Yet more aerosols: Comment on Shindell and Faluvegi

In our paper, we wanted to characterize the geographic forcing/response relationship more clearly. Prior studies had looked at particular scenarios or time periods when forcings were typically changing over much of the world (albeit most strongly in certain regions). So we put idealized forcings from GHGs, aerosols, and ozone in the tropics, mid-latitudes and polar regions to see what would happen. The results showed that the temperature response in the tropics, like the global mean, is only mildly sensitive to the location of forcing. That is, you get an enhanced tropical response to forcing in the Northern Hemisphere extratropics (where you can activate strong positive feedbacks like snow/ice albedo), but the enhancement is only 40-50% over that found with forcings applied elsewhere. In contrast, the extratropical zones are much, much more sensitive to local radiative forcing than to tropical forcing or to forcing in the opposite hemisphere. So to quote from the paper

“global and tropical mean temperature trends during the twentieth century would have been quite similar if short-lived-species radiative forcing had been distributed homogeneously rather than being concentrated in the northern extratropics. Regional concentration of forcing contributed to the departures of Northern Hemisphere mid-latitude and Arctic temperature trends from the global or Southern Hemisphere extratopical means, however.”

We then used the regional forcing/response relationships to derive the aerosol forcing needed to explain the observed global and regional temperature trends. Our results have a substantial uncertainty range which arises primarily from the influence of unforced, internal variability. The global mean preindustrial to present-day aerosol forcing we calculate is -1.31 +- 0.52 W/m2, consistent with the IPCC AR4 range of -0.6 to -2.4 W/m2.

We also estimated aerosol forcing for the tropics and Northern Hemisphere mid-latitudes for several time periods, and compared with historical emissions estimates to tie the forcings to sulfate or black carbon (BC) aerosols when possible. The results show, for example, that nearly all CMIP3 models require strong aerosol cooling at Northern Hemisphere mid-latitudes during the 1931-1975 period to capture both the global mean trends and the NH mid-latitude versus Southern Hemisphere extratropics temperature trends (many CMIP3 models had both sulfate and BC, but not necessarily the correct amounts as modeling their forcing directly is quite uncertain, hence we compared the CMIP3 models’ responses to non-aerosol forcings with observations to see how well they could do without aerosols). During the last 3 decades (1976-2007), the best fit to the temperature responses in the models require negative forcing from tropical aerosols but positive forcing from Northern Hemisphere mid-latitude aerosols. It’s the latter, the positive Northern Hemisphere mid-latitude aerosol forcing that leads to the strong warming impact on the Arctic as well, as the Arctic responds to mid-latitude and local forcing, but the local forcing is primarily driven by mid-latitude emissions that are transported to the Arctic, so the overall climate response ends up being closely tied to Northern Hemisphere mid-latitude emissions. Given the strong sensitivity of the Northern Hemisphere extratropical zones to aerosol forcing, it’s then understandable that those areas could have cooled during the mid 20th century when the aerosol forcing we calculate was substantially larger than greenhouse gas forcing (in absolute magnitude).

A big uncertainty is still the influence of unforced internal variability, which we estimated from coupled ocean-atmosphere climate runs. Though that contribution is large, it was still not large enough to account for many of the mid-latitude and Arctic temperature trends without including aerosol forcing. For many cases, the influence of aerosols and internal variability were comparable in size. Though the influence of internal variability leads to a substantial uncertainty range in our results, they are nonetheless useful as other techniques of estimating aerosol forcing of climate have comparably large or larger uncertainties. These include ‘forward’ modeling from emissions to concentration to optical properties (e.g. see [Schulz et al., 2006]), and various estimates based at least in part on satellite observations (see this previous post).

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