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What Is — and What Isn’t — Evidence of Global Warming

All the evidence we've heard regarding global warming never constituted, in any manner, actual evidence that it was taking place.

by
William M. Briggs

Bio

November 25, 2009 - 12:22 am
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For years, it was as if there was a contest for the most outlandish claim of what might happen if AGW were true. But no statement of what might happen if AGW is true is evidence for AGW. Those prognostications were only evidence of the capacity for fanciful speculation. Merely this and nothing more.

So if observations of what happens when it’s hot outside don’t verify AGW, and if predictions of what might happen given AGW were true do not verify AGW, what does? Why did people get so excited?

In the late 1990s, some places on Earth were hotter than they were in the late 1980s. These observations were indirect — and not direct — evidence of AGW. The Earth’s climate has never been static; temperatures sometimes rise and sometimes fall. So just because we see rising temperatures at one point does not prove AGW is true. After all, temperatures have been falling over the last decade, and AGW supporters still say their theory is true. Rising — or falling — temperatures are thus consistent with many theories of climate, not just AGW.

Climate scientists then built AGW models, incorporating the observed temperatures. They worked hard at fitting those models so that the models could reproduce the rising temperatures of the 1990s, while at the same time fitting the falling temperatures of the 1970s, etc. They had to twist and tweak — and with the CRU emails, it now appears they twiddled. They had to cram those observations into the models and, by God, make them fit, like a woman trying on her favorite jeans after Thanksgiving.

They then announced to the world that AGW was true — because their models said it was.

But a model fitting old data is not direct evidence that the theory behind the model is true. Many alternate models can fit that data equally well. It is a necessary requirement for any model, were it true, to fit the data, but because it happens to is not a proof that the model is valid.

For a model to be believable it must make skillful predictions of independent data. It must, that is, make accurate forecasts of the future. The AGW models have not yet done so. There is, therefore, no direct evidence for AGW.

The models predicted warmer temperatures, but it got cooler. One of the revealed CRU emails found one prominent gentlemen saying, “We can’t account for the lack of warming at the moment and it is a travesty that we can’t.”

It is. But only if you were concerned that the AGW theory will be nevermore.

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William M. Briggs is a statistical consultant in New York and San Francisco. He is an American Meteorological Society member and serves on their Probability & Statistics Committee. His specialty is on the philosophy of evidence, forecast evaluation, and marketing.
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