), and organic carbon (OC)] and their comprehensive climatic effects were simulated and discussed, using the updated aerosol-climate online model of BCC_AGCM2.0.1_CUACE/Aero. From 1850 to 2010, the total ERF of these anthropogenic aerosols was −2.49 W m−2, of which the aerosol–radiation interactive ERF (ERFari) and aerosol–cloud interactive ERF (ERFaci) were ∼ −0.30 and −2.19 W m−2, respectively. SF was the largest contributor to the total ERF, with an ERF of −2.37 W m−2. The ERF of BC and OC were 0.12 and −0.31 W m−2, respectively. From 1850 to 2010, anthropogenic aerosols brought about a decrease of ∼2.53 K and ∼0.20 mm day−1 in global annual mean surface temperature and precipitation, respectively. Surface cooling was most obvious over mid- and high latitudes in the northern hemisphere (NH). Precipitation change was most pronounced near the equator, with decreased and increased rainfall to the north and south of the equator, respectively; this might be largely related to the enhanced Hadley Cell in the NH. Relative humidity near surface was increased, especially over land, due to surface cooling induced by anthropogenic aerosols. Cloud cover and water path were increased, especially in or near the source regions of anthropogenic aerosols. Experiments based on the Representative Concentration Pathway (RCP) 4.5 given in IPCC AR5 shows the dramatic decrease in three anthropogenic aerosols in 2100 will lead to an increase of ∼2.06 K and 0.16 mm day−1 in global annual mean surface temperature and precipitation, respectively, compared with those in 2010."

[Richard Rathbone]

i.e if we'd cleaned up our aerosol act we'd already have broken 3 degrees?

[AbruptSLR]

i.e if we'd cleaned up our aerosol act we'd already have broken 3 degrees?

The attached NOAA images shows the projected temperature increases (baselined to 2000) for the different RCP scenarios.  As the scenario numbers (e.g. 4.5) is the assumed radiative forcing, if you clean-up the aerosols faster then you need to clean-up the emissions faster to maintain the assumed forcing (otherwise you need to follow a different pathway say RCP 8.5).

Per the paper: "Experiments based on the Representative Concentration Pathway (RCP) 4.5 given in IPCC AR5 shows the dramatic decrease in three anthropogenic aerosols in 2100 will lead to an increase of ∼2.06 K and 0.16 mm day−1 in global annual mean surface temperature and precipitation, respectively, compared with those in 2010."

[Richard Rathbone]

Its the 2010 number  (2.53K higher without aerosols) that seems incredible to me.

[jai mitchell]

Its the 2010 number  (2.53K higher without aerosols) that seems incredible to me.

that doesn't sound right to me either, if it is, knowing that aerosols are preventing approximately 50% of the total atmospheric forcing from GHGs then our current climate response to a complete cessation of fossil fuels today would lead to a 4C increase in temperatures above pre-industrial.

I would have to assume that they mean, with feedbacks that this is by 2100. . .not sure though.

[Steven]

Its the 2010 number  (2.53K higher without aerosols) that seems incredible to me.

That paper seems to be an outlier.  Their climate model seems to have an excessively strong aerosol-cloud interaction.  In the abstract they say:

From 1850 to 2010, the total ERF [Effective Radiative Forcing] of these anthropogenic aerosols was −2.49 W m⁻², of which the aerosol–radiation interactive ERF (ERFari) and aerosol–cloud interactive ERF (ERFaci) were ~ −0.30 and −2.19 W m⁻², respectively.

http://onlinelibrary.wiley.com/doi/10.1002/joc.4613/abstract


Their estimate of −2.19 W m⁻² for aerosol-cloud interactive ERF is unrealistic.  (And hence their estimate of −2.49 W m⁻² for total aerosol forcing is also unrealistic.)

For comparison, the IPCC's estimate for the effective radiative forcing from aerosol-cloud interaction for the year 2011 (compared to pre-industrial) is  –0.45 W m⁻², with 90% confidence interval –1.2 to 0.0 W m⁻²:



(Source: IPCC AR5 Fig. 8.15).

[AbruptSLR]

That paper seems to be an outlier.  Their climate model seems to have an excessively strong aerosol-cloud interaction.  In the abstract they say:

While it certainly is possible that the Zhang et al (2016) paper is an outlier and should be discounted.  It is also possible that it represents an upper bound value.  Certainly, citing AR5 values as poof that Zhang is an outlier cannot be valid as AR5 merely catalogs the findings available well before its publication.  Furthermore, it is well understood that AR5 included values of climate sensitivity that were too low by unbiased standards; which raises the possibility that aerosol negative forcing is stronger than AR5 considered.  For example, Marvel et al (2015) found that the most likely value for TCR is 1.7C (which is above the AR5 most likely value); but the the following Storelvmo et al (2015)research cites a most likely value for TCR of 1.9C (with a 95% CL interval of 1.2C to 2.7C); which again raises the possibility that aerosol negative forcing is stronger than AR5 considers.

