Support the Arctic Sea Ice Forum and Blog

Author Topic: The Science of Aerosols  (Read 41840 times)

Ken Feldman

  • ASIF Citizen
  • Posts: 132
    • View Profile
  • Liked: 11
  • Likes Given: 47
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

  • ASIF Upper Class
  • Posts: 1913
    • View Profile
  • Liked: 14
  • Likes Given: 5
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
Haiku of Past Futures
My "burning embers"
are not tri-color bar graphs
+3C today

Ken Feldman

  • ASIF Citizen
  • Posts: 132
    • View Profile
  • Liked: 11
  • Likes Given: 47
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

  • ASIF Upper Class
  • Posts: 1913
    • View Profile
  • Liked: 14
  • Likes Given: 5
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 »
Haiku of Past Futures
My "burning embers"
are not tri-color bar graphs
+3C today

jai mitchell

  • ASIF Upper Class
  • Posts: 1913
    • View Profile
  • Liked: 14
  • Likes Given: 5
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
Haiku of Past Futures
My "burning embers"
are not tri-color bar graphs
+3C today

Ken Feldman

  • ASIF Citizen
  • Posts: 132
    • View Profile
  • Liked: 11
  • Likes Given: 47
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

  • ASIF Upper Class
  • Posts: 1913
    • View Profile
  • Liked: 14
  • Likes Given: 5
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

Quote
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!
Haiku of Past Futures
My "burning embers"
are not tri-color bar graphs
+3C today

Ken Feldman

  • ASIF Citizen
  • Posts: 132
    • View Profile
  • Liked: 11
  • Likes Given: 47
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

  • ASIF Upper Class
  • Posts: 1913
    • View Profile
  • Liked: 14
  • Likes Given: 5
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
Haiku of Past Futures
My "burning embers"
are not tri-color bar graphs
+3C today

Ken Feldman

  • ASIF Citizen
  • Posts: 132
    • View Profile
  • Liked: 11
  • Likes Given: 47
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.

Csnavywx

  • ASIF Citizen
  • Posts: 465
    • View Profile
  • Liked: 6
  • Likes Given: 0
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

  • ASIF Upper Class
  • Posts: 1913
    • View Profile
  • Liked: 14
  • Likes Given: 5
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
Haiku of Past Futures
My "burning embers"
are not tri-color bar graphs
+3C today

Ken Feldman

  • ASIF Citizen
  • Posts: 132
    • View Profile
  • Liked: 11
  • Likes Given: 47
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

  • ASIF Upper Class
  • Posts: 1913
    • View Profile
  • Liked: 14
  • Likes Given: 5
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
Haiku of Past Futures
My "burning embers"
are not tri-color bar graphs
+3C today

gerontocrat

  • ASIF Governor
  • Posts: 2919
    • View Profile
  • Liked: 235
  • Likes Given: 4
Re: The Science of Aerosols
« Reply #164 on: July 09, 2018, 09:34:54 AM »
As anthropogenic aerosol emissions decrease, relative importance of Northern Oceans heat uptake increases. Important for Arctic Sea Ice, methinks?

https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-18-0170.1

Evolving Relative Importance of the Southern Ocean and North Atlantic in Anthropogenic Ocean Heat Uptake

Jia-Rui Shi*, Shang-Ping Xie, and Lynne D. Talley
Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, USA

