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Satellites and global warming

August 25th, 2002

At the urging of Ken Parish, I’m returning to the topic of global warming, but I’ll do my best to keep things civilised. One issue raised by Ken is the discrepancy between ground level and satellite measurements of global warming. I have followed this one fairly closely and on one crucial aspect of debate, I have a fair bit of professional expertise.
The big expert on satellite measurements of climate is John R. Christy of the University of Alabama. His data, which started in 1979, initially appeared to show a cooling trend in the troposphere. Since satellite data seemed free of many of the errors that affect surface measurements, these results were seized on by global warming ‘sceptics”.
However, it was discovered in 1998 that the satellite data had problems of their own, arising from a failure to correct for the gradual decay in their orbits. Correcting for this, and adding more data, the satellite data now shows a slight warming trend, but not as much as the surface data. These facts were enough for an NAS panel, including Christy, to publish a report Reconciling Observations of Global Temperature Change which concluded that
“Despite differences in temperature data, strong evidence exists to show that the warming of the Earth’s surface is undoubtedly real, and surface temperatures in the past two decades have risen at a rate substantially greater than average for the past 100 years”

Of course, some sceptics could not bear to give up their best bit of evidence and have put a lot of weight in the remaining discrepancy. Speaking as someone who has taught and researched the statistical analysis of time series, I can say that putting this kind of weight on 20 years of inconclusive data is not justified, even if such events as the eruption of Mt Pinatubo in 1981 had not added to the usual background noise.

The satellite data does raise some issues – for example it shows that the link between tropospheric and surface level temperatures is not as tight as was once thought, but the idea that it represents serious evidence against the hypothesis of human-induced global warming has been thoroughly refuted.

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