A recurring theme in my past columns is that a moderate degree of global warming is likely to be beneficial to mankind. Al Gore, on the other hand, says climate change is already causing catastrophic results. In testimony before Congress last March, he stated, "droughts are [already] becoming longer and more intense". But the findings of a group of University of Wisconsin-Madison researchers say otherwise.

The scientists, led by Gemma Narisma, examined 100 years of global rainfall data. Using sophisticated wavelet analysis methods, they identified 30 cases of severe droughts lasting 10 or more years. The results showed the number of droughts dropping sharply over time. From 1900-1920, seven droughts, another seven from from 1920-1940, and eight from 1940-1960. But after that, the picture changes. In the period 1960-1980, only five droughts were recorded, and from 1980-2000 (the warmest period of all), only three occurred. Furthermore, of the most severe droughts, none began in the last 30 years..

The researchers found another surprising result. Changes in rainfall levels are not only much more common than previously thought, but they tend to occur in a very abrupt, unexplained manner. More proof that climate change is part of nature.

The work represents the first systematic survey of abrupt climate changes that have occurred in recent history. Professor Johnathan Foley, who also participated in the research, says the study is important, "because previous work largely focused on ancient climates or theoretical changes in future climates".

The findings are published in Geophysical Research Letters.