https://doi.org/10.1175/JCLI-D-18-0170.1

Quote
Abstract
Ocean uptake of anthropogenic heat over the past 15 years has mostly occurred in the Southern Ocean, based on Argo float observations. This agrees with historical simulations from the Coupled Model Intercomparison Project phase 5 (CMIP5), where the Southern Ocean (south of 30±S) accounts for 72%±28% of global heat uptake, while the contribution from the North Atlantic north of 30±N is only 6%. Aerosols preferentially cool the Northern Hemisphere, and the effect on surface heat flux over the subpolar North Atlantic opposes the greenhouse gas (GHG) effect in nearly equal magnitude. This heat uptake compensation is associated with weakening (strengthening) of the Atlantic Meridional Overturning Circulation (AMOC) in response to GHG (aerosol) radiative forcing. Aerosols are projected to decline in the near future, reinforcing the greenhouse effect on the North Atlantic heat uptake. As a result, the Southern Ocean, which will continue to take up anthropogenic heat largely through the mean upwelling of water from depth, will be joined by increased relative contribution from the North Atlantic due to substantial AMOC slowdown in the 21st century. In the RCP8.5 scenario, the percentage contribution to global uptake is projected to decrease to 48%±8% in the Southern Ocean and increase to 26%±6% in the northern North Atlantic. Despite the large uncertainty in the magnitude of projected aerosol forcing, our results suggest that anthropogenic aerosols, given their geographic distributions and temporal trajectories, strongly influence the high latitude ocean heat uptake and interhemispheric asymmetry through AMOC change.
"Para a Causa do Povo a Luta Continua!"
"And that's all I'm going to say about that". Forrest Gump
"Damn, I wanted to see what happened next" (Epitaph)

jai mitchell

  • ASIF Upper Class
  • Posts: 1913
    • View Profile
  • Liked: 14
  • Likes Given: 5
Re: The Science of Aerosols
« Reply #165 on: July 10, 2018, 05:55:48 PM »
We looked at the results of Durack et. al. (2014) here. 

The results of the paper in ASLR's post above (Aerosol impact in the NH OHC) was discussed at that time.

https://forum.arctic-sea-ice.net/index.php/topic,1011.msg38424.html#msg38424

Quote
3.  The basic model of understanding appears to be severely underestimating northern hemisphere aerosol effects.
Haiku of Past Futures
My "burning embers"
are not tri-color bar graphs
+3C today

rboyd

  • ASIF Middle Class
  • Posts: 656
    • View Profile
  • Liked: 0
  • Likes Given: 15
Re: The Science of Aerosols
« Reply #166 on: July 22, 2018, 10:56:00 PM »
When we burn coal, the aerosol effect is pretty immediate and then stabilizes due to the low transit time of the aerosols in the atmosphere. The CO2 effect is cumulative, given the long transit time of CO2 in the atmosphere.

Therefore, even if we just stabilize the level of coal burning then rate of climate change should increase (no increases in aerosols to offset the cumulative increases in CO2). The impact of aerosol (SO2) scrubbers is even worse, as we continue with CO2 but greatly reduce the aerosols. If China goes this way to clean up their air we will rapidly find out how much climate dimming is produced by aerosols. Probably not a good outcome for global climate change, nor for regional weather patterns.
With the new maritime SO2 emission standards, this is already being put into practice for maritime traffic globally.

Overall, everything (increasing CO2 emissions, fugitive methane, SO2 reductions) seems to be pointing to an acceleration in climate change. Add another El Nino and the next decade could become the "climate shock" decade.

gerontocrat

  • ASIF Governor
  • Posts: 2919
    • View Profile
  • Liked: 235
  • Likes Given: 4
Re: The Science of Aerosols
« Reply #167 on: August 04, 2018, 10:18:52 AM »
Neven is in danger of becoming (in)famous.

https://www.theguardian.com/environment/climate-consensus-97-per-cent/2018/aug/03/pollution-is-slowing-the-melting-of-arctic-sea-ice-for-now

Pollution is slowing the melting of Arctic sea ice, for now
Human carbon pollution is melting the Arctic, but aerosol pollution is slowing it down


Quote
I asked Arctic writer Neven Curlin what his thoughts were. He maintains the go-to site for updated news on the Arctic and its ice. He told me,

Arctic sea ice loss is already bad news, and this research comes on top of it. It’s amazing to think that the loss could’ve been even faster, if it hadn’t been for this dampening effect. If reducing the emissions of aerosols leads to an even faster warming of the Arctic, this will only further decrease the temperature gradient between the pole and the equator, likely adding to the destabilisation of Northern Hemisphere weather patterns. Never mind the longer term risks tied to sea level rise, methane release and changes to ocean currents. Not reducing aerosols isn’t an option, either, and so we find ourselves in quite a predicament. Hopefully future research will show that the number is actually lower.

