The world's largest lakes, including Lake Tahoe, have been warming rapidly for 25 years as the global climate changes, NASA scientists report.

And throughout the Northern Hemisphere, surface water temperatures of many lakes have been rising even faster than the warming air above them, according to observations by ultra-sensitive satellites.

In a report just published in the journal Geophysical Research Letters, Philipp Schneider and Simon Hook of NASA's Jet Propulsion Laboratory in Pasadena say the warming rate of all the major lakes observed by the satellites has averaged nearly a full degree Fahrenheit per decade. For some lakes in the Northern Hemisphere, the average increases reached nearly two degrees Fahrenheit per decade, they said.

"This is just one of several lines of evidence that global warming is really taking place," Hook said. "The evidence is striking and worldwide."

A geologist who designs high-resolution, Earth-observing instruments for satellites, Hook frequently measures surface temperatures at Lake Tahoe directly in order to validate the accuracy of the satellite observations.

The warming trend there has been particularly striking, indicating that the lake's surface temperature has often risen twice as fast as the air temperatures above it, he said.

Geoffrey Schladow, director of the Tahoe Environmental Research Center at UC Davis, is familiar with the report on global temperatures. Tahoe has long been a bellwether for understanding the effects of warming lakes generally, he said.

As lakes like Tahoe grow warmer, the regular mixing of water between the surface and the bottom slows, Schladow said. Dangerous chemicals like heavy metals and phosphorous, which normally are locked in bottom sediments, become soluble, so they pollute the entire lake.

"The result is to change the lake's entire ecology," he said.

Robert Coats, a UC Davis water-quality researcher and consulting hydrologist, has used temperature data gathered in the more than 40 years since Charles Goldman founded the original Tahoe Research Group at Davis. He has calculated that since 1968 the entire lake - not just the surface - has warmed by nearly a full degree Fahrenheit.

That increased warming in lakes like Tahoe and the loss of oxygen in their depths, Coats said, could ultimately turn those lake bottoms into "dead zones" - much like those now found in offshore ocean regions where oxygen has been depleted and many life-forms have disappeared.

Three satellites flying in polar orbits continuously measure the warming temperatures of the world's largest lakes, and they have already found similar warming trends over the oceans and land surfaces. Two satellites operate for the National Oceanic and Atmospheric Administration and one for the European Space Agency. All are equipped with sophisticated instruments called Advanced Very High Resolution Radiometers, which have long been used in other orbiting spacecraft examining Earth's features.

For their report, Schneider and Hook selected 176 of the world's 364 largest lakes and gathered measurements only at night and only from selected lake areas, far from surrounding land.

The results, they said, proved consistent with the most recent estimates of the Intergovernmental Panel on Climate Change, the U.N.-sponsored scientific organization that has reported on global warming regularly since 1990.