Geology in Real Time: Vanishing Arctic Ponds

Okay, technically this is a matter of geomorphology, but however you style it, arctic ponds and lakes are vanishing and it’s a real concern.

As anyone who has tried to walk across arctic and subarctic tundra can tell you, it a sloppy wet business. In fact, ponds and lakes constitute 20–40% of Arctic lowlands, the largest surface water fraction of any terrestrial biome. Those ponds and lakes provide crucial habitat for wildlife, supply water for remote Arctic communities and play an important, if incompletely understood, role in carbon cycling and the regional energy balance.

Climate scientists had assumed that the rapidly increasing warmth in the arctic and subarctic – an area warming as much as five times faster than global averages – would increase the size of existing lakes and ponds. Increased melt water occurred as expected and the fresh water thawed the margins of lakes, melting the permafrost, and triggering subsidence. That’s certainly happening.

But the greater observed effect is that arctic and subarctic ponds and lakes are vanishing.

Drained arctic lake. Photo: David Swanson, National Park Service

Inventories from satellite photography document that in the last ten years at least 192 larger lakes in the Alaska arctic have vanished. At one level, it’s pretty easy to understand. The area is underlain by permafrost, permanently frozen soils.

Why, yes, there are a lot of arctic and subarctic ponds

The interstices in the lake bed sediments are filled with ice; they are impermeable. Hence, water pools on top. The lake beds partially thaw each summer, but the thawing doesn’t penetrate completely through the frozen layer. So the lakes became permanent, or as permanent as any lake can be.

But as the area has warmed as a consequence of anthropogenic climate change, several mechanisms have operated to alter that millennia-old stability. First, as expected, longer summers have meant that the lake surfaces are open water longer than before. Open water is darker than ice, let alone snow-covered ice, and absorbs more sunlight. So the lake water is warmer and warmer longer than formerly. Which means the seasonal thawing goes deeper. But the surprise mechanism is rainfall. Precipitation that formerly fell as snow – frozen water – now falls as rain. The rain is much warmer than the snow, so it warms the lake water, triggering still more thawing. When the thaw zone under the lake penetrates through the permafrost under the lake bottom, what was impermeable is suddenly permeable. The lake sinks through its floor.

In Alaskan arctic national parks, for example, from 2000-2017, the average rate of water loss was about 700 hectares (1,730 acres) per decade. Some 760 hectares (1,878 acres) of lake surface disappeared in 2018 alone, a more than tenfold increase.

The loss of these lakes is concerning for several reasons. It’s critical breeding habitat for several bird species, including the near-threatened Yellow-billed Loon. It’s foraging habitat for shorebird species, including the critically endangered Spoon-billed Sandpiper in eastern Siberia. The loss of habitat is a grave threat to those species’ survival.

In the bigger picture, it also means climate models that didn’t include disappearing lakes are wrong, or at least incomplete, and need to be re-done. While the greenhouse gas impacts of the vanishing lakes is still being puzzled out, it seems likely that there is another feedback loop in the exposed, frost-free, dried out lakebed, where carbon captured for millennia will be converted to CO₂ and methane at accelerating rates. If nothing else, the drier landscape is going to be more prone to wildfires.

Another incipient catastrophe to add to you worry list.