SAN FRANCISCO, CA, DECEMBER 16, 2015 — Lakes across the globe have been warming along with the rest of the planet, report authors of a new study, but at a rate surprisingly higher than surface trends observed so far.

“These results suggest that large changes in our lakes are not only unavoidable, but are probably already happening,” said lead author Catherine O’Reilly, an associate professor of geology at Illinois State University.

The study, published in Geophysical Research Letters, is the combined effort of more than 60 scientists around the world. They measured surface temperatures of 235 lakes over the period of 1985 to 2009, combining satellite measurements and on-site lake recordings by boats and buoys.

Temperatures figure
Map of trends in lake summer surface temperatures from 1985 to 2009. Most lakes are warming, and there is large spatial heterogeneity in lake trends. Note that the magnitudes of cooling and warming are not the same. Credit: Geophysical Research Letters.

The vast majority of the lakes studied were seen to be warming, particularly in the polar regions. As a set, the warming of summertime surface temperatures increased at a statistically significant rate of +0.34 degrees C, +0.61 degrees F, per decade over the study period, relative to the mean over the same time interval.

This rate is much higher than the +0.19 degrees C per decade global surface warming trend measured by NASA and the +0.17 degrees C per decade trend measured by NOAA.

Besides demonstrating the rapid pace of modern warming driven by greenhouse gases, the results have implications for animals that inhabit the lakes and for humans living nearby. “Society depends on surface water for the vast majority of human uses,” said Stephanie Hampton, a co-author and director of Washington State University’s Center for Environmental Research, Education and Outreach. “Not just for drinking water, but manufacturing, for energy production, for irrigation of crops.”

“Protein from freshwater fish is especially important in the developing world,” she said.

The health of lake ecosystems can depend critically on lake temperatures. Warming can kill fish, especially in ecosystems with high nutrient pollution. Together these create the conditions for sometimes-toxic algal blooms.

Other Factors Also at Play, Leading to Variations

Lake warming depends on several factors in addition to global warming, and these local changes lead to variations in warming rates. For instance, cloud cover can reduce the incidence of sunlight, melting glaciers contribute cooler freshwater, and decreases in ice cover mean some lakes absorb more sunlight than when less covered by ice. The area and maximum depth of lakes also modulate surface temperatures.

The local lake measurements have confirmed satellite observations of lakes at high latitudes in the north and south, where surface warming trends as high as 0.72 degrees C per decade (1.3 degrees F per decade).

These warmings imply a 20 percent increase in summer surface algal blooms over the next century, and a 5 percent increase in toxic blooms. They imply a 4 percent increase in lake methane emissions, a powerful greenhouse gas, over the next decade.

Rapid warming of earth's lakes means changes 'already under way' for lake resources, productivity. Click To Tweet

Higher temperatures also bring increased evaporation rates, with corresponding decreases in lake levels, a factor now in drought-stricken regions such as Lake Mead on the Nevada-Arizona border. Those lower lake levels in turn can bring economic losses, and in some cases complete ecosystem collapse. In some cases, scientists have observed productivity of some lakes already dropping.

“The widespread warming reported here,” the study concludes, “suggests that large changes in Earth’s freshwater resources and their processes are not only imminent but already under way.”

Photo: Lake Mead (copyright protected).

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