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Ken Feldman

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Re: The Science of Aerosols
« Reply #150 on: June 04, 2018, 08:58:30 PM »
Here's a 2015 study that shows the responses of three climate models to an idealized removal of all aerosols:

https://www.atmos-chem-phys.net/15/8201/2015/acp-15-8201-2015.pdf

Table 2 on page 8207 of that journal summarizes the results.  The formatting of the table doesn't translate, so here's an excerpt:

Emissions  Model           Temp Change (C)
SO2          HadGEM            0.838
SO2          ECHAM-HAM      0.831
SO2          NorESM             0.396
SO2          Mean                0.688

The effects for Organic Carbon was less warming (mean of 0.132) and for Black Carbon was slight cooling (mean of -0.044).

This is for the instantaneous removal of all anthropogenic aerosols, which won't happen (less than half of aerosols are now coming from utilities and industries).  And it doesn't include the responses from natural aerosols which may increase as a result of climate change.

So while the reduction of anthropogenic aerosols due to a decrease in fossil fuel burning may result in a slight increase in temperature, it probably will be far less than the 2 to 4 degrees I keep seeing people post in the ASI forums.
« Last Edit: June 04, 2018, 09:06:33 PM by Ken Feldman »

jai mitchell

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Re: The Science of Aerosols
« Reply #151 on: June 06, 2018, 06:41:09 AM »
the 2 to 4 degrees is in the Arctic only which experiences much greater impacts
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Ken Feldman

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Re: The Science of Aerosols
« Reply #152 on: June 06, 2018, 07:15:43 PM »
the 2 to 4 degrees is in the Arctic only which experiences much greater impacts

The recent studies I posted about  upthread show much smaller impacts, less than 1 degree C.  Are you confusing the overall warming impacts (including polar amplification due to warm water transport to the Arctic and changing air currents) from increased greenhouse gas concentrations with the increase due to reduced aerosols?  If so, you're double counting the impacts, as these studies are using GCMs that take into account these effects.

jai mitchell

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Re: The Science of Aerosols
« Reply #153 on: June 07, 2018, 07:05:43 PM »
Earlier papers that project aerosol forcing have real problems since many (most!) of the ESM models did not include key (known) atmospheric and atmospheric chemistry interactions with aerosols.  These models underestimate the aerosol effect.  This has been well known even before the publication of AR5 as satellite observations indicated much greater effects than were being modeled.

This total indirect effect is comprised of First (FIE) and Second (SIE) indirect effects, both are negative (cooling). 

The lack of these mechanisms in some models and the poor representation (compared to direct observations in others) led to the great uncertainty bars in the AR4 and AR5 (image below) for this effect.  The total indirect effect here is labeled "Cloud Adjustments due to Aerosols" with a median value of about 0.56 Watts/m^2.

Recent observations from the Satellite record indicate that the FIE component itself is underestimated by approximately 23% which has a cascading local effect based on relative humidity of several watts per meter squared.  See: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018GL077679

Quote
One‐unit enhancement in aerosol scattering coefficient by swelling effect is found to lead to a systematic underestimation of the first indirect effect (FIE) by about 23% that can result in an underestimation in the FIE‐related radiative forcing by several W/m2 depending on aerosol properties and relative humidity.

Recent observations from the satellite record performed by a different team of scientists shows that the FIE effect is approximately double the total effect shown in the graphic below (and cited as the median value of aerosol cloud impacts in AR5) See : http://www-k12.atmos.washington.edu/~dennis/McCoy-2017-Theglobalaerosol-cloud.pdf

Quote
Using preindustrial emissions models, the change in Nd between preindustrial and present day is estimated. Nd is inferred to have more than tripled in some regions. Cloud properties from Moderate Resolution Imaging Spectroradiometer (MODIS) are used to estimate the radiative forcing due to this change in Nd. The Twomey (FIE) effect operating in isolation is estimated to create a radiative forcing of -0.97 ± 0.23 W m^2 relative to the preindustrial era.

The problem (and this will be cross posted in the "Conservative Scientists" thread) is that these more recent papers that rely on models specifically tuned to include the total effects of aerosols show much higher cooling impacts, especially in the  Arctic than your examples.  see: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GL076079

Quote
We note that in two models, Arctic warming due to aerosol reductions reaches 4°C in some locations (Figures S2–S5). The four‐model mean increase for the 60°N–90°N region is 2.8°C.

note:  Even the four models used in this paper severely underestimate the FIE as shown in the first papers (23%) cited which was published only 1 month ago

Image of average model (4 model) response to aerosols removal found here: https://wol-prod-cdn.literatumonline.com/cms/attachment/46814f2f-f617-4dea-83ce-0ab4c61244bf/grl56865-fig-0002-m.jpg

You can download the Supplementary information with the individual model results of aerosol removal on temperatures (figures S2-S5) here:
https://agupubs.onlinelibrary.wiley.com/action/downloadSupplement?doi=10.1002%2F2017GL076079&attachmentId=2186427861

It is strongly urged that you limit your research for best accuracy to papers less than 2 years old since the modeling capabilities have increased significantly since 2015.  I understand that this has produced a lot of confusion in the discussion since the understanding of these aerosol impacts are changing very rapidly.

