The world's rainfall patterns are changing, and they are doing so because of human-caused climate change, a new study finds.
The pattern found "is one predicted by climate models when they include anthropogenic, human influence on the climate system," Environment Canada's Francis Zwiers, director of the climate research division, told CTV.ca on Monday.
"And it's one that they fail to predict if you don't include human influence on the climate system in the climate models."
Environment Canada researchers analyzed global rainfall patterns over land from 1925 to 1999, breaking it down into bands of 10 degrees in latitude. The bulk of Canada's landmass sits between the 40th and 70th parallels.
They found 10 per cent more rain and snow in northern regions including Canada, Russia and Europe, and the southern tropics region below the equator.
Meanwhile, there has been drying away from the equator to 30 degrees North, including Mexico, Central America, sub-Saharan Africa, southern India and Southeast Asia.
Researchers say the main cause behind the shifting patterns is human activity, including a steady rise in greenhouse gas emissions and sulphate aerosols from the burning of fossil fuels.
Sulphate aerosols have a cooling effect. Zwiers wants to separate out the effect of sulphate aerosols in future research.
However, the overall effect is still one of warming, and that warming is changing things.
"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," Zwiers and colleague Xuebin Zhang said in a statement.
Piece of the puzzle
Zwiers said these results "are one additional piece of the puzzle" for the computerized climate models that scientists are using to predict the effect of climate change -- as well as mirroring the past.
"We're able to separate out the effect of humans and what is the effect of other external influences on the climate," he said.
Climate models have been good at modelling temperature and surface pressure, which affects storm patterns, he said.
If anything, the models have underestimated the changing patterns of rainfall and surface pressure, Zwiers said.
"There's still work for us to do to better understand why that's the case," he said.
"In the case of rainfall, we might have some worries about how well we observe rainfall," he said, noting land only represents 30 per cent of the Earth's surface. In addition, rain gauges aren't evenly distributed.
"What we are quite certain about is this pattern of change ... is very robust. And that's a feature you'd expect if there was human influence on the climate system."
If one just looks at the effects of things like volcanic eruptions or the sun, "you're not able to explain what you've seen," Zwiers said.
The full report will be published by the journal Nature on Thursday.
The future
As to the future, Zwiers said we can expect more of the same.
"What this means for Canada, we will receive more (precipitation) in the future than in the past," he said.
"But it looks like we will be receiving the lion's share of that in the wintertime, and that summer precipitation will be reduced, particularly in the southern and western parts of the country."
This will affect agriculture and forestry in those areas, with crops and forests being subjected to more frequent droughts, he said.
Some climate models suggest that not only will summers be hotter and drier in a warming world, when rainfall comes, it will do so in the form of intense cloudbursts and not gentle, soaking rains.
"That's one of the ironies," Zwiers said. "Even though climate models project that southern parts of the countries will become drier, they also project that extreme events will become more intense."
What today might be a once-in-a-100-year event could come every 50 years in the future, he said.
"People will suffer from more frequent droughts in the summertime, and at the same time, will suffer from more intense extremes when it does rain."