Storelvmo, Trude; Leirvik, Thomas; Phillips, Petter; Lohmann, Ulrike; Wild, Martin (2015), "Disentangling Aerosol Cooling and Greenhouse Warming to Reveal Earth's Climate Sensitivity", GU General Assembly 2015, held 12-17 April, 2015 in Vienna, Austria. id.4326, Bibliographic Code: 2015EGUGA..17.4326S

Abstract: "Earth's climate sensitivity has been the subject of heated debate for decades, and recently spurred renewed interest after the latest IPCC assessment report suggested a downward adjustment of the most likely range of climate sensitivities. Here, we present a study based on the time period 1964 to 2010, which is unique in that it does not rely on global climate models (GCMs) in any way. The study uses surface observations of temperature and incoming solar radiation from approximately 1300 surface sites, along with observations of the equivalent CO2 concentration (CO2,eq) in the atmosphere, to produce a new best estimate for the transient climate sensitivity of 1.9K (95% confidence interval 1.2K - 2.7K). This is higher than other recent observation-based estimates, and is better aligned with the estimate of 1.8K and range (1.1K - 2.5K) derived from the latest generation of GCMs. The new estimate is produced by incorporating the observations in an energy balance framework, and by applying statistical methods that are standard in the field of Econometrics, but less common in climate studies. The study further suggests that about a third of the continental warming due to increasing CO2,eq was masked by aerosol cooling during the time period studied."

Furthermore, the following linked Zhang et al (2015) research shows that the uncertainties of aerosol indirect effects (AIE) is larger in local dynamic regimes than for the global average case.  As the dynamic area are more subject to non-linear positive feedback acceleration, this implies that we are at greater risk of accelerating ECS than previously understood:

Zhang, S., Wang, M., Ghan, S. J., Ding, A., Wang, H., Zhang, K., Neubauer, D., Lohmann, U., Ferrachat, S., Takeamura, T., Gettelman, A., Morrison, H., Lee, Y. H., Shindell, D. T., Partridge, D. G., Stier, P., Kipling, Z., and Fu, C.: On the characteristics of aerosol indirect effect based on dynamic regimes in global climate models, Atmos. Chem. Phys. Discuss., 15, 23683-23729, doi:10.5194/acpd-15-23683-2015, 2015.


http://www.atmos-chem-phys-discuss.net/15/23683/2015/acpd-15-23683-2015.html

Also I feel that it is premature to brand Zhang et al (2016) as an outlier, when researchers like Myhre et al (2015) cann't even cut the uncertainty associated with TCR in half before 2030.

Gunnar Myhre, Olivier Boucher, François-Marie Bréon, Piers Forster & Drew Shindell, (2015), "Declining uncertainty in transient climate response as CO2 forcing dominates future climate change", Nature Geoscience, doi:10.1038/ngeo2371

http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2371.html

Abstract: "Carbon dioxide has exerted the largest portion of radiative forcing and surface temperature change over the industrial era, but other anthropogenic influences have also contributed. However, large uncertainties in total forcing make it difficult to derive climate sensitivity from historical observations. Anthropogenic forcing has increased between the Fourth and Fifth Assessment Reports of the Intergovernmental Panel of Climate Change (IPCC) although its relative uncertainty has decreased. Here we show, based on data from the two reports, that this evolution towards lower uncertainty can be expected to continue into the future. Because it is easier to reduce air pollution than carbon dioxide emissions and because of the long lifetime of carbon dioxide, the less uncertain carbon dioxide forcing is expected to become increasingly dominant. Using a statistical model, we estimate that the relative uncertainty in anthropogenic forcing of more than 40% quoted in the latest IPCC report for 2011 will be almost halved by 2030, even without better scientific understanding. Absolute forcing uncertainty will also decline for the first time, provided projected decreases in aerosols occur. Other factors being equal, this stronger constraint on forcing will bring a significant reduction in the uncertainty of observation-based estimates of the transient climate response, with a 50% reduction in its uncertainty range expected by 2030."

