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Lake Tahoe, Sand Harbor, NV..

Lake Tahoe, Sand Harbor, NV

Photogrph by Stephen Hall, My Shot

Richard A. Lovett

for National Geographic News

Published December 2, 2010

This story is part of a special news series on global water issues.

A loss of oxygen and the deterioration of food chains have transformed Africa’s Lake Tanganyika and Russia’s Lake Baikal. Scientists have pointed to global warming, and now a new study finds that a similar fate may be in store for many of the world's freshwater bodies.

In the last 25 years, the world's largest lakes have been steadily warming, confirms the new study, some by as much as 4°F (2.2°C). In some cases that is seven times faster than air temperatures have risen over the same period.

It's an important find, scientists say, because lake ecology can be extremely temperature-sensitive. "A small change in temperature can have quite a dramatic effect,"

says study author Simon Hook, a geologist and remote sensing expert at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California.

In many lakes, warming waters could kill native fish, clog pristine waters with algae, and expose fish and other aquatic species to more toxic pollution.

Hook adds: "It may allow nonnative species to come in—fish that prefer warmer water, or a nonnative plant. They may not be the species you want."

For example, the invasive zebra mussel, found now throughout the Great Lakes region, seems to prefer the warmer waters of Lake Superior’s sheltered harbors, explains Steve Colman, director of the Large Lakes Observatory at the University of Minnesota in Duluth. Colman was not involved in the study. If other areas of the lake warmed, he says, the mussels, which cling to intake pipes, motors, and more, and threaten to kill off native and commercially important species, could spread. (Read more about Colman’s work on Lake Superior.)

Small temperature changes could also spur algal blooms that could make a lake toxic to fish, Hook and his colleague Philipp Schneider, also of the JPL, said in a statement.

Taking the Earth’s Temperature

The two scientists measured lake temperatures by digging through 25 years of stored data from satellites that had imaged the entire surface of the Earth (sometimes several times a day).

Eventually they whittled the data down to nighttime measurements of thermal infrared emissions for 104 lakes large enough so that the satellites got good readings of their water surfaces, rather than a mix of water and shoreline. These included the Great Lakes, Tahoe, Baikal, and Tanganyika.

Thermal infrared radiation is a type of light emitted by warm objects. Because it varies with temperature, it can be used as a long-distance thermometer, accurate to within a few tenths of a degree.

Schneider and Hook found that for the lakes they studied, midsummer surface temperatures have, on average, warmed at the rate of 0.81°F per decade (0.45°C)—or slightly more than 2°F (1.1°C) during the 25-year study period.

Some lakes warmed at more than twice that rate. In general, lakes in northern regions—the United States, Canada, and Europe—warmed the most quickly.

"That's consistent with what you'd expect from global warming," Hook said. "In the equatorial regions we see very little change."

(Learn more about the link between global warming and water resources with National Geographic’s Climate Change and Water Quiz.)

Ecological Effects

The satellite data alone aren't sufficient to determine all of the ecological impacts of the lakes studied. "[That] cannot be determined without specific analysis of each lake," says Steven Running, an ecologist and expert on global ecosystem monitoring at the University of Montana who was not part of the study team. Running was part of a team of scientists awarded the Nobel Prize for his work with the Intergovernmental Panel on Climate Change.

Beyond the possible consequences Colman, Hook, and Schneider have named, increasing water temperatures could have an impact on a host of issues related to toxicity, warns Marjorie Brooks, an aquatic ecologist at Southern Illinois University in Carbondale.

To begin with, she said by email, "Warmer water has lower oxygen." But at the same time, she said, warmer conditions increase most aquatic animals' metabolic rates, increasing their need for oxygen.

That, she added, not only stresses them, but it can make some toxic chemicals, such as metals and organophosphate pesticides more toxic, even if contamination levels remain unchanged. This occurs because the higher metabolic rates increase the amount of chemicals that the animals are exposed to, either as they "breathe" water for oxygen, or are forced to eat more food to fuel their higher metabolic rates.

The next step for the study, Hook says, is to go to some of the more rapidly changing lakes and determine precisely what type of ecological changes have been occurring.
The study was published November 24 in the online edition of the journal Geophysical Research Letters.

(See photos of floods, droughts, and other ways climate change is affecting water resources.)

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