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