(postscript)  I note that the Wang paper that you cited on the previous page was published only last January and holds a much lower (by an order of magnitude!) cooling effect from Sulfates.  I was confused about the CESM use of tracers for SO2 and did some background research.  The CESM version 1.2.0 was released in 2013 and subsequent releases have only been for technical glitches (apparently)  the Aerosol component (CAM5) included new organic coupling.  However, the indication is that the CESM model does not include more recent developments in aerosol-cloud interactions.  The use of synthetic tracers is apparently an attempt to adjust the discrepancy between modeled and observed sulfate loading see lecture notes here: http://www.cesm.ucar.edu/events/tutorials/2016/lecture5-tilmes.pdf

AHA!! yes indeed, this recent paper shows that the CESM model projects the total indirect effect about 1/2 the observed effect produced from only the FIE!  So the CESM severely underestimates this cloud effect.  https://www.osti.gov/pages/servlets/purl/1375377
« Last Edit: June 07, 2018, 07:39:24 PM by jai mitchell »
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jai mitchell

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Re: The Science of Aerosols
« Reply #154 on: June 08, 2018, 07:57:21 PM »
WRT CESM as a model for aerosol response.

The long awaited release of CESM2 has occurred today.  This means that the previous papers using this model will likely have very different results since the process took over 4 years to produce the new version, using much greater computing capacity and much more detailed modules.  Here is the ins and outs of the new CESM2

http://www.cesm.ucar.edu/models/cesm2/whatsnew.html
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Ken Feldman

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Re: The Science of Aerosols
« Reply #155 on: June 10, 2018, 12:55:10 AM »
Jai,

Those are all good papers, based on recent modelling.  There are other good papers based on recent modelling that show the aerosol -cloud interactions may be over estimated.

For example, this 2017 paper that used observations of increased aerosol loading from a volcanic eruption:

https://www.nature.com/articles/nature22974

Quote
Aerosols have a potentially large effect on climate, particularly through their interactions with clouds, but the magnitude of this effect is highly uncertain. Large volcanic eruptions produce sulfur dioxide, which in turn produces aerosols; these eruptions thus represent a natural experiment through which to quantify aerosol–cloud interactions. Here we show that the massive 2014–2015 fissure eruption in Holuhraun, Iceland, reduced the size of liquid cloud droplets—consistent with expectations—but had no discernible effect on other cloud properties. The reduction in droplet size led to cloud brightening and global-mean radiative forcing of around −0.2 watts per square metre for September to October 2014. Changes in cloud amount or cloud liquid water path, however, were undetectable, indicating that these indirect effects, and cloud systems in general, are well buffered against aerosol changes. This result will reduce uncertainties in future climate projections, because we are now able to reject results from climate models with an excessive liquid-water-path response.

jai mitchell

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Re: The Science of Aerosols
« Reply #156 on: June 11, 2018, 02:58:37 AM »
Ken,

Excellent paper, however it does support a higher sensitivity for FIE than the current AR5.  See below:

https://ora.ox.ac.uk/objects/uuid:a63e1dbb-1671-4ed7-b19d-3fc7fdca5eff/download_file?file_format=application/pdf&safe_filename=MAIN_TEXT_affiliation_and_aknowledgement_changes_accepted.pdf&type_of_work=Journal%20article

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Despite such massive emissions and large anomalies in reff, we estimate a moderate globalmean radiative forcing of -0.21 ± 0.08 W.m-2 (1 standard deviation, Supplementary S15) for
September-October which equates to a global annual mean effective radiative forcing of -0.035 ± 0.013 W.m-2 (1 standard deviation) assuming that a forcing only occurs in September and October 2014. Global emissions of anthropogenic SO2 currently total around 100 TgSO2/year and the Intergovernmental Panel on Climate Change17,47 suggests a best estimate for the aerosol forcing of -0.9 W.m-2 , yielding a forcing efficiency of -0.009 W.m-2 318 /TgSO2. The emissions for September and October 2014 total approximately 4 TgSO2, thus the global annual mean radiative forcing efficiency for the 2014-15 eruption at  Holuhraun yields a forcing efficiency of -0.0088 ± 0.0024 W.m-2 320 /TgSO2 (1 standard deviation). The similarity is remarkable, but may be by chance given the modelled sensitivity to emission location and time (Supplementary S12).

