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Climate myths: The lower atmosphere is cooling, not warming

17:00 16 May 2007 by Phil McKanna

See all climate myths in our special feature.

Increasing levels of greenhouse gases should warm the Earth's surface and the lower atmosphere and cool the upper layer. So is this happening as the theory and models predict?

Satellites and weather balloon measurements show that the stratosphere, the layer from 10 to 50 kilometres above the Earth, is indeed cooling (although this is partly due to the depletion of the ozone layer).

In 1992, however, an analysis of satellite data by John Christy at University of Alabama in Huntsville, US, conducted that the lower part of the troposphere - the first 10 kilometres of atmosphere - had cooled relative to the surface since 1979, when the first satellites capable monitoring temperature measurements were launched. This trend seemed to continue into the late 1990s and also seemed to be supported by balloon measurements.

This was not quite the "nail in the coffin" for global warming that some sceptics claimed. If the satellite data was correct, it meant there was something wrong with the existing models of climate change. But it made little sense for the lower atmosphere to be cooling even as the surface warmed, suggesting the problem lay with the data. The jury was out until the issue could be resolved one way or another.

Slowing satellites

The answer came in a series of studies published in 2005 (see Sceptics forced into climate climbdown).

One study in Science revealed errors in the way satellite date had been collected and interpreted. For instance, the orbit of satellites gradually slows, which has to be taken into account because it affects the time of day at which termperature recording are taken. This problem was always recognised, but the corrections were given the wrong sign (negative instead of positive and vice versa).

A second study, also in Science, looked at the weather balloon data. Measurements of the air temperature during the day can be skewed if the instruments are heated by sunlight. Over the years the makers of weather balloons had come up with better methods of preventing or correcting for this effect, but because no one had taken these improvements into account, the more accurate measurements appeared to show daytime temperatures getting cooler.

The corrected temperature records show that tropospheric temperatures are indeed rising at roughly the same rate as surface temperatures. Or, as a 2006 report by the US Climate Change Science Program (pdf) puts it: "For recent decades, all current atmospheric data sets now show global-average warming that is similar to the surface warming." This one appears settled.

There is still some ambiguity in the tropics, where most measurement show the surface warming faster that the upper troposphere, whereas the models predict faster warming of the atmosphere. However, this is a minor discrepancy compared with cooling of the entire troposphere and could just be due to the errors of margin inherent in both the observations and the models.

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