The tropical lapse rate quandary

Guest commentary by Steve Sherwood

There are four independent instrumental records of sufficient length and potential accuracy to tell us about 20th-century climate change. The two longest ones are of temperature near the Earth’s surface: a vast network of weather stations over land areas, and ship data from the oceans. While land surface observations go back hundreds of years in a few places, data of sufficient coverage for estimating global temperature have been available only since the end of the 19th century. These have shown about a 0.7 C warming over land during the last century, with somewhat less increase indicated over oceans. The land records contain artifacts due to things like urbanization or tree growth around station locations, buildings or air conditioners being installed near stations, etc., but laborious data screening, correction procedures, and a-posteriori tests have convinced nearly all researchers that the reported land warming trend must be largely correct. Qualitative indicators like sea ice coverage, spring thaw dates, and melting permafrost provide strong additional evidence that trends have been positive at middle and high northern latitudes, while glacier retreat suggests warming aloft at lower latitudes.

The other two climate records, so-called “upper air” records, measure temperatures in Earth’s troposphere and stratosphere. The troposphere—that part of the atmosphere that is involved in weather, about 85% by mass—is expected to warm at roughly the same rate as the surface. In the tropics, simple thermodynamics (as covered in many undergraduate meteorology courses) dictates that it should actually warm faster, up to about 1.8 times faster by the time you get to 12 km or so; at higher latitudes this ratio is affected by other factors and decreases, but does not fall very far below 1. These theoretical expectations are echoed by all numerical climate models regardless of whether the surface temperature changes as part of a natural fluctuation, increased solar heating, or increased opacity of greenhouse gases.

It turns out that the upper-air records have not shown the warming that should accompany the reported increases at the surface. Both the Microwave Sounding Unit (MSU) satellite (analyzed by the University of Alabama in Huntsville by John Christy and Roy Spencer) and weather balloon data (trends reported by a number of researchers, notably Jim Angell at NOAA) have failed to show significant warming since the satellite record began in late 1978, even though the surface record has been rising at its fastest pace (~0.15 C/decade) since instrumental records began. On the other hand both records have shown dramatic cooling in the stratosphere, where cooling is indeed expected due to increasing greenhouse gases and decreasing ozone (which heats the stratosphere due to its absorption of solar ultraviolet radiation). The sondes in particular have shown a lot more cooling than the satellites, almost certainly too much, leading one to wonder whether their tropospheric trends are also too low.

The non-warming troposphere has been a thorn in the side of climate detection and attribution efforts to date. Some have used it to question the surface record (though that argument has won few adherents within the climate community), while others have used it to deny an anthropogenic role in surface warming (an illogical argument since the atmosphere should follow no matter what causes the surface to warm). The most favored explanation has been that the “lapse rate,” or decrease in temperature as you go up in the atmosphere, has actually been increasing. This would contradict all of our climate models and would spell trouble for our understanding of the atmosphere, especially in the tropics.

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