March 11, 2005

Less Cooling, Less Warming

Filed under: Climate Models

Newly published results show that the degree to which sulfate aerosols lead to surface cooling is overestimated in current climate models. This result further undermines what little support remains for the IPCC’s high-end warming projections.

In order to produce extreme temperature rises during this century, the Intergovernmental Panel on Climate Change (IPCC), in its 2001 Third Assessment Report (TAR), relies on a theory in which cooling from sulfate aerosols largely counteracts greenhouse warming. Then, they construct emissions scenarios in which sulfate aerosols are progressively removed under the guise of pollution reduction measures, and voila, greenhouse warming is unmasked and the earth warms dramatically.

The IPCC had to dream up this bit of chicanery, because the planet was not heating up as much as climate models that only included the effects of increasing greenhouse gases predict it should. In their 1995 Second Assessment Report (SAR), the IPCC wrote:

When increases in greenhouse gases only are taken into account…most [climate models] produce a greater mean warming than has been observed to date, unless a lower sensitivity [to the greenhouse effect] is used…There is growing evidence that increases in sulfate aerosols are partially counteracting the [warming] due to increases in greenhouse gases.

Admitting to a lower climate sensitivity would reduce the need for urgent action to curb the burning of fossil fuels, therefore, the IPCC strongly embraced the sulfate hypothesis. The IPCC planted the seed for the sulfate hypothesis in its SAR, and then, 5 years later, plucked its fruit:

The globally averaged surface temperature is projected to increase by 1.4 to 5.8ºC over the period 1990 to 2100. These results are for the full range of 35 SRES scenarios, based on a number of climate models. Temperature increases are projected to be greater than those in the SAR, which were about 1.0 to 3.5ºC based on six IS92 scenarios. The higher projected temperatures and the wider range are due primarily to the lowered projected SO2 emissions [which lead to sulfate aerosols] in the SRES scenarios relative to the IS92 scenarios.[emphasis added]

However, the sulfate hypothesis just doesn’t hold water. No clearer example of the failure of this hypothesis can be found than that of a simple examination of the observed patterns of temperature change. Figure 1 (taken from the IPCC TAR) shows the patterns of temperature change during the past 25 years. Notice that the regions with the greatest temperature rise are eastern North America, Europe, and eastern Asia—the three leading industrialized areas of the world. Compare that with the IPCC’s description of the influence of adding sulfates to climate models previously only run with greenhouse gas enhancements:

The biggest difference between the 1% [per year] CO2-increase experiments, which have no sulphate aerosol, and the SRES experiments is the regional moderating of the warming over industrialized areas, in the SRES experiments, where the negative forcing [cooling] from sulphate aerosols is greatest .[emphasis added]

The observations show the exact opposite!

IPCC Temperatures

Figure 1. Observed surface temperature trends, 1976-2000 (Source: IPCC, TAR).

One explanation for why the observed temperature change patterns don’t match the ones forecast by the sulfate hypothesis, is that sulfate cooling is being more than offset by warming from other aerosol particles, primarily soot. In this case, soot emissions would lead to regional warming in heavily industrialized regions—akin to what is being observed. The IPCC paid little attention to the role of soot in 2001, allowing it to offset only about one-sixth of the cooling from sulfate. However, since the publication of the IPCC TAR in 2001, evidence has been growing that soot is responsible for more warming that the IPCC allowed for and that sulfate is responsible for less cooling.

Background into the soot findings can be found in some of our earlier reports . Basically, the amount of positive climate forcing (which leads to warming) thought to be attributable to soot has been tripled (from an IPCC TAR value of 0.2 W m-2). The focus here is on a new finding concerning the overestimation of the cooling impact of sulfate. Published in Geophysical Research Letters are the recent findings from Leon Rotstayn and Yangang Liu. These researchers reexamined the physics and the computational scheme used in state-of-the-art climate models to calculate the cooling effect of sulfate aerosols. There were motivated by the fact that, according to them, “Global estimates of the indirect aerosol effect much larger than 1 W m-2 in magnitude are difficult to reconcile with observations, yet climate models give estimates between -1 and -4.4 W m-2.” After carefully developing an updated scheme for handling how sulfate particles interact with water droplets in clouds, they find that the degree to which sulfate particles brighten clouds (reflecting to space incoming solar radiation) is much less than currently modeled—in fact, about 33% less. This brings the modeled forcing from sulfates closer to observed values (e.g. Lohmann and Lesins, 2002).

Taken together, the findings concerning soot and sulfates mean that the role of increases in greenhouse gases in warming the earth can be more clearly defined and that it is much less than climate models assume. This provides support for a lower climate sensitivity than is typically used in climate models. Furthermore, less sulfate cooling and more soot warming means that measures to lower air pollution in the future, which will undoubtedly include limits on both sulfate and soot—and their offsetting climate effects—will do little to impact the course of future temperature rises.

Thus, what we see is what we get, and what we see is a temperature increase of about 0.17ºC per decade. This leads to a total average temperature rise during the 21st century of between 1.5 and 2.0ºC—or values that lie near the low end of the IPCC range of projections. Associated with low-end warming are low-end impacts and virtually the end of global warming scare stories. This being the case, you can be sure that the IPCC and the rest of the global alarmist collection will not let go of the sulfate hypothesis without a fight. But, as is the case with the hockey stick temperature reconstruction, its hold of the spotlight is quickly slipping away.


Intergovernmental Panel on Climate Change, 2001. Climate Change 2001: The Scientific Basis. Houghton, J.T. et al., (eds). Cambridge University Press, Cambridge, UK, 881pp.

Lohmann, U., and Lesins, G., 2002. Stronger constraints on the anthropogenic indirect aerosol effect. Science, 298, 1012-1015.

Rotstayn, L.D., and Liu, Y., 2005. A smaller global estimate of the second indirect aerosol effect. Geophysical Research Letters, 32, L05708, doi:10.1029/2004GL021922.

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