Changes in rainfall man-made, Canadian scientists say
Human activity is changing rainfall patterns throughout the world, bringing more precipitation to temperate and northern regions while providing less to subtropical and tropical regions north of the equator, according to a study to be published Thursday in the journal Nature.
The study is the first to confirm a link between rainfall patterns and global warming, according to co-author Francis Zwiers, the director of the Climate Research Division of Environment Canada.
Previous studies haven't shown a positive link between human-induced changes and precipitation on a global scale because changes in precipitation in different regions cancel each other out, Zwiers told CBC News.
But by looking at each latitudinal region individually, larger trends were discovered that could not be accounted for by naturally occurring changes such as increased volcanic activity.
Instead, the burning of fossil fuels appears to a leading cause, the authors said.
The scientists looked at data sets of rainfall patterns over a 75-year period from 1925 to 1999 and compared them with 14 climate models to find the closest fit.
Even climate models that accounted for both the natural andhuman-produced greenhouse gas emissions underestimated the expected changes in precipitation patterns, the authors found.
"The observed changes, which are larger than estimated from model simulations, may have already had significant effects on ecosystems, agriculture and human health in regions that are sensitive to changes in precipitation" such as the Sahel region south of the Sahara desert in Africa, wrote lead authors and Xuebin Zhang and Zwiers from Environment Canada's Climate Research Division.
The authors found global warming contributed significantly to increases in precipitation in the Northern Hemisphere's mid-latitudes, a region between 40 and 70 degrees north.
Dry regions to become drier
But in the subtropical and tropical regions between 30 degrees north and the equator, precipitation decreased, threatening in some cases already vulnerable regions.
"The area that will see the greatest impact if these trends continue is sub-Sahara Africa, an area that is already climatologically dry and where agriculture is marginal at best," said Zwiers.
Regions in the Southern Hemisphere's tropics became wetter, according to the study.
Should the current trends in precipitation continue, Zwiers said Canada should expect to see more precipitation in the winter and less in the summer, with the overall effect being an overall increase in precipitation.
Zwiers said the study should give scientists more confidence in the predictive powers of climate models.
"This is the opening chapter in what should be a long series of reports on the changes in precipitation caused by climate change," Zwiers told CBC News.