Lakes are warming across the globe

Throughout the past quarter-century, inland lakes have been experiencing a small, steadily rising nighttime fever. Globally, the average increase has hovered around 0.045 degrees Celsius per year, but in some regions the increase has been more than twice that — or about 1 °C per decade.

In some regions — notably the Northern Hemisphere’s mid and upper latitudes — lakes have been warming faster than have surrounding air temperatures.

No question, the upticks have been small. They also appear to have been inexorable since at least 1985. Indeed, these observations, published November 24 in Geophysical Research Letters, represent “a new independent data source for assessing the impact of climate change throughout the world,” according to the study’s authors, NASA scientists Philipp Schneider and Simon Hook of the Jet Propulsion Laboratory at Caltech.

JPL’s thermal survey of 167 major lakes relied on satellites. Like hovering thermometers, those eyes in the sky have been collecting infrared — heat — measurements of surface features. Nighttime lake surfaces were surveyed during summer months in the Northern Hemisphere and in January through March in the Southern Hemisphere.

Overall, lakes in northern Europe appeared to be warming fastest. In North America, lakes in the U.S. Southwest warmed somewhat faster than did the Great Lakes.

An unrelated study due out soon in the same journal has mapped changes in the existence and size of 2,938 lakes throughout China. The new analysis compared data collected between the 1960s and ’80s with others from the mid-2000s. And it finds not only a 13 percent reduction in China’s overall surface area covered by lakes but also a vanishing of almost 250 discrete water bodies. Those lake losses do not just reflect the drying up of some little ponds, the authors note, because their analysis surveyed only lakes that initially had been at least 1 square kilometer in size.

The objective was to generate “the first comprehensive view of decadal-scale lake changes, which may be used to examine how climate change and human activities may have affected lakes in the entire China,” explain Ronghua Ma and Guishan Yang of the Chinese Academy of Sciences in Nanjing and their colleagues.

Most lake disappearances occurred in northern provinces and autonomous regions. Although the evidence is “limited,” the authors note, in these areas lake reductions or losses “might be associated primarily with climate change.” By contrast, lake losses or contraction in southern provinces appear most likely due to the effects of human influences, which presumably would reflect diversion for irrigation and drinking water or landscape remodeling for urban development.