SAN FRANCISCO, CALIFORNIA—The world’s lakes are warming faster than both the oceans and the air around them, a global survey of hundreds of lakes shows. The rapid temperature rise could cause widespread damage to lake ecosystems, say scientists who presented the findings today at the American Geophysical Union meeting here. The global effects could be even more serious, because higher lake temperatures could trigger the conversion of billions of tons of carbon stored in lake sediments to methane and carbon dioxide (CO2), in a feedback effect that could accelerate global warming.
The temperature increase—a summertime warming of about a third of a degree per decade over 25 years—is “pretty modest,” says lake biologist Peter Leavitt of the University of Regina in Canada, who did not participate in the study. “But you don’t need 2°- to 3°- increases in lake temperatures to have profound impacts.”
Many studies have shown that individual lakes are warming up in the summer. (Scientists rarely track lake temperatures in winter, when ice makes measurements more challenging.) The new study encompassed 235 lakes around the world, combining measurements by hand, which reach the depths of lakes, with satellite readings, which provide global coverage of lake surfaces. Over the study’s time period, between 1985 and 2009, a few lakes cooled whereas others warmed sharply, but the average warming of 0.34°C per decade was more than twice the 0.12°C per decade measured in the oceans over a similar period.
That the oceans lag isn’t unexpected, given their enormous mass, says Catherine O’Reilly, lead author on the study, which involved 64 scientists collecting data on six continents. But many lakes are warming even faster than surface air temperatures, which warmed by 0.25°C per decade between 1979 and 2012. “I would have expected that, on average, lakes would be warming more slowly than air,” says O’Reilly, a freshwater ecologist at Illinois State University in Normal.
A shorter ice season because of warmer winters may help explain why. “Normally, ice is a good insulator protecting lakes from atmospheric heating,” Leavitt says. With ice melting earlier, lake water is exposed to warm spring air for longer. That probably explains why lakes that normally freeze in winter are warming by 0.48°C per decade, about twice as fast as lakes that don’t freeze.
Another factor that may be accelerating the lake warming is a decline in cloudiness in some temperate areas, due at least in part to climate change. Clearer air allows more sunlight to strike lakes’ surfaces. And as lakes soak up more heat from the sun and the air, their waters become more stratified, with the less dense warm water floating on top of more dense cold water. The stratification prevents deep, cold waters from mixing into surface layers and cooling them in summer.
The rapid summertime warming bodes ill for lake species. Freshwater fish that like the cold, such as lake trout, could suffer. So could species that rely on increasingly threatened lake ice. The Baikal seal in Russia’s Lake Baikal gives birth on the ice, points out biologist Stephanie Hampton of Washington State University, Pullman. Adds O’Reilly: “Large changes in our lakes are not only unavoidable, but are probably already happening.”
Meanwhile, in warmer places, says Leavitt, “strong stratification and warm surface waters are the recipe for blooms of noxious and possibly toxic cyanobacteria, particularly in regions where agriculture and urbanization have fertilized the lakes and estuaries.”
Warming lakes may have global implications as well, say the researchers, whose study will be published today in Geophysical Research Letters. As aquatic organisms die, their carbon-rich remains fall into the water column, where they can be stored in sediments or broken down by microorganisms into gases. “Lakes are already massive furnaces for processing terrestrial organic matter” and creating greenhouse gasses, Leavitt says. “Warming these regions further is likely to increase their role in combusting carbon to CO2.”
The authors say their findings point to the need to update global climate models so they better predict lake warming. They’d also like to see better remote sensing techniques that can measure the temperature of smaller lakes, missed by current satellites.