jeudi 28 avril 2016

Smog around climate change III : Dimming is back!

Does China really care about climate change and not only on air pollution? According to LI Yijing[i] there is no doubt. After her presentation[ii] on March 11th, 2016 at PolyU HK, she just replied “yes of course!” to my question, without any other attempt to justify her very confident answer. It is probably true that China (i) would like to be the leader in something like reducing global warming and (ii) already suffers from the adverse consequences of climate change, but it is even more convincing to observe that China faces an air pollution catastrophe and should urgently deal with it. And this is my concern.

Fighting air pollution implies reducing SO2, one of the 6 air pollutants entering the Air Quality Index as computed by China's Ministry of Environmental Protection (MEP), others being ozone, lead, carbon monoxide, nitrogen oxides and PM. It is one of the 6 major air pollutants as listed by the EPA as well. SO2 is a gas produced from burning coal, mainly in thermal power plants. There is scientific evidence proving that short-term exposure to SO2 (from 5mn to 24h) causes adverse respiratory effects.  SO2 is affecting radiative forcing as well but in a surprising way: due to a dimming effect it is actually cooling the climate!
A recent paper published in Nature[iii] argue that if SO2 emissions were to go back to their 1980 level China contribution to radiative forcing of the climate would be 13%+/- 4% instead of the current 10+/-4%. In fact, in China, the cooling effect of SO2 (-0.11 Wm-2radiative forcing) currently nearly compensates for the warming effect of CH4 (0.13 Wm-2radiative forcing). This illustrates the “masking” effect of cooling aerosols such as sulphated ones. And China is effectively reducing these emissions. During the 10th Five-Year Plan period (2001-2005) the MEP set a goal to reduce SO2 emissions by 10% by 2005 compared to 2000. It was not achieved, but there is a significant decline starting after 2005 (around 10% between 2006 and 2008, according to Lu et al. 2010)[iv], In the same paper, authors stress that whether an air quality improvement in China occurring through SO2 emissions reduction would imply a rise of China’s contribution to the global radiative forcing depends on the reduction of other pollutants such as ozone precursors and black carbon that may be co-emitted and have a warming effect. But what I noticed[v] is that Particulate Organic Matters are cooling the climate as well…

[i] China Executive Leadership Academy Pudong, Shanghai, 201204;
[iii] The contribution of China’s emissions to global climate forcing” , by Bengang Li, Thomas Gasser, Philippe Ciais, Shilong Piao, Shu Tao, Yves Balkanski, Didier Hauglustaine, Juan-Pablo Boisier,  Zhuo Chen, Mengtian Huang, Laurent Zhaoxin Li, Yue Li, Hongyan Liu, Junfeng Liu, Shushi Peng, Zehao Shen, Zhenzhong Sun, Rong Wang, Tao Wang, Guodong Yin, Yi Yin, Hui Zeng, Zhenzhong Zeng & Feng Zhou, Nature 531, p357-362  January 2016.
[iv] Lu Z, Streets D.G., Zhang Q., Wang S., Carmichael G.R., Cheng Y.F., Wei C., Chin M., Diehl T., and Tan Q. (2010), “Sulfur dioxide emissions in China and sulfur trends in East Asia since 2000”Atmospheric Chemistry and Physics, 6311-6331.
[v] See figure 1 p358.

2 commentaires:

  1. Very true and very interesting, but not cutting back air-pollution (or releasing extra sulphur into the air) would be combatting the symptoms instead of the source of the problem. Many people (including my brother) have argued enthusiastically for such geo-engineering in recent years, but would this not 1) create new problems (such as acid rain through H2SO4 formation) and 2) allow carbon dioxide levels to rise uncontrollably, thus only worsening the problem on the long run. In my opinion, the only way to thoroughly deal with this issue is to try and tackle both issues head-on: led China clean up its air pollution (and control its carbon-emissions), if temperatures rise significantly on the short run, than this will only help scaring the rest of the world into action.

  2. Completely agree, thanks for the comment! It gives me the opportunity to make clear that my conclusion is certainly NOT that China should not tackle the air pollution issue (actually I don't even think they have the choice). It is rather: we should not rely on expectations that China environmental policy is going to contribute a lot to climate change mitigation...except maybe if we believe it could initiate a cooperative behavior of the other (big) countries.