Thursday, July 26, 2007

Humans have shifted global precipitation patterns

Environment News Service: For the first time, climate scientists have clearly detected the human fingerprint on changing global precipitation patterns over the past century.

Their study to be published in tomorrow's issue of the journal "Nature" demonstrates that "human activities have contributed significantly to shifts in global precipitation patterns over the past century," including increased rain and snowfall in northern regions, drier conditions in tropical areas north of the equator, and increased rainfall in the southern tropics.

Human-induced changes have not previously been detected in global studies of precipitation, partly because drying in some regions cancels moistening in others, reducing the global signal. Here the scientists used the patterns of the changes in different latitude bands instead of the global average.

...According to the study, over the past century, climate records indicate there have been sizable shifts in precipitation patterns around the globe as a result of the emission of greenhouse gases and sulphate aerosols..

Looking at average conditions over broad regions of the globe, and comparing them to changes anticipated due to human influence on climate, scientists have determined that human-induced climate change has caused most of the observed increase in precipitation north of 50° latitude, a region that includes Canada, Russia and Europe, as well as in the southern hemisphere.

Human-induced climate change has also made important contributions to the drying observed in a broad region north of the equator that includes Mexico, Central America and northern Africa.

These shifts may have already had significant effects on ecosystems, agriculture and human health, especially in regions that are sensitive to changes in precipitation, such as the Sahel region in northern Africa.

The evidence suggests that natural factors, such as volcanic activity, have also contributed to the changes in global precipitation patterns over the past century, although to a much smaller extent than human activity.

The study compared observed precipitation changes with those produced by complex computer climate models that were used to estimate the effects of human activities over the past century.

In recent years, scientists have become increasingly sophisticated in combining different global climate models to increase the accuracy of their results. In this study, 14 different models were used. As a result, the scientists have considerable confidence in the findings of this study.

This study has also given scientists increased confidence in their ability to predict future changes in global climate. By using computer models to simulate climate change that has already occurred, the researchers have demonstrated the reliability of these models.

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