About a month or so ago, Science magazine published a paper by Susan Solomon and colleagues that concluded that aerosols in the upper atmosphere that were unaccounted for in earlier estimations, have, over the past 10 years or so, acted to offset about 0.07°C of warming that would have otherwise occurred. In other words, we shouldn’t be so hard on the climate models for failing to anticipate the dearth of warming over the past 10-15 years.
Or should we?
It turns out, that what the paper really says, is that the amount of global warming that should have occurred over the past 10-15 years (that is, if the climate models were getting things correct) is about 25% greater than the model-expected warming from the combination of increases in greenhouse gases and lower atmospheric pollution alone. Which means that the observed warming during this same time—which has been close to nil—is even harder to explain and makes the models look even worse.
But, of course, that is not at all how the results were spun to the press.
First, a bit of background. When large volcanic eruptions occur, they can inject particulates into the upper atmosphere (stratosphere) where they are largely unaffected by the weather (which occurs in the lower level of the atmosphere called the troposphere) and thus can remain suspended for months to years, rather than days to weeks (the typical time that a particular aerosols particle remains suspended in the troposphere). These suspended particulates in the stratosphere primarily act as dirty little mirrors and reflect away some of the incoming radiation from the sun. This leads to a general cooling of the earth’s average temperature. The more aerosols in the stratosphere, the more cooling. Fairly major volcanic eruptions (such as El Chichon in 1982 and Mt. Pinatubo in 1991) can lead to perhaps a degree or so of cooling of the earth’s average surface temperature for maybe a year or so. The cooling effect attenuates as the aerosols eventually are removed from the stratosphere.