In Arctic Lakes, Scientists Watch For a Shifting Carbon Cycle
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Lake SS85 is one of hundreds of lakes dotting this 90-mile-wide fringe of land between the towering Greenland Ice Sheet and the Labrador Sea. For centuries, 85 and its aquatic neighbors have been ice-covered most of the year. But as the climate has warmed, high-latitude lakes — from the northern United States and Canada to Scandinavia and Siberia — have started to thaw, on average, a week earlier and freeze 11 days later than they did a century ago, according to Sapna Sharma, a biologist at York University in Toronto. The rate of ice loss has sextupled over the past 25 years. Northern lake temperatures are rising more than twice as fast as the global lake average, Sharma says. And nowhere is the climate changing faster than in the Arctic.
The boreal forests and unglaciated polar lowlands are Earth’s most lake-rich biome, hosting nearly half of the planet’s lakes by surface area. While precise data are sparse, a 2015 satellite-based inventory estimates some 3.5 million lakes cover a total of around 150,000 square miles in the Arctic. But due to the difficulty of conducting research in the remote north, relatively little is known about how these vast freshwater ecosystems are responding to the sweeping changes underway.
One of scientists’ key questions is how rising temperatures, shrinking ice seasons, and the increasing precipitation projected for many parts of the Arctic might affect lakes’ carbon cycles. Put simply, this cycle describes the actions of aquatic microbes that break down organic material — exhaling carbon dioxide and other greenhouse gases — and phytoplankton that take up carbon dioxide to build their skeletons — releasing oxygen. Lakes that breathe out more carbon dioxide than they take in are net carbon sources, while those that on balance remove carbon dioxide from the atmosphere are sinks.
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“We are trying to understand the carbon budget in the Arctic,” says Hazuková. The stakes are high: That ledger of sinks and sources informs the models that scientists use to project the Earth’s future climate. Currently, however, the estimate “pretty much only focuses on soils and vegetation,” she says. “Freshwaters are just not included at all.”
Some of those freshwater systems are changing “very, very quickly,” says John Smol, a paleolimnologist at Queens University in Kingston, Ontario. Earlier thaws and later freezes expose lakes to more light, heat, and contact with the outside world. The impacts compound at high latitudes, such as the lakes on Canada’s Ellesmere Island that Smol has studied for decades. The summer ice-free period up there used to be six weeks at most, he says. With 24-hour daylight during the Arctic summer, less time under ice cover opens lakes to significantly more time under the sun.
Arctic lakes are diverse, however, and climate change is manifesting differently across regions. In areas where rapidly thawing permafrost releases once-frozen stores of plants and other organic material into lakes, microbes are feasting on those extra helpings of carbon and belching out carbon dioxide and methane. Thermokarst lakes such as Alaska’s Big Trail Lake visibly boil with escaping greenhouse gases. Across the boreal region, the total annual carbon dioxide emissions from lakes is equivalent to that of forest fires, according to a study conducted in 2017 (before the recent extreme wildfire seasons).
But those amped-up emissions may be offset, at least in part, by lakes that emit little or even sponge up carbon. In a 2019 survey of Alaska’s Yukon River Basin, biogeochemist Matthew Bogard found lakes in that flat, dry region produce “negligible” CO2 emissions. That’s because those lakes have little hydrologic connection to the surrounding landscape, which means almost no organic material is delivered to the lakes through outside water flowing in, explains Bogard, who is now at the University of Lethbridge in Alberta.
Data on Arctic emissions are patchy overall, Bogard acknowledges. “We need more data from understudied regions.”
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Except this summer isn’t normal. Across the Northern Hemisphere, 2023 will turn out to be the hottest summer on record, according to the European Union’s Copernicus Climate Change Service. In West Greenland, we see day after day of rain. Everyone in Kangerlussuaq is talking about the extraordinary weather. Longtime resident Vivi Grønvald tells me she’s never seen a summer this wet. “It’s like we haven’t had a summer at all,” she laments. The period from May to July ends up breaking West Greenland precipitation records dating back to 1940, climatologist Sean Birkel, developer of the University of Maine’s Climate Reanalyzer, found in an August analysis. Birkel linked the season’s extreme precipitation to large circulation anomalies, including unusually weak North Atlantic winds, likely related to the 2023 El Niño.
For lake scientists, all that rain makes for murky work. Usually, these lakes are crystal clear, says project co-lead Jasmine Saros, a University of Maine ecologist who has worked in the area for more than a decade. But this year, the water is the color of coffee. “This is the first time I’ve seen these lakes like this,” Saros says. “So dark.”
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“The data that we got so far from the carbon sensors shows all the lakes were carbon sources,” Hazuková says, leaning over a computer screen filled with numbers. “Between April and now, they were carbon sources the whole time.”
That’s the opposite of what the researchers expected. “The reason why we started this study is that we thought these lakes were going to be sinks of carbon, at least during the summer … because they are not receiving organic matter to fuel respiration,” Hazuková reflects. “But what we saw this year was just unprecedented.”
So unprecedented, in fact, that Hazuková and Saros return to Kangerlussuaq in August for another look. They speed-hike the same route, covering around 60 miles in a week. The West Greenland weather has returned to its usual rainless days of long summer sun. The lakes are still brown, but their carbon dioxide levels have dropped, and several are once again behaving like sinks, says Hazuková.
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https://undark.org/2023/09/26/arctic-lakes-carbon/A couple of years ago there was a paper looking at the output in Alaskan lakes and the balance of CO2 and CH4 plus they sampled the waters to see what was involved. It was complicated.
In the grand scheme of things this is the time when the northern forest burn and the lakes thaw but it is interesting to find out more about what is going on.
