Humans Have Shifted Global Precipitation Patterns

NORWICH, UK, July 25, 2007 (ENS) - 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.

Londoners shopping in the rain. (Photo courtesy Freefoto)
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.

Authors of the study, "Detection of Human Influence On 20th-Century Precipitation Trends," include climate scientists from Canada, the United States, Japan and the UK.

The UK authors are Peter Stott at the Met Office Hadley Centre and Nathan Gillett at the University of East Anglia in Norwich.

Dr. Nathan Gillett is a lecturer at the University of East Anglia (Photo courtesty UEA)
U.S. authors include Susan Solomon of the NOAA Earth System Research Laboratory in Boulder, Colorado, Gabriele Hegerl of the Nicholas School for the Environment and Earth Sciences at Duke University, North Carolina and F. Hugo Lambert, Department of Geography, University of California-Berkeley.

In the study, which the University of East Anglia says "breaks new ground in climate change research," the scientists studied the combined effect that changes in greenhouse gases and sulphate aerosol concentrations in the atmosphere have had on global precipitation over land during the past century.

Greenhouse gases and sulphate aerosols are produced primarily by burning fossil fuels such as coal, oil and gasoline. Greenhouse gas concentrations in the atmosphere have increased steadily over the past century.

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.

South Africa's Graskop rainforest in the rain. (Photo courtesy
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.

The paper’s authors include Xuebin Zhang and Francis Zwiers of the Climate Research Division, Environment Canada, Toronto; and Toru Nozawa, National Institute for Environmental Studies, Tsukuba, Ibaraki, Japan.

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