He also reiterated the importance of this topic. He told me that we need more young minds to study the cryosphere; we need more data to help us understand the long-term trends. To gather that data, we need more and better equipment. This is a great example of how a small investment now can pay huge dividends in the future.

I plonked this into the comments thing of the article as the link was just to Neven.typepad.

Quote
The link in the article  is to Neven's blog. The mass of data and comment on all things environmental are to be found on  https://forum.arctic-sea-ice.net/index.php  "The Arctic Sea Ice Forum" founded by Neven and open to all.

"Para a Causa do Povo a Luta Continua!"
"And that's all I'm going to say about that". Forrest Gump
"Damn, I wanted to see what happened next" (Epitaph)

Rob Dekker

  • ASIF Upper Class
  • Posts: 1796
    • View Profile
  • Liked: 64
  • Likes Given: 49
Re: The Science of Aerosols
« Reply #168 on: August 12, 2018, 08:44:46 AM »
Neven published this post in the ASIB :

http://neven1.typepad.com/blog/2018/08/aerosols-and-arctic-sea-ice-loss.html

which is essentially a summary of an article in the Guardian, it which he was quoted :

https://www.theguardian.com/environment/climate-consensus-97-per-cent/2018/aug/03/pollution-is-slowing-the-melting-of-arctic-sea-ice-for-now

The interesting claim from that article is this one :

Quote
So how much of an effect do aerosols have? It turns out 23% of the warming caused by greenhouse gases was offset by the cooling from aerosols.

I always like to check the science on such claims, and after Michael Sweet in the ASIB comment section found a free copy of the paper (Mueller et al 2018), I decided to review it :

https://dspace.library.uvic.ca/bitstream/handle/1828/7669/Mueller_Bennit_MSc_2016.pdf?sequence=1

I admit that I know very little about aerosols, and have not been following the literature about it.
So it may very well be that the better informed people on this fine forum find all of the following rather boring. But for me, it was pretty exciting and educative.

What I really wanted to know was how they determined that aerosols had a significant impact on Arctic Sea Ice decline.

Overall, I find the paper extremely thorough, and well argued.
I especially like their careful and formal handling of uncertainty in the data, and I learned a lot just reading the methods they use.

At the core, their method is pretty straightforward : They use CMIP5 GCM simulations of ALL, GHG only and NAT only forcings and use (multi-variable) linear regression to tease out these signals from the observed SIE over the 1953-2012 period. Something like this :

  SIEobs. = βoant*SIEoant + βnat*SIEnat + βghg*SIEghg

where SIEnat is the CMIP5 simulation of SIE (Sea Ice Exent) with Natural forcings only, SIEghg is the CMIP5 simulation of SIE with well mixed GHGs only, and SIEoant is the CMIP5 simulation of SIE with "everything else" (which is mostly aerosols).
In CMIP5, there is no "OANT" simulation, so they use OANT = ALL - GHG - NAT. Which makes sense. Just remember that OANT is basically "everything else" that is not GHG or Natural driven. That's mostly aerosols, but not exclusively.

SIEobs is the observed Arctic Sea Ice extent in September.

For SIEobs, they use three different SIE data sets : HadISST2 (which is a bit dated), Walsh and Chapman (WC) which is a great dataset, which we extensively discussed in the comment section here :
http://neven1.typepad.com/blog/2016/01/september-arctic-sea-ice-extent-1935-2014.html
and a new dataset by Piron and Pasalodos (PP) which I did not know about before and will certainly take a look at, especially since they date back to 1933.
PP and WC are apparently very similar for the 1953-2012 period that Mueller et al used.

The β's are scaling factors.

