June 10, 2009

Sulfates and Global Warming

Usually when we think of global warming, we are led to believe that it is caused primarily by increasing greenhouse gases. After all, that is what all the fuss is about in Washington DC these days. But is that entirely true?

After all there are lots of other things going on all the while. For instance, to what degree has the global temperature record in recent decades been influenced by the variability in aerosol emissions?

This question has been the subject of a series of articles in recent years by Martin Wild and colleagues which look at the impacts of (primarily sulfate) aerosols on the earth’s climate. They typically conclude that sulfate aerosols play a larger role in multi-decadal climate fluctuations than the climate models generally give them credit for. And that models’ inability to properly handle the climate aspects of aerosols “may hamper the predictive skills of these models to project near future climate evolution.”

In their latest paper, just-appearing in the Journal of Geophysical Research, Wild and colleagues makes some interesting observations about aerosol influences on the global temperature history in recent decades and what it means for greenhouse gas-induced warming:

The compensating [warming/cooling] tendencies [from aerosols] in various regions of the globe may tentatively indicate that the overall surface solar radiation signal inferred from the ground-based observations did not undergo dramatic changes since the year 2000. This fits to the general picture provided by the satellite community, which suggest that the planetary albedo as well as the background aerosol burden of the atmosphere may not have undergone substantial changes between 2000 and 2005, at least globally [Loeb et al., 2007a, 2007b; M. Mishchenko, personal communication, 2007]. This is also in line with recent Earthshine observations, which indicate a fairly stable planetary albedo after 2000 [Pallé et al., 2009]. Further, air pollution control measures seem to have saturated lately in some of the industrialized regions, which may prevent further brightening in these areas [e.g., Ruckstuhl et al., 2008; D. G. Streets et al., submitted paper, 2009]. One may also speculate that the recent lack of a significant overall brightening may favor a more moderate temperature increase in the early 2000 compared to the 1990s, when brightening has more substantially added to the greenhouse-induced warming [Wild et al., 2007, 2008]. Overall global warming since the turn of the millennium may therefore be more readily attributable to the enhanced greenhouse effect, and no longer suppressed by surface solar dimming as in the period from the 1950s to 1980s or enhanced by surface solar brightening as from the 1980s to 2000. However, further investigations based on both modeling and observational approaches will be required to get more insight into the origins and impacts of the changes documented in this study. [emphasis added]

So what Wild and colleagues are saying is that 1) the observed global warming rate during the 1980s and 1990s was elevated as a result of a decreasing atmospheric aerosol burden (which led to more solar radiation reaching the surface, aka “solar brightening”)—so all the talk about how fast the planet was warming up because of greenhouse gas concentration increase is a bit overblown, and 2) that since the turn of the century, the climate influence from aerosols has been relatively stable (in net) and so the true impact from increasing greenhouse gases should now be more evident. And, as readers of this blog are quite aware—there is no significant trend in global temperatures this century!

Now, we are not suggesting that this will continue indefinitely, but we are suggesting that the rate of temperature increase produced by greenhouse gas increases appears to be much less than climate models are indicating (or at least given credit for)—for the past several decades, and likely into the future.

It is high time that people start getting their heads out of their models and start looking at the observations. The picture grows clearer every day—carbon dioxide emissions have much less climate impact than advertised by those folks clamoring for their regulation.

References:

Wild, M., et al. (2005), From dimming to brightening: Decadal changes in surface solar radiation, Science, 308, 847–850, doi:10.1126/science.1103215.

Wild, M., A. Ohmura, and K. Makowski (2007), Impact of global dimming and brightening on global warming, Geophysical Research Letters, 34, L04702, doi:10.1029/2006GL028031.

Wild, M. (2009), How well do IPCC-AR4/CMIP3 climate models simulate global dimming/brightening and twentieth-century daytime and nighttime warming? Journal of Geophysical Research, 114, D00D11, doi:10.1029/2008JD011372.

Wild, M., B. Trüssel, A. Ohmura, C. N. Long, E. G. Dutton, G. König-Langlo, and A. Tsvetkov (2009), Global dimming and brightening: An update beyond 2000, Journal of Geophysical Research, 114, doi:10.1029/2008JD011382.

Wild, M. (2009), Global dimming and brightening: A review. Journal of Geophysical Research, 114, doi:10.1029/2008JD011470, in press.




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