Finally, I note that the linked 2015 The Guardian article points to evidence that climate sensitivity is likely higher than most mainstream climate scientists are willing to admit to publicly; which again raises the possibility that aerosol negative radiative forcing may be higher than the range that AR5 cataloged from several year old research.

http://www.theguardian.com/environment/climate-consensus-97-per-cent/2015/apr/30/overlooked-evidence-global-warming-may-proceed-faster-than-expected

[jai mitchell]

It must be here stated that the GHG Radiative Forcing values include the lapse rate and water vapor feedbacks in their calculation.  These associated feedbacks represent about 3/4 of the total forcing produced by a given amount of GHG emissions.

To my current understanding, I have looked but not found definitive proof, the radiative forcing values associated with aerosols are ONLY associated with immediate effects and the lapse rate and water vapor feedbacks are not included in their calculation.

If the total anthropogenic aerosol radiative forcing term is -0.5 W/m^2 then these feedbacks would produce the effective warming of +2.0 W/m^2 if those aerosols are removed.

If indeed the anthropogenic aerosol values are -2.4 W/m^2 then with the removal of those emissions, within about 10 years, we would see an effective forcing of +9.6 W/m^2.

iff this is true then our collective geese are truly cooked.

[AbruptSLR]

It must be here stated that the GHG Radiative Forcing values include the lapse rate and water vapor feedbacks in their calculation.  These associated feedbacks represent about 3/4 of the total forcing produced by a given amount of GHG emissions.

To my current understanding, I have looked but not found definitive proof, the radiative forcing values associated with aerosols are ONLY associated with immediate effects and the lapse rate and water vapor feedbacks are not included in their calculation.

If the total anthropogenic aerosol radiative forcing term is -0.5 W/m^2 then these feedbacks would produce the effective warming of +2.0 W/m^2 if those aerosols are removed.

If indeed the anthropogenic aerosol values are -2.4 W/m^2 then with the removal of those emissions, within about 10 years, we would see an effective forcing of +9.6 W/m^2.

iff this is true then our collective geese are truly cooked.

jai,

Thank you for the clarifications.  As Zhang et al (2016) has been peer reviewed, it is advisable to evaluate its implications seriously, rather than to simply label it an "outlier" as Steven's post implied.

Best,
ASLR

[Steven]

It must be here stated that the GHG Radiative Forcing values include the lapse rate and water vapor feedbacks in their calculation.

No, they are not included.  Forcings and feedbacks are treated separately.  The IPCC graphic that I posted upthread shows forcings, no feedbacks.  The concept of Effective Radiative Forcing (ERF) does include some "rapid adjustments", but those rapid adjustments are different from the usual feedbacks such as the water vapor and lapse rate feedbacks.

From IPCC AR5:

7.1.3   Forcing, Rapid Adjustments and Feedbacks


Forcings associated with agents such as greenhouse
gases (GHGs) and aerosols act on global mean surface temperature
through the global radiative (energy) budget.  Rapid adjustments
(sometimes called rapid responses) arise when forcing agents, by alter-
ing flows of energy internal to the system, affect cloud cover or other
components of the climate system and thereby alter the global budget
indirectly. Because these adjustments do not operate through changes
in the global mean surface temperature (ΔT), which are slowed by the
massive heat capacity of the oceans, they are generally rapid and most
are thought to occur within a few weeks. Feedbacks are associated
with changes in climate variables that are mediated by a change in
global mean surface temperature

...

In principle rapid adjustments are independent of ΔT, while feedbacks
operate purely through ΔT.

...

Furthermore, one
can distinguish between the traditional concept of radiative forcing
(RF) and the relatively new concept of effective radiative forcing (ERF)
that also includes rapid adjustments.

http://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter07_FINAL.pdf

[Steven]

For example, Marvel et al (2015) found that the most likely value for TCR is 1.7C (which is above the AR5 most likely value); but the the following Storelvmo et al (2015)research cites a most likely value for TCR of 1.9C (with a 95% CL interval of 1.2C to 2.7C); which again raises the possibility that aerosol negative forcing is stronger than AR5 considers.

Those Marvel and Storelvmo papers are both in good agreement with the IPCC consensus.  According to the IPCC AR5 report: "The transient climate response is likely in the range of 1.0°C to 2.5°C (high confidence) and extremely unlikely greater than 3°C".

Also I feel that it is premature to brand Zhang et al (2016) as an outlier, when researchers like Myhre et al (2015) cann't even cut the uncertainty associated with TCR in half before 2030.