Here it gets interesting.
If a β factor is close to unity (1), that suggests that the simulation is very consistent with the actual observed SIE. If a β factor is much different from 1, there may be something fishy going on. For example, if a β factor is close to 0, the signal is not detected at all. That would mean the simulated signal is not detectable in real live observations. If the β factor is much bigger than 1, there may be more causes for the signal in the observed data set than the simulations suggest.
Now just keep that in mind for a moment, because I will get back to that.

In my opinion, the real impressive part (the awe factor) in this paper is the way in which they deal with uncertainties. They have truly set up a Detection and Attribution mechanism, where the calculate formally how the uncertainties in the estimations propagate through the system. And there are many uncertainties to deal with : uncertainties in the GHG / aerosol / NAT forcings, uncertainties in the modeling of their effect on Arctic SIE, the uncertainties in the SIE record etc etc.

There are several formal statistical methods they use (like regularized optimal fingerprinting (ROF), and the residual consistency test (RCT)) that I can learn from, and could apply to my own method of predicting SIE in September based on earlier (June) data :
https://forum.arctic-sea-ice.net/index.php/topic,103.msg162418.html#msg162418

When they apply these methods, the signals for GHG increases, Natural forcing and OANT (mostly aerosols) clearly are present in all 3 SIE data sets. They all come out of the noise, with a 90% certainty. That's impressive.

So overall, I really like this paper.

The only question I have is regarding the β factor they obtain for OANT (everything else but GHG and NAT forcings). I attached the results, from Figure 3.3 in the paper.

This suggests that the OANT signal has a β factor of about 1.7 or 1.8. That means that the OANT (aerosols mostly) signal shows up 1.7 to 1.8 stronger in the actual SIE record than the simulations suggest. So either aerosols have a much stronger effect on SIE than simulations suggest, or there is another signal present in reality (maybe something like land snow-cover or so) which is similar to the aerosol, which is there in reality, but is not properly taken into account by the GCM CMIP5 simulations.

Also, I don't see the 23% number from the Guardian anywhere in the paper.
All I see is a 30% number (from the conclusions) :
Quote
OANT has offset about 30% of the decline that would have been
expected in the absence of OANT forcing due to the combined climate response from
GHG and NAT forcing.
I suspect that the difference (23% versus 30%) is caused by the fact that aerosols do not fully cover the OANT (everything except for GHG and NATural) forcings.

So, overall a great paper, with the notion that maybe they overestimated the influence of aerosols on Arctic Sea Ice extent by a factor of 1.7 - 1.8.
« Last Edit: August 12, 2018, 09:12:49 AM by Rob Dekker »
This is our planet. This is our time.
Let's not waste either.

AbruptSLR

  • ASIF Emperor
  • Posts: 15874
    • View Profile
  • Liked: 94
  • Likes Given: 6
Re: The Science of Aerosols
« Reply #169 on: August 20, 2018, 07:08:10 PM »
The linked reference indicates that climate models need to account for the geographic distribution of anthropogenic aerosol emissions in order to correctly simulate the associated radiative forcing impacts on global warming:

Geeta G. Persad & Ken Caldeira (2018), "Divergent global-scale temperature effects from identical aerosols emitted in different regions", Nature Communications, volume 9, Article number: 3289, DOI: https://doi.org/10.1038/s41467-018-05838-6

https://www.nature.com/articles/s41467-018-05838-6

Abstract: "The distribution of anthropogenic aerosols’ climate effects depends on the geographic distribution of the aerosols themselves. Yet many scientific and policy discussions ignore the role of emission location when evaluating aerosols’ climate impacts. Here, we present new climate model results demonstrating divergent climate responses to a fixed amount and composition of aerosol—emulating China’s present-day emissions—emitted from 8 key geopolitical regions. The aerosols’ global-mean cooling effect is fourteen times greater when emitted from the highest impact emitting region (Western Europe) than from the lowest (India). Further, radiative forcing, a widely used climate response proxy, fails as an effective predictor of global-mean cooling for national-scale aerosol emissions in our simulations; global-mean forcing-to-cooling efficacy differs fivefold depending on emitting region. This suggests that climate accounting should differentiate between aerosols emitted from different countries and that aerosol emissions’ evolving geographic distribution will impact the global-scale magnitude and spatial distribution of climate change."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

diablobanquisa

  • ASIF Citizen
  • Posts: 119
    • View Profile
  • Liked: 6
  • Likes Given: 1
Re: The Science of Aerosols
« Reply #170 on: September 07, 2018, 11:14:36 AM »