So the values measured are slightly below the global mean AR5 value, but:

Quote
The global ERF from HadGEM3 over the September-October 2014 period is estimated at -0.21 W.m-2 . . . .We also investigate whether a fissure eruption of this magnitude could have a more significant radiative impact if the timing/location of the eruptions were different (Supplementary S12). Our simulations suggest that for contrasting scenarios the global ERF would i) strengthen to -0.29 W.m-2 (+40%) if the eruption commenced at the beginning of  June, ii) strengthen to -0.49 W.m-2 (+140%) if the fissure eruption had occurred in an area of South America where it could affect clouds in a stratocumulus-dominated regime

In other words, the constraint fits if it involves a far nothern hemisphere loading and the fall period, the effect is greatly exacerbated during both summer and in regions with higher relative humidity (i.e. the tropics). 

cheers!
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Ken Feldman

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Re: The Science of Aerosols
« Reply #157 on: June 11, 2018, 07:06:43 PM »
Jai,

The point of the volcanic study was that it shows that aerosol impacts on cloud properties from other studies may be overestimated.  From the paper:

Quote
Changes in cloud amount or cloud liquid water path, however, were undetectable, indicating that these indirect effects, and cloud systems in general, are well buffered against aerosol changes. This result will reduce uncertainties in future climate projections, because we are now able to reject results from climate models with an excessive liquid-water-path response.

This may be because the cloud impacts are much lower than the direct impacts of increased reflection of sunlight.  Here's a 2018 paper that reviews the current science related to aerosols:

https://link.springer.com/article/10.1007/s40641-018-0089-y

Quote
The first scale is energetic: the ERFaci is approximately two orders of magnitude smaller than the shortwave cloud radiative effect (e.g., [153]). The path to a 1% effect goes partly through large perturbations that occur rarely, or over limited areas (shiptracks, closing of open cells; [47]), and small perturbations that occur frequently, posing challenges for observability. For example, [122] indicate that shiptracks, the most eminently observable manifestation of ACI, exert a paltry 0.5 mW m−2 of forcing globally. The challenge is therefore to determine the meteorological conditions under which aerosol perturbations manifest as energetically significant, along with their geographical coverage and frequency of occurrence.

The second set of scales is spatiotemporal: the scales relevant for ACI range from the microscale through cloud-process scales for cloud-top turbulent entrainment and cloud updrafts. However, the aerosol perturbations at cloud-scale affect the regional and global circulation, and these regional- through global scale changes feed back as meteorological influences on cloud processes [112, 133, 150, 151]. This means that constraining ERFaci requires understanding the microscale, the cloud process scale, and the global scale, as well as the interactions between scales.

And improvements in comparing model outputs to observations are leading to lower estimates of forcing for aerosol-cloud impacts:

Quote
As discussed in “Why Are ERFaci Estimates so Challenging?”, progress is being made on understanding the discrepancy between GCM and observational estimates of ERFaci, which was large in AR5 (ERFari+aci = −0.93 to−0.45~W~m −2 
 to−0.45~W~m−2
 with a median of −0.85 W m−2 for studies using the satellite record, compared against − 1.68to−0.81~W~m −2 
to−0.81~W~m−2
 with a median value of − 1.38 W m−2 for GCM studies; [18]). Gryspeerdt et al. [52] show that the choice of N a  proxy can significantly reduce the discrepancy; their best estimate of RFaci based on a GCM-observation combination is −0.4 W m−2. Christensen et al. [28] and Neubauer et al. [102] take a different approach, investigating the effects of reducing near-cloud biases in satellite aerosol observations consistently between observations and modeling. This simplification of ACI, where the effect of clouds on aerosols is reduced, succeeds at bringing the GCM and observations into agreement and leads to a reduction in the intrinsic ERFaci to −0.28 ± 0.26 W m−2 from −0.49 ± 0.18 W m−2 when no removal of near-cloud aerosol observations is performed. However, the distant aerosol field can also be expected to have less causal connection with the aerosol that perturbed the cloud; the resulting forcing estimate should probably be considered an upper (i.e., least negative) bound.

jai mitchell

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Re: The Science of Aerosols
« Reply #158 on: June 11, 2018, 07:53:59 PM »
Ken,

Thanks for those papers, they are very good and provide great news, if their constraints prove to be true,  I remain somewhat skeptical of the overall results, knowing that the disentanglement of aerosol impacts on regional preciptitation, cloud height and reflectivity is very difficult to disengage and the larger effects of global atmospheric circulation patterns (and the potential for increased Relative Humidity) that would result from a complete removal of aerosols produces another massive amount of uncertainty.

