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2010 Literature Review Archives - Radiative Forcing

Chen, W.-T., Y.H. Lee, P.J. Adams, A. Nenes and J.H. Seinfeld. 2010. Will black carbon mitigation dampen aerosol indirect forcing? GRL Vol 37, L09801.

Reducing emissions of black carbon and organic carbon is shown to have a potentially strong warming effect on climate through reductions in cloud albedo and lifetime. This could offset the climate cooling effect of black carbon emission reductions through reduced direct radiative forcing.

There is growing interest in the potential for mitigating climate warming through reductions in black carbon emissions, since black carbon is known to be a strong absorber of solar radiation. However, in the atmosphere, black carbon is part of a complex aerosol mix and aerosols are implicated in both direct radiative forcing effects (absorption and scattering of radiation) and indirect effects (cloud radiative forcing effects). This study by Chen et al. investigated both the direct and indirect aerosol forcings associated with a reduction in black carbon emissions (and organic carbon emissions). They assume in their analyses that a reduction in black carbon mass emissions would be accompanied by a decrease in primary particulate number emissions which would ultimately lead to a reduction in global cloud condensation nuclei (CCN) concentrations. They also assume that black carbon, through internal mixing with other aerosol components, will contribute to the CCN population. They evaluate the impact of two mitigation scenarios using the GISS Middle Atmosphere Model III: a 50% reduction (from present day) in primary black carbon and organic carbon mass and number emissions from fossil fuel combustion alone, and then from all primary carbonaceous sources (fossil fuels, domestic biofuels and biomass burning). They find a strong indirect warming effect (a reduced cloud albedo effect) which in their experiments is larger than the cooling effect from reduced BC direct radiative forcing.  The estimated changes in cloud radiative forcing were found to be +0.13 ± 0.33 W/m2 (fossil fuel sources only) and +0.31 ± 0.33 W/m2 (all carbonaceous sources). The net climatic effect of both mitigation scenarios, including changes in direct and indirect forcing, were positive (i.e., a warming): +0.06 W/m2 for fossil fuel sources only and +0.19 W/m2 for all sources.


Doherty, S.J., S.G. Warren, T.C. Grenfell, A.D. Clarke and R.E. Brandt. 2010. Light-absorbing impurities in Arctic snow. Atmos. Chem. and Physics 10:11647-11680.

 

A new extensive survey of impurities in Arctic snow reveals that snow albedo is affected by black carbon and other impurities, such as ‘brown’ organic carbon and dust. However, since black carbon in Arctic snow is shown to be no higher now than in the mid-1980s, it is unlikely that black carbon in Arctic snow has contributed to the rapid decline of Arctic sea ice in recent years.

 

Snow and ice are highly reflective surfaces with high albedos. Because albedo is so high, it can be reduced by impurities in the snow or ice and this reduced reflection of solar radiation has been shown to effect Arctic climate. Pioneering work to measure soot in Arctic snow was undertaken in the mid-1980s. The results of a new survey of absorptive impurities in Arctic snow have been reported recently in the journal Atmospheric Chemistry and Physics, expanding and updating the earlier work. Snow samples were collected from across the circumpolar Arctic during 1998 and between 2005-2009 on tundra, glaciers, ice caps, sea ice, frozen lakes and in boreal forest. The sampling strategy was designed to be able to measure, also, whether melting snow was depositing impurities on underlying ice by sampling from the tundra in spring, but from the Arctic Ocean and the Greenland Ice Sheet in both spring and summer. About 1200 snow samples were analyzed using filter transmission methods. The results of this work confirm that in addition to black carbon, other impurities in Arctic snow are significant for the surface energy budget of the Arctic. They show that about 20-50% of the measured light absorption by particles in the snowpack is from non-black carbon constituents, mainly brown (organic) carbon and dust. Concentrations of impurities are highest and most variable in the eastern Arctic (Scandinavia, Russia and Svalbard) and lower and less variable in the western Arctic (Canada and Alaska), with intermediate values for snow-covered sea ice and in bare sea ice on the Arctic Ocean. The new measurements provided here are somewhat lower than those reported from the earlier survey from the mid-1980s but cannot be rigorously compared due to methodological differences. Nonetheless, the black carbon content of Arctic snow appears to be no higher, and is suggested to be lower, than in the earlier survey. On the basis of these and other data that provide evidence of declining levels of black carbon in the atmosphere, the authors conclude that it is doubtful that black carbon in Arctic snow has contributed to the rapid decline of Arctic sea ice in recent years.


Gillett, N.P. and H.D. Matthews. 2010. Accounting for carbon cycle feedbacks in a comparison of the global warming effects of greenhouse gases. Environmental Research Letters, Vol 5, doi:10.1088/1748-9326/5/3/034011.


A recent modeling study suggests that the global warming effect of methane and nitrous oxide (CH4 and N2O) may be 20% higher than indicated using the current metric of 100 year global warming potential (GWP).

Non-CO2 greenhouse gases (GHG) are estimated to have contributed 37% of the total greenhouse gas forcing over the historical period.  The warming potential of non-CO2 GHGs must therefore be considered when developing multi-gas strategies to mitigate human-induced climate change.  Current global warming potentials used in the Kyoto protocol and the IPCC assessments do not account for carbon-cycle feedbacks on the climate response to non-CO2 GHGs (although they do account for carbon-cycle feedbacks on the climate response to CO2 itself).  Two Canadian scientists evaluated the robustness of the standard metrics developed to compare the global warming potential of CH4 and N2O to CO2.  Gillett and Matthews (2010) use simulations from the coupled carbon version of the UVic Earth System Climate model (which has a relatively strong carbon-cycle feedback) to evaluate the feedbacks of warming induced by CH4 and N2O on the carbon cycle.  Climate warming related to increased atmospheric concentrations of CH4 and N2O reduces the efficiency of natural carbon sinks (land and ocean) thereby increasing the concentration of CO2 in the atmosphere (a positive feedback).  Gillett and Matthews (2010) found that the radiative forcing and temperature responses to CH4 and N2O are inflated by around 20% in the modeling experiments that include the carbon-cycle.  The authors note that although their results are based on a single model, they do suggest a low bias in metrics of the global warming effect of non-CO2 GHGs and recommend that carbon-climate feedbacks be included in calculations of metrics such as global warming potential.


Kopp, R.E. and D.L. Mauzerall. 2010. Assessing the climatic benefits of black carbon mitigation. Proceedings of the National Academy of Sciences, Vol 107(26), pp 11703-11708, doi:10.1073/pnas.0909605107.

A recent study reconciles previous estimates of the radiative forcing effects of carbonaceous aerosols (black carbon and organic carbon) on climate.  These estimates are used to assess the opportunity cost of neglecting to reduce carbonaceous aerosol emissions in terms of achieving a specific emission target.


Recent scientific and political attention has focused on the effectiveness of reducing atmospheric concentrations of short-lived air pollutants with high radiative forcing (RF) such as black carbon (BC) as a means of limiting global warming.  Kopp and Mauzerall (2010) attempt to better quantify the effects of carbonaceous aerosols (CA) on the Earth’s radiative balance by comparing the results of four previous studies.  This meta-analysis involved converting the existing results to common units and included estimation of processes and terms omitted in the original studies.  The “best” estimate of the global effective RF of CA from contained combustion of fossil fuels and biofuels (based on 1996 emission levels) from this analysis is 0.22 W m-2; “low” and “high” estimates are given as 0.02 W m-2 and 0.37 W m-2 respectively.  Based on these estimates of radiative forcing, the authors consider the impacts of four different emission scenarios of CA on a 21st century CO2 emission target (sulphate aerosols are not considered).  The target used is a CO2 equivalent of 500 ppm by 2100 (effective RF of 3.1 Wm-2), which would offer a 50% chance of limiting equilibrium warming to <2.5oC above pre-industrial temperatures.  The results suggest that failure to reduce CA emissions from contained combustion of fossil fuels and biofuels to below 1996 levels by 2100 would require CO2 emissions to be cut to half of 2005 levels 8 years earlier (range of 1-15 years).


Ramana, M.V., V. Ramanathan, Y. Feng, S-C. Yoon, S-W. Kim, G. R. Carmichael and J.J. Schauer. 2010. Warming influenced by the ratio of black carbon to sulphate and the black-carbon source. Nature Geoscience, Vol 3, pp 542-545, doi:10.1038/ngeo918.

A recent study observed greater warming for aerosol plumes with higher ratios of black-carbon to sulphate aerosols and found that fossil-fuel-dominated black-carbon plumes were approximately 100% more efficient warming agents than bimomass-burning-dominated plumes.


Ramana and colleagues investigated the extent of warming induced by black-carbon (BC) aerosols in relation to their combustion source and the ratio of black carbon to sulphate.  The study is based on surface and aircraft-based measurements of aerosol plumes from different source regions in Asia; primary black-carbon combustion sources differ between the source regions.  The study recorded, amongst other variables, the BC-to-sulphate ratio, the vertical profile of warming and BC concentration, and derived the solar-absorption efficiency of plumes from the different source regions.  Measurements were conducted on Cheju Island, downwind of eastern China over parts of 2007 and 2008.  Aerosol plumes from Beijing showed the highest BC-to-sulphate ratio and exerted a positive influence on net warming.  Overall, the results showed that for all plumes the solar-absorption efficiency increased significantly with increases in the BC-to-sulphate ratio.  However, the rate of increase varied by source region and was greatest for fossil-fuel dominated plumes from East Asia.  These findings support model-based studies that have concluded that fossil-fuel based BC is a more effective warming agent than BC from biomass burning.  The authors conclude that climate-change mitigation policies should aim at reducing BC emissions from fossil fuels and that efforts to decrease sulphur dioxide emissions should be accompanied by larger percentage reductions in BC with the goal of reducing the BC-to-sulphate emission ratio.