For SIEobs, they use three different SIE data sets : HadISST2 (which is a bit dated), Walsh and Chapman (WC) which is a great dataset, which we extensively discussed in the comment section here :
http://neven1.typepad.com/blog/2016/01/september-arctic-sea-ice-extent-1935-2014.html
and a new dataset by Piron and Pasalodos (PP) which I did not know about before and will certainly take a look at, especially since they date back to 1933.
PP and WC are apparently very similar for the 1953-2012 period that Mueller et al used.



Hi Rob,

Just to point out that the Piron and Pasalodos (PP) dataset is our time series, the one that we discussed nicely and extensively here: http://neven1.typepad.com/blog/2016/01/september-arctic-sea-ice-extent-1935-2014.html

(Journal article: https://doi.org/10.5281/zenodo.44756 , NetCDF file with the gridded data: https://doi.org/10.5281/zenodo.44757 , CSV file with the extent values: https://doi.org/10.5281/zenodo.44758)

Glad to see that our data are useful!


AbruptSLR

  • ASIF Emperor
  • Posts: 15874
    • View Profile
  • Liked: 94
  • Likes Given: 6
Re: The Science of Aerosols
« Reply #171 on: September 11, 2018, 07:32:08 PM »
The linked reference provides evidence that CMIP5 model projections 'have underestimated the cooling effect that aerosol particles have had on climate in recent decades"; which 'suggests that the models are not sensitive enough to increasing greenhouse gas concentrations in the atmosphere'.  In other words, this reference finds that the CMIP5 models (as a group) underestimate both TCR & ECS:


Trude Storelvmo et al. (29 August 2018), "Lethargic response to aerosol emissions in current climate models", Geophysical Research Letters, https://doi.org/10.1029/2018GL078298

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018GL078298

"Abstract
The global temperature trend observed over the last century is largely the result of two opposing effects – cooling from aerosol particles and greenhouse gas (GHG) warming. While the effect of increasing GHG concentrations on Earth's radiation budget is well‐constrained, that due to anthropogenic aerosols is not, partly due to a lack of observations. However, long‐term surface measurements of changes in downward solar radiation (SDSR), an often‐used proxy for aerosol radiative impact, are available worldwide over the last half‐century. We compare SDSR changes from ∼1,400 stations to those from the CMIP5 global climate simulations over the period 1961‐2005. The observed SDSR shows a strong early downward trend followed by a weaker trend‐reversal, broadly consistent with historical aerosol emissions. However, despite considerable changes to known aerosol emissions over time, the models show negligible SDSR trends, revealing a lethargic response to aerosol emissions, and casting doubt on the accuracy of their future climate projections.

Plain Language Summary
Observations of incoming solar radiation, as measured at approximately 1400 surface stations worldwide, show a strong downward trend from the 1960s to the 1980s, followed by a weaker trend reversal thereafter. These trends are thought to be due to changes in the amount of aerosol particles in the atmosphere, and we find support for that here in the temporal evolution of anthropogenic aerosol emissions. This is expected because aerosol particles reflect and/or absorb sunlight back to space, and have a net cooling effect on Earth's climate. However, we find that the current generation of climate models simulate negligible solar radiation trends over the last half‐century, suggesting that they have underestimated the cooling effect that aerosol particles have had on climate in recent decades. Despite this, climate models tend to reproduce surface air temperature over the time period in question reasonably well. This, in turn, suggests that the models are not sensitive enough to increasing greenhouse gas concentrations in the atmosphere, with important implications for their ability to simulate future climate."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson