Human activity is altering the world's precipitation patterns, bringing more rainfall to Canada, Northern Europe and Russia and drier weather to tropical and subtropical areas north of the equator, according to the first major international study that examines these changes over the past century.
Global warming has been blamed for higher temperatures and warmer oceans. But this is the first research paper that makes a link to rainfall patterns, with widespread implications for how people will adapt now that they've messed with Mother Nature.
"It's the first time that we've detected in precipitation data a clear imprint of human influence on the climate system," Francis Zwiers, one of the lead authors of the study and director of the climate research division at Environment Canada, said in an interview Monday.
"Temperature changes we can cope with. But water changes are much more difficult to cope with. That will have economic impacts, and impacts on food production, and could ultimately displace populations."
The study, to appear Thursday in the science journal Nature, comes as record rainfalls wreak havoc in Britain and force thousands from their homes.
The global-warming models that scientists developed to forecast climates have predicted that one effect of a warmer world would be a shift in precipitation patterns. Current observations, the study found, confirm what the models predicted.
Dr. Zwiers and his colleagues gathered global rainfall data for about 80 years, starting in 1925, and then compared it to 14 complex computer climate models.
Natural factors, such as volcanic activity, contributed to changes in precipitation patterns. But nature's work pales in comparison to what humans have done, in the form of steady increases in greenhouse gases and sulphate aerosols produced by burning fossil fuels, the study found.
Dr. Zwiers, one of Canada's most respected thinkers on global warming, explained this human activity is causing a stronger water cycle, moving more water vapour away from the warmest parts of the planet and pushing it toward the poles. This is what is making wet areas wetter, and dry areas drier.
Furthermore, higher use of fossil fuels in the Northern Hemisphere appears to be nudging the central rain band off the equator and driving it farther south, he said.
The study found that human-induced climate change has caused most of the precipitation increases in mid-latitude areas, including Canada, as well as south of the equator. Global warming has also resulted in drier conditions just north of the equator, including Mexico, Central America and northern Africa.
One of the ironies for Canada is that while there is more overall precipitation through the year, future projections show that the summers will be particularly dry for the southern Prairies, Dr. Zwiers said. That means residents will have to spend more on irrigation or switching crops.
Gordon McBean, a professor at the University of Western Ontario and the former assistant deputy minister of Meteorological Service of Canada, said the study is proof that global warming is not only about changing temperatures.
"The reality is that the climate is changing. It's not just warming, it's getting wetter in certain places and unfortunately also drier in other places," he said.
Dr. McBean said scientists are arming individuals and governments with information to make regulatory changes and plan development.
"We need strategies both within Canada and … working globally on making informed choice. The scientific community is providing information which individuals, governments at all levels need to factor into their choices," he said.
Dr. Zwiers said the results from this study have given climate scientists increased confidence in their ability to predict changes in precipitation patterns.