Friday, August 12, 2005

Data Hot and Cold

Yesterday’s Economist featured quite an interesting discussion of three studies which have important implications, if borne out, for skeptics of global warming through human agency.
As even most casual observers of the controversy know, while computer models consistently predict warming of greater or lesser extent, the actual data have been in disagreement. While measurements of surface temperatures have seemed to confirm some models’ predictions of rising heat, at least in part, measurements of temperature collected by balloons and satellites for the troposphere – the lower layer of Earth’s atmosphere – have been contradictory, even showing somewhat of a cooling trend, particularly in the tropics.
Of the three papers discussed, the third appears least significant. Authored by a group from Lawrence Livermore National Laboratory, it simply points to the agreement of 19 different computer models, which all predict warming in the troposphere, and at the anomalous data, then argues that there must be something wrong with the data. Well, maybe. But the other two studies have something more interesting to say.
The first, conducted by Steven Sherwood of Yale University and colleagues, evaluates the data collected by weather balloons. The balloons are launched simultaneously twice each day from sites around the globe, once at midnight and once at noon Greenwich Mean Time. The raw data collected by these balloons is corrected to eliminate any solar heating of the thermometers so that all temperatures recorded will reflect readings taken in the shade. As the Economist explains, “because weather stations around the world release their balloons simultaneously, some of the measurements are taken in daylight and some in darkness. By comparing the raw data, the team was able to identify a trend: recorded night-time temperatures in the troposphere (night being the ultimate form of shade) have indeed risen. It is only daytime temperatures that seem to have dropped. Previous work, which has concentrated on average values, failed to highlight this distinction, which seems to have been caused by over-correction of the daytime figures. When the team corrected the erroneous corrections, the result agreed with the models of the troposphere and with records of the surface temperature. The improvement was particularly noticeable in the tropics, an area that had previously appeared to have high surface temperatures but far cooler tropospheric temperatures than had been expected.”
In a similar fashion, temperature data collected by satellites are also adjusted for the effect of intermediary layer of the atmosphere (the stratosphere) upon the measurements taken of the troposphere. In a second study, these data are also challenged. “Carl Mears and Frank Wentz of Remote Sensing Systems, a firm based in Santa Rosa, California, think that this trend [the cooling of the troposphere relative to surface temperature readings], too, is an artefact. It is caused, they believe, because the orbital period of a satellite changes slowly over that satellite’s lifetime, as its orbit decays due to friction with the outer reaches of the atmosphere. If due allowance is not made for such changes, spurious long-term trends can appear in the data. Dr Mears and Dr Wentz plugged this observation into a model, and the model suggested that the apparent cooling the satellites had observed is indeed such a spurious trend. Correct for orbital decay and you see not cooling, but warming.”
To be sure, none of these three studies address themselves to the question of human agency. But they do raise serious questions about the data which skeptics have emphasized most in their challenge to the reigning orthodoxy of global warming. The debate goes on.


Here’s the quick reaction of the Cato Institute’s senior fellow for environmental studies, Patrick J. Michaels: “The newly published research indicates that satellite, weather balloon and surface temperature trends in recent years are all nearly the same, placing much greater confidence in the amount of global warming that is occurring. These three different ways of measuring temperature have all converged on a warming rate that is at or near the low limit for warming given by scientists on the United Nations Intergovernmental Panel on Climate Change. These results reassure the arguments of those who say that global warming is likely to be modest and they argue strongly against the alarmist point of view on climate change.”


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