Study: Humans have shifted global precipitation patterns

Humans are directly affecting global rainfall patterns and have been doing so for most of a century, according to a new study that gives the first solid proof that people are causing critical climate change. Their study published in the July 26 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. Climate models have, for a number of years, suggested that human activity has led to changes to the distribution of rain and snow across the globe. However, the computer models have been unable to pinpoint the extent of our influence, partly because drying in some regions has cancelled out moistening in others. In this study, 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 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. 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 at least 50 percent, and as much as 85 percent, of average rainfall increases at northern latitudes between 1925 and 1999 could be attributed to human activity. The team's data showed that in northern countries, average annual precipitation increased by 2.4 inches between 1925 and 1999. 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. The rainfall data confirmed what the scientists had speculated could occur due to human activity, and in some cases the weather changes went beyond what scientists had predicted. "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 report states. 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. 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. Gordon McBean, former head of the Meteorological Service of Canada and a professor at the University of Western Ontario said the report reinforces that climate change is about much more than rising temperatures. "The reality is that the climate is changing. It's not just warming. It's getting wetter in certain places and unfortunately also dryer in other places," he said. The study looked at annual average rainfall on land, not at sea, and did not look at extreme weather events like drought and flooding, the frequency of which is expected to increase due to global warming.