I enjoyed reading the reference document from your paper above.  it is found here.  https://www.atmos-chem-phys.net/17/13151/2017/acp-17-13151-2017.pdf
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Ken Feldman

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Re: The Science of Aerosols
« Reply #159 on: June 11, 2018, 09:27:34 PM »
Ken,

Thanks for those papers, they are very good and provide great news, if their constraints prove to be true,  I remain somewhat skeptical of the overall results, knowing that the disentanglement of aerosol impacts on regional preciptitation, cloud height and reflectivity is very difficult to disengage and the larger effects of global atmospheric circulation patterns (and the potential for increased Relative Humidity) that would result from a complete removal of aerosols produces another massive amount of uncertainty.

I enjoyed reading the reference document from your paper above.  it is found here.  https://www.atmos-chem-phys.net/17/13151/2017/acp-17-13151-2017.pdf

For those who don't have time to follow the link Jai posted, here is the key takeaway from the study:

Quote
These new estimates suggest that
aerosol effects on the radiative properties of clouds are even
smaller than previously demonstrated from satellite-based
studies. This new methodology therefore further widens the
gap between the satellite and the very strong forcing estimates
derived using most GCMs.

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Re: The Science of Aerosols
« Reply #160 on: June 12, 2018, 06:43:35 AM »
I am inherently more interested in their effects in convection-prone areas. From a meteorologist's perspective and anecdotal experience, they seem to have significant effects on deep convection (often as an enhancement -- especially in oceanic environments where boundary layer moisture restriction is less of an issue).

jai mitchell

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Re: The Science of Aerosols
« Reply #161 on: June 12, 2018, 10:29:16 PM »
And it doesn't include the responses from natural aerosols which may increase as a result of climate change

Ocean acidification and tropical forest loss is projected to decline Dimethyl Sulfide emissions by a significant amount
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Ken Feldman

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Re: The Science of Aerosols
« Reply #162 on: June 13, 2018, 12:30:45 AM »
Here's a good overview of some of the possible changes to natural aerosol emissions due to climate change:

https://link.springer.com/article/10.1007/s40641-018-0086-1

It's a long paper, as it goes through each time of natural aerosol and discuss the current understanding of how they are impacted by changes in temperature, wind speed and precipitation or moisture content.  Here is the abstract:

Quote
Purpose of Review

Climate factors may considerably impact on natural aerosol emissions and atmospheric distributions. The interdependencies of processes within the aerosol-climate system may thus cause climate feedbacks that need to be understood. Recent findings on various major climate impacts on aerosol distributions are summarized in this review.

Recent Findings

While generally atmospheric aerosol distributions are influenced by changes in precipitation, atmospheric mixing, and ventilation due to circulation changes, emissions from natural aerosol sources strongly depend on climate factors like wind speed, temperature, and vegetation. Aerosol sources affected by climate are desert sources of mineral dust, marine aerosol sources, and vegetation sources of biomass burning aerosol and biogenic volatile organic gases that are precursors for secondary aerosol formation. Different climate impacts on aerosol distributions may offset each other.

Summary

In regions where anthropogenic aerosol loads decrease, the impacts of climate on natural aerosol variabilities will increase. Detailed knowledge of processes controlling aerosol concentrations is required for credible future projections of aerosol distributions.

jai mitchell

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Re: The Science of Aerosols
« Reply #163 on: June 15, 2018, 03:25:57 AM »
Ocean acidification and warming impacts on global DMS production

https://www.mpimet.mpg.de/fileadmin/staff/ilyinatatiana/SixAllNatureCC2013.pdf

Global warming amplified by reduced sulphur fluxes as a result of ocean acidification

Quote
Marine  DMS  emissions  are  the  largest  natural source of atmospheric sulphur and changes in their strength have the potential to alter the Earth’s radiation budget.  Here we  establish  observational-based  relationships  between  pH changes and DMS concentrations to estimate changes in future DMS emissions with Earth system model  climate simulations. Global  DMS  emissions  decrease  by  about  18(± 3)%  in  2100 compared with pre-industrial times as a result of the combined effects of ocean acidification and climate change. The reduced DMS   emissions   induce   a   significant   additional   radiative forcing, of which 83% is attributed to the impact of ocean acidification, tantamount to an equilibrium temperature response between  0.23  and  0.48 K.  Our  results  indicate  that  ocean acidification  has  the  potential  to  exacerbate  anthropogenic warming through a mechanism that is not considered at present in projections of future climate change
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