Chip Levy of GFDL recently gave an informal talk at U T Austin. I think it’s reasonable to share his conclusions:
For the last year or two, I have been responsible for CCSP SAP3.2, one of the ~ 22 reports that is the US government’s version of IPCC [trips to DC instead of Nobel prizes]. Our report, SAP 3.2 [don’t you love acronyms], focused on the impact of short-lived air pollutants on future climate, and was based on the A1B standard scenario.
I would like to talk with you about our major conclusions:
1. Nobody [IPCC or CCSP] paid much attention to the projections for emissions of short-lived pollutants and their precursors. They had better do so in the future.
2. Three credible groups [GFDL, GISS, NCAR] working more or less at the state-of-the-art, came up with very different versions of future emissions of SO2, BC, OC and NOx distributions of the pollutants and their radiative impacts.
3. 2 of the groups found that short-lived pollutants were responsible for ~20% of the surface temperature increase by 2050, when all emissions [well mixed greenhouse gases and short-lived pollutants] followed the A1B scenario.
4. Running out to 2100 with a standard version of A1B, we found that short-lived pollutants [primarily SO2 and BC] were responsible for ~40% of the summertime warming over central North America and led to a significant decrease in precipitation and increase in soil drying.
5. The primary increase in radiative forcing from pollutants was over Asia while the primary summertime climate response was over the central US. The general disconnect between the regional locations of the pollutants and their radiative forcing and the regional climate response was quite robust. Air Quality and Climate need to talk.
Levy stressed the last point was not anticipated prior to their research. The largest impact of expected particulate emissions increases in China and India in their study was increased drying in the central US agriculture belt! We discussed whether North American atmospheric dynamics is effectvely captured in global scale models. There is some plausible argument that they aren’t, and that the effect of the Gulf of Mexico is underrepresented because it is only coarsely captured on GCM scales. On the other hand, a marked drying tendency in the corn and grain growing areas of the US is a feature robust across the better GCMs.
We scrupulously avoided discussing the political implications of this result if it holds true, except in agreeing that there would surely be some. Again, we can’t guess the future of civilization well enough to predict the future of climate well, even if our models are perfect.