Gunnar Myhre, Olivier Boucher, François-Marie Bréon, Piers Forster & Drew Shindell, (2015), "Declining uncertainty in transient climate response as CO2 forcing dominates future climate change", Nature Geoscience, doi:10.1038/ngeo2371

http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2371.html

Abstract: "Carbon dioxide has exerted the largest portion of radiative forcing and surface temperature change over the industrial era, but other anthropogenic influences have also contributed. However, large uncertainties in total forcing make it difficult to derive climate sensitivity from historical observations. Anthropogenic forcing has increased between the Fourth and Fifth Assessment Reports of the Intergovernmental Panel of Climate Change (IPCC) although its relative uncertainty has decreased. Here we show, based on data from the two reports, that this evolution towards lower uncertainty can be expected to continue into the future. Because it is easier to reduce air pollution than carbon dioxide emissions and because of the long lifetime of carbon dioxide, the less uncertain carbon dioxide forcing is expected to become increasingly dominant. Using a statistical model, we estimate that the relative uncertainty in anthropogenic forcing of more than 40% quoted in the latest IPCC report for 2011 will be almost halved by 2030, even without better scientific understanding. Absolute forcing uncertainty will also decline for the first time, provided projected decreases in aerosols occur. Other factors being equal, this stronger constraint on forcing will bring a significant reduction in the uncertainty of observation-based estimates of the transient climate response, with a 50% reduction in its uncertainty range expected by 2030."

What Myhre et al. actually say is that the stronger constraint on anthropogenic forcing by 2030 will reduce the uncertainty for TCR by almost 50%, even without better scientific understanding.  In addition, it's possible that advances in scientific understanding could reduce the uncertainty even further.

[AbruptSLR]

Its the 2010 number  (2.53K higher without aerosols) that seems incredible to me.

That paper seems to be an outlier.  Their climate model seems to have an excessively strong aerosol-cloud interaction.  In the abstract they say:

From 1850 to 2010, the total ERF [Effective Radiative Forcing] of these anthropogenic aerosols was −2.49 W m⁻², of which the aerosol–radiation interactive ERF (ERFari) and aerosol–cloud interactive ERF (ERFaci) were ~ −0.30 and −2.19 W m⁻², respectively.

http://onlinelibrary.wiley.com/doi/10.1002/joc.4613/abstract


Their estimate of −2.19 W m⁻² for aerosol-cloud interactive ERF is unrealistic.  (And hence their estimate of −2.49 W m⁻² for total aerosol forcing is also unrealistic.)

For comparison, the IPCC's estimate for the effective radiative forcing from aerosol-cloud interaction for the year 2011 (compared to pre-industrial) is  –0.45 W m⁻², with 90% confidence interval –1.2 to 0.0 W m⁻²:



(Source: IPCC AR5 Fig. 8.15).

I note that per the linked pdf (& attached image) that Hua Zhang was one of the lead others of the WG1 AR5 Chapter 8, and thus likely has some idea of what he is talking about, and should not be dismissed as an outlier.  Furthermore, the Effective Radiative Forcing likely includes the aerosol-cloud interaction and the aerosol-radiation interaction:

http://www.climatechange2013.org/images/report/WG1AR5_Chapter08_FINAL.pdf

[Steven]

I note that per the linked pdf (& attached image) that Hua Zhang was one of the lead others of the WG1 AR5 Chapter 8, and thus likely has some idea of what he is talking about, and should not be dismissed as an outlier.

http://www.climatechange2013.org/images/report/WG1AR5_Chapter08_FINAL.pdf

Actually Hua Zhang is female.  Moreover, no one in this thread has claimed that Zhang et al. are outliers who have no idea what they are talking about.  The point is that in this particular Zhang et al. 2016 paper, their climate model has a total anthropogenic aerosol forcing that is aberrant compared to the scientific consensus.  (More precisely, the contribution from aerosol-radiation interactions in their climate model is in good agreement with the consensus, but the contribution from aerosol-cloud interactions is clearly not.)  If you read the Zhang et al. 2016 paper (pdf-file here), you'll see that they discuss this a bit further in the last paragraph of Section 3.2 of the paper:

ERFaci can be considered as the residual between ERF
and ERFari ∼ −2.19 W m−2, which is close to the anthropogenic
aerosol indirect effects (AIE) in Wang et al.
(2014) and the simulated AIE by CAM5 (Ghan et al.,
2012), but much larger than the values given by IPCC
(2013) [−0.45 (−1.2, 0) W m−2]. Hoose et al. (2009)
found that the AIE was very sensitive to the lower bound
of cloud droplet number concentration (CDNCmin), and
the absolute value of AIE decreased rapidly when the
CDNCmin increased from 0 to 40 cm−3. Many models
have adopted different CDNCmin, but CDNCmin was set
at 0 in this study, as the prescription of CDNCmin is still
physically unclear (Hoose et al., 2009). This is the main
reason for the much larger simulated ERFaci.