Friday, May 2, 2008

Predictions of Short-Term Cooling, and the Complexity of Global Warming

In an article surely to be a favorite for crank quote-mining, scientists using retroactively tested models, predict a slight cooling of Europe and North America over the next decade caused by shifting ocean currents. This will temporarily mute the effect of global warming, but unfortunately, it will be a temporary respite.

To understand how this is possible, consider the climate of England and Newfoundland. They are at about the same latitude, and yet Newfoundland has much a colder climate. The reason is the gulf stream, which flows from the Gulf of Mexico across the Atlantic to England, bringing warm water into the otherwise cold North Sea. Change the flow of the gulf stream, and you get a colder England.

Of course the Global Warming denialists will jump all over this study as proof that AGW has been falsified. As one of the scientists put it:

"Too many think global warming means monotonic relentless warming everywhere year after year," Dr. Trenberth said. "It does not happen that way."

Indeed, it is interesting to note how often one sees a comment like "How can you predict weather 100 years from now if you can't even predict the weather tomorrow?" Such comments reveal a lack of understanding of the law of large numbers. To illustrate the point, consider the difficulty in predicting the next roll of the dice in contrast to predicting the total of 1,000 such rolls. While the roll of one die will vary from 1 - 6 with equal probability, the total of 1,000 rolls will come very close to the expected mean of 3,500.

Likewise with temperature. Of course there will always be variance in temperature, both temporally and geographically. Climate change refers to the total figures, not the short-term fluctuations. This is why one year's data means nothing, and why recent charges that global warming has stopped based on an unusually high 1998 and unusually low 2007 are fraudulent. Expect more such fraudulent claims if the predictions of these scientists come to pass.

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