Lakes around the world are warming surprisingly quickly due to climate change, threatening the global water supply. And lakes in Canada are some of the fastest-warming in the world, a new study shows.

The warming waters can lead to problems like toxic algae blooms that make water undrinkable, declines in fish populations that people rely on for food and other serious problems, warns the international team of researchers that released the study this week.

"If air temperatures continue to increase and this influences water supply and water quality, that has a huge implication for humans as we need fresh water to survive," said Sapna Sharma, a researcher at Toronto's York University who was one of the lead authors of the report.

Sapna Sharma, a researcher at Toronto's York University who was one of the lead authors of the report, said Canadian lakes are warming at twice the global average rate. (Courtesy Sapna Sharma)

The study looked at 235 lakes on six continents representing half the world's freshwater supply. Their surface temperatures between 1985 and 2009 had been measured both directly and using satellites.

The lakes had different sizes, depths, locations and other characteristics, but despite their variability, "over 90 per cent of them had a clear signal of warming," said Sharma. "I didn't expect to see that."

The study found that on average, lakes were warming at a rate of 0.34 C per decade — faster than either the ocean (increasing 0.12 C per decade) or the air (warming by 0.25 C per decade), the researchers reported in the journal Geophysical Research Letters and announced at the American Geophysical Union meeting San Francisco Wednesday.

"Canadian lakes and ice-covered lakes were warming twice as fast as air temperatures and most of the other lakes in the study," Sharma said in an interview with CBC News.

The study found that the rate of warming averaged 0.72 C per decade at high latitudes.

Lake Superior warming extra fast

One of the big surprises, Sharma added, was that Lake Superior had one of the fastest rates of warming in the world.

While smaller lakes might be expected to warm more quickly, a couple of factors are having a particularly strong effect on lakes like Superior.

Cliffs run along the coast of Caribou Island on Lake Superior, which was found to be one of the fastest-warming lakes in the world. (Rolf Hicker Photography)

One is that lakes that are normally ice-covered in winter are melting earlier in the spring, exposing the lake to warmer air temperatures for a longer period of time.

Another, ironically, is that decreased pollution in North America is leading to less smog and cloud cover.

"So more solar radiation is hitting the lakes and water temperatures are warming faster than you'd just expect simply [from] climate change," Sharma said. "But hopefully, this will be a short-lived phenomenon."

The paper predicts a lot of negative effects linked to the warming:

Algal blooms that suck the oxygen out of the lake water, choking out other organisms, are expected to increase 20 per cent over the next century.

Algal blooms that are toxic to fish and humans are expected to increase by five per cent over the next century.

An increase in emissions of the potent greenhouse gas methane is expected to increase four per cent over the next decade.

Increased evaporation will cause a drop in lake water levels.

Much of this dry land would be under water when Lake Powell in Arizona is full. Lakes are warming surprisingly quickly, leading to increased evaporation and contributing to lower water levels. (Rick Wilking/Reuters) Sharma said the warming also decreases the amount of habitat available to native cold water fish such as lake trout and walleye, while increasing the likelihood that invasive species will thrive.

Africa's Lake Tanganyika hasn't warmed nearly as much as Lake Superior, but is already seeing declines in fish populations that local people rely on for food, Sharma added.

In North America, lake levels are already dropping in some of the Great Lakes and algae blooms have already made tap water undrinkable for days at a time in places like Toledo, Ohio.