 

Schmidt,G.A., R.A. Ruedy, R.L. Miller, and A.A. Lacis. 2010. Attribution of the present day total greenhouse effect. JGR 115, D20106, doi:10.1029/2010JD014287; Also, Lacis,A.A., G.A. Schmidt, D. Rind, and R.A. Ruedy. 2010. Atmospheric CO2: Principal control knob governing Earth’s temperature. Science 330:356-359.


Two recent studies confirm that while only 25 percent of the Earth’s planetary greenhouse effect is caused by the presence of long lived greenhouse gases (particularly CO2), the natural greenhouse effect would collapse without these gases.  Furthermore, CO2 concentrations are the primary control for the magnitude of this effect.


Public discussions about the natural greenhouse effect and climate sensitivity to rising CO2 concentrations often indicate a misunderstanding of the roles of long lived greenhouse gases (LLGHGs) relative to those of water vapour and cloud feedbacks within the climate system.  Two new studies undertaken by scientists at the NASA Goddard Institute for Space Studies have provided some updated estimates for these roles and reinforce the central role of LLGHGs in the greenhouse effect.  In one of these, a team of scientists led by Gavin Schmidt undertake a review of related scientific literature and use the radiation component of their GISS global climate model to examine the role of each of the key components of the greenhouse effect for current and 2xCO2 conditions.  They find that for current conditions, water vapour represents 50% of the effect, clouds 25% and CO2 20%.  The remaining five percent is due to the minor roles played by other radiation absorbers.  While the total effect increases significantly under doubled CO2 conditions, the ratios essentially remain the same.  The second study, led by Andrew Lacis, emphasizes the importance of the initial radiative forcing caused by CO2 and the other minor LLGHGs in sustaining the natural greenhouse effect and in causing changes in its magnitude.  They show that, without this initial forcing, the greenhouse effect would collapse, leaving the Earth a frozen planet.  Increases in CO2 are also the primary driver of enhanced greenhouse effects and the resulting rise in surface temperatures.  That is, while the roles of water vapour and cloud effects are very important in the net greenhouse effect, they function as feedbacks rather than primary drivers of change.


Shindell, D., M. Schulz, Y. Ming, T. Takemura, G. Faluvegi and V. Ramaswamy. 2010. Spatial scales of climate response to inhomogeneous radiative forcing. JGR 115, D19110, doi: 10.1029/2010JD014108.

Study provides quantitative estimates of the distances over which the effect of emissions of short-lived aerosols are shown to influence surface temperatures.  This influence extends out in all directions about 4,500 km from the location of forcing, and even longer in an east-west direction.

Understanding and modeling the climate response to emissions of short-lived aerosols, which produce non-uniform radiative forcing, remains challenging. Earlier studies have shown that the climate response to such forcing can extend well beyond the location of the forcing. However, the ‘zones of influence’ from aerosol forcing remain poorly quantified. Shindell and co-authors present the results of a study that aimed to improve quantification of these ‘zones of influence’. They analyse output from four different global climate models (GCMs) from experiments that investigated the radiative forcing and climate response to changes in aerosol forcing from pre-industrial times to present day conditions (three of which were performed for the IPCC Fourth Assessment while one was a more recent experiment). They propose as a measure of the ‘zones of influence’ the enhancement in autocorrelation of the response field (i.e., temperature) relative to the input field (i.e., radiative forcing). The most consistent results among models were found when the analyses were restricted to the latitudinal band between 30°S - 60°N. Three out of four models showed nearly identical distance of influences in the meridional direction (north-south) within this band with strong influences apparent on surface temperatures out to 3,500 km (about 30° latitude). The distance of influence was even greater in the zonal (east-west) direction.  The limits of influence in a circular zone out from the location of forcing were thus constrained mainly by the sensitivity to the meridional response, with the radius of influence demonstrated to be about 4,500 km over land, in all directions. The authors suggest that the length scale of influence of forcing, in areas where substantial forcing exists (i.e., 30°S - 60°N), may be a fairly robust feature across global climate models. These results confirm that short-lived pollutants have climate effects at great distances from their point of emission, extending beyond even the distances that the pollutants are transported, and provide quantitative estimates of the extent of that influence.


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