Because of the enormous credibility of the Intergovernmental Panel on Climate Change’s reports, and because they suggest that human-induced climate change is a very real risk, opponents of action on climate change must attack the IPCC or completely cede the scientific high ground in the debate.

With the release of the latest IPCC report, a whole new crop of specious skeptical arguments has arisen. Here’s a good example, which appeared in this week’s Weekly Standard:

One possible reason for the timing is that there appear to be some significant retreats from the 2001 IPCC report. The IPCC has actually lowered its estimate of the magnitude of human influence on warming, though we shall have to wait for the full report in May to understand how and why. Only readers with detailed knowledge of the 2001 report would notice these changes, which is why most news accounts failed to report them.

As with most skeptical arguments, there is a grain of truth here, sitting under a mountain of deception.

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The truth is that the IPCC did indeed lower its estimate of radiative forcing of the planet due to humans from 2.4 to 1.6 watts per square meter. Radiative forcing is the change in downward infrared flux caused by human activities, and it is a standard measure of how humans have impacted the climate system.

The reason for this revision is not that we think CO2 and other well-mixed greenhouse gases are less effective at warming. Rather, we have learned that tiny particles suspended in the atmosphere, known as aerosols, are reflecting sunlight back to space. The cooling associated with this reduction of sunlight has been offsetting some of the heat trapped by CO2.

Aerosols are a primary component of air pollution, and breathing them has significant negative health effects. Many of their sources are activities found in the developing world, where air quality regulations are lax or non-existent. As these places get serious about reducing air pollution, emissions of aerosols will decrease.

This, in turn, will decrease the aerosol burden in the atmosphere. That’s because aerosols only stay in the atmosphere for a few days or weeks — compared to CO2’s lifetime of a century or two — so a decrease in aerosol emission is followed relatively rapidly by a decrease in atmospheric concentration.

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As the abundance of aerosols in the atmosphere decreases, its offsetting effects on CO2 will also decrease. Our climate will then be exposed to the full climatic effects of CO2.

That will result in additional warming.

In other words, the IPCC’s 2007 revision in radiative forcing doesn’t mean climate change is less serious than previously thought; on the contrary, it means there is warming in the pipeline we have yet to experience.

This argument, like its forebears, is cleverly constructed to mislead.

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