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Lake warming: The implications are both global and dire

Lakes holding half of the world’s freshwater are warming faster than oceans or air, suggesting that large changes in freshwater resources and processes are already under way, researchers say.

Lakes holding more than half the world’s surface freshwater have been warming up at alarming rates, according to new research backed by NASA and the National Science Foundation.

Indeed, the average increase of .61 degrees Fahrenheit per decade (.34° Celsius) exceeds the warming rates of either Earth’s atmosphere or its oceans.

If you saw this study in the news late last week, perhaps it was the Strib’s brief story about Lake Superior’s standing as one of the world’s fastest-warming lakes, or a slightly broader piece in the Duluth and St. Paul papers, all focused primarily on changes in ice-cover in Superior and the other Great Lakes.

Nothing awful or even very remarkable there, I guess; provincialism has been a hallmark in most coverage of this story, whether in  the Reno Gazette-Journal’s focus on Lake Tahoe or in the Tel Aviv newspaper Haaretz’s emphasis on Lake Kinneret, which you might have known as the Sea of Galilee.

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But the implications of this trend could hardly be more global, and more dire, given the potential impacts outlined by the researchers in the journal Geophysical Research Letters:

The global average lake summer surface water warming rate found here implies a 20% increase in algal blooms and a 5% increase in toxic blooms over the next century, as well as a 4% increase in methane emissions from lakes during the next decade.

Increased evaporation associated with warming can lead to declines in lake water level, with implications for water security,  substantial economic consequences, and in some cases, complete ecosystem loss.

Already, changes in thermal structure and mixing have decreased productivity of some lakes, which threaten human communities that depend on fisheries as a nutritional and economic resource. Lakes with high rates of surface temperature change may appear more likely to experience major ecosystem changes, but we caution that even lakes with low rates of change may be under ecosystem stress if the initial water temperatures are already near physiological maxima.

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

Also, unfortunately, complicated by a range of factors that make future changes really, really hard to predict.

235 lakes in sample

As with so many advances in earth science these days, this research drew upon a marriage of modern-day satellite sensing with old-time measurements by earthbound technicians.

As NASA explained in a press release announcing the paper, whose publication was timed to last week’s national conference of the American Geophysical Union in San Francisco, nothing beats satellites for large-scale data gathering across the globe. But their sensing is limited to surface temperatures, and their data sets go back only 30 years or so.

So the research team, led by Catherine M. O’Reilly of Illinois State University, looked for lakes where satellite readings could be augmented with on-site measurements that sometimes go back a century or more. They settled on a sample of 235 lakes with good data available for at least the 25-year period from 1985 through 2009, and focused on trends for three-month periods in summer, the exact dates varying somewhat with location.

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One scientist on the team, Simon Hook of NASA’s Jet Propulsion Laboratory in Pasadena, California, had already found rapid warming in certain high-latitude lakes, as measured by satellites.

And the new paper found that these lakes – including our Great Lakes and others in northern Europe, notably Lake Baikal in Siberia  – are warming at more than twice the average rate – 1.3 degrees per decade versus .61 degrees, with Lake Superior warming at more than three times average, 2.09 degrees.

Complicating factors

This was just one example of how lake warming driven by climate change is modified by interaction with location factors, including latitude and cloud cover, as well as differences in the lakes’ size, shape and depth, or morphology.

In general, and for reasons still not completely understood, lakes that are colder and deeper to begin with seem to be warming more rapidly, perhaps because periods of ice cover are shortening. Of all the Great Lakes only Erie, the shallowest, wasn’t warming at above-average rates.

Ice-covered lakes are typically warming faster than ambient air temperatures, and lake morphology affected the strength of this response. The world’s deepest ice-covered lakes warmed twice as fast as the overlying air temperatures, consistent with previous single-lake studies.  

For these large, deep lakes, the combination of shorter ice duration and rising air temperatures can lead to earlier summer stratification that results in surface waters warming more rapidly than air, whereas in smaller, shallower lakes, surface water temperatures should more closely track changes in air temperature. In addition, summer [solar] shortwave radiation trends were significantly greater for ice-covered lakes, and the lakes exhibiting the highest warming rates also experienced substantial decreases in summer cloud cover.. . .

A more detailed exploration of the interplay between these various climate drivers, by investigating the impact of temperature-related changes in ice-cover relative to increases in shortwave radiation, for example, would be beneficial for improving our ability to predict lake changes.

Though the researchers feel their work “signals the urgent need to incorporate climate impacts into vulnerability assessments and adaptation efforts for lakes,” there’s no discussion in the paper or the coverage it has inspired of the shape such efforts might take.

In the 10 percent or so of lakes that actually cooled during the study period, increased rates of glacial melt are one likely factor, but here and there a reforestation effort  or land-use change might have had some influence, according to a fine piece by Alexandra Witze for Nature’s online news site.

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And though the researchers have generally maintained their professional detachment from the undeniably gloomy import of these findings, the occasional wistful comment got through. As co-author Stephanie Hampton of Washington State University told Witze, she and her fellow researchers at Lake Baikal “used to hold informal competitions about who could swim the longest in the frigid waters without a wetsuit.”

“They still go out for a swim,” she says. “But it’s not the competition it used to be.”

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The full paper, “Rapid and highly variable warming of lake surface water around the globe,” can be accessed without charge here.