|June 24, 2020|
Why the Arctic is warming so fast, and why that's so alarming
|ON SATURDAY, THE residents of Verkhoyansk, Russia, marked the first day of summer with 100 degree Fahrenheit temperatures. Not that they could enjoy it, really, as Verkhoyansk is in Siberia, hundreds of miles from the nearest beach. That's much, much hotter than towns inside the Arctic Circle usually get. That 100 degrees appears to be a record, well above the average June high temperature of 68 degrees. Yet it's likely the people of Verkhoyansk will see that record broken again in their lifetimes: The Arctic is warming twice as fast as the rest of the planet---if not faster---creating ecological chaos for the plants and animals that populate the north.|
"The events over the weekend---in the last few weeks, really---with the heatwave in Siberia, all are unprecedented in terms of the magnitude of the extremes in temperature," says Sophie Wilkinson, a wildfire scientist at McMaster University who studies northern peat fires, which themselves have grown unusually frequent in recent years as temperatures climb.
The Arctic's extreme warming, known as Arctic amplification or polar amplification, may be due to three factors. One, the region's reflectivity, or albedo---how much light it bounces back into space---is changing as the world warms. "What we've been seeing over the last 30 years is some relatively dramatic declines in sea ice in the summertime," says University of Edinburgh global change ecologist Isla Myers-Smith, who studies the Arctic.
Since ice is white, it reflects the sun's energy, something you're already probably familiar with when it comes to staying cool in the summer. If you had to pick the color of T-shirt to wear when going hiking on a hot day, she says, "most of us would pick the white T-shirt, because that's going to reflect the sun's heat off of our back." Similarly, Myers-Smith says, "If the sea ice melts in the Arctic, that will remove that white surface off of the ocean, and what will be exposed is this darker ocean surface that will absorb more of the sun's heat."
That's warming the region's waters, and potentially raising temperatures on land as well. Sea ice is also returning later in the autumn because temperatures are taking longer to drop, in part because the heat trapped in the deiced ocean is taking longer to dissipate. "Even though the ocean will refreeze in the wintertime," Myers-Smith says, "it's a thinner layer that will potentially melt off the next summer, rather than what it used to be in the past, which is this much larger ice pack of sea ice that stayed all summer long."
This dovetails with the second factor: changing currents. Ocean currents normally bring in warmer water from the Pacific, and colder water exits out of the Arctic into the Atlantic. But those currents may be changing because more melting ice is injecting the Arctic Ocean with freshwater, which is less dense than saltwater, and therefore floats above it. The missing ice also exposes the surface waters to more wind, speeding up the Beaufort Gyre in the Arctic, which traps the water it would normally expel into the Atlantic. This acceleration mixes up colder freshwater at the surface and warmer saltwater below, raising surface temperatures and further melting ice.
Ocean currents influence the weather, a third factor. More specifically, they drive the powerful polar jet stream, which moves hot and cold air masses around the Northern Hemisphere. This is a product of the temperature differences between the Arctic and the tropics. But as the Arctic warms, the jet stream now undulates wildly north and south. This has been injecting the Arctic with warm air in the summer and the US with extremely cold air in the winter, like during the "polar vortex" of January 2019.
"What's happening right now in Siberia is this high-pressure system and this warm air mass is being moved up from the south," says Myers-Smith. "And then it's just sort of stalling out there and sitting there. And we've seen those kind of weather patterns more frequently in recent years." Having that warm air hanging over the Arctic during the summer further imperils sea ice that should be lasting through the season, as well as frozen soil known as permafrost (more on that in a second).
These warm air masses can also arrive in the wintertime, with serious consequences for Arctic ecosystems. If all that snow on the ground starts to melt, and then freezes once more, it'll form impenetrable layers of ice. "There have been some pretty dramatic diebacks of reindeer and caribou in various places, because you get these thick ice layers and they can't dig through to get to the plants," says Myers-Smith.
And the ecological ripple effects don't end there. Sea ice tends to produce fog because it cools the local climate and creates a variation between the temperature of the air and of the ocean. When it's cooler, plants grow more slowly. The fog also changes the light conditions---it's more diffuse than direct sunlight. If the fog is super thick, plants won't get as much light. "But if it's lighter fog, it might actually help the plants a bit, because plants do better at photosynthesizing when they have more diffuse light," says Myers-Smith. Losing sea ice, then, will have ripple effects across the land as well, with ecological consequences that Myers-Smith and her colleagues are just beginning to explore.
What they have been finding is that, indeed, the Arctic is greening as it warms. Having a newly verdant north sounds lovely, but could in fact be a serious problem for the planet. It's not so much that invasive plant species are moving into the Arctic but that the community of native species is changing. Taller shrubs are growing more abundant, for instance, which traps more snow against the ground in the winter so it doesn't blow across the tundra. This insulating layer may mean that the cold can't penetrate into the soil, potentially exacerbating the thawing of permafrost, which releases greenhouse gases that further warm the planet.
When this permafrost thaws, it can change the salinity and general chemistry of the water flowing through an Arctic environment. "These northern soils, they also contain vast stores of mercury that has been frozen for a long time," says the University of Alberta's David Olefeldt, who studies permafrost. "We don't know really to what degree mercury will be mobilized and be able to move downstream, where of course it can move into food webs and fish, which then would influence indigenous communities and local land use."
Olefeldt and his colleagues are finding that some permafrost is thawing so quickly that it's collapsing and carving massive holes in the landscape, a phenomenon known as thermokarst. "It turns into squishy wetlands rather than firm ground, which is something that affects mobility of both people and animals that are being herded," says Olefeldt. "In large parts of the Arctic, you have caribou or reindeer herding, which will be impacted if the ground loses its firmness."
Here's another twist: More plant growth in the Arctic means that the vegetation is sequestering more CO2 via photosynthesis. But on the whole, scientists think that it doesn't balance out the effects of the greenhouse gases released when permafrost thaws. "Yes, there is more carbon in these plants as you get more shrubs and more growth and less bare ground," says Myers-Smith. "But we are also through permafrost thaw and other factors losing carbon from the soils as well. And the amount we're losing is probably not being offset by the increased plant growth."
Another question that Myers-Smith and her colleagues are looking into is what that shift in vegetation might mean for wild animal species. Moose and beavers, for example, rely on woody shrubs for food---and in the beaver's case, building material. "Both of those species have been seen more frequently in recent years in tundra locations. They seem to be moving their ranges northward," Myers-Smith says. "That has implications for the wildlife species that inhabit tundra ecosystems as well. So there's potential interesting interactions at play there." For instance, beavers might compete with local species for food, and alter the flow of water in these habitats by building dams.
On top of having to deal with newcomers, native Arctic animal species are not equipped to deal with such crippling heat. "The kind of temperatures that they're seeing in Siberia right now, up to 100 degrees Fahrenheit, that is a temperature which would stress out most Arctic animals pretty severely," says Myers-Smith.
Oddly enough, Arctic plants may be better equipped to ride out the sizzling heat. The climate in this part of Siberia is similar to parts of Alaska's interior, where the freezing temperatures of winter naturally swing into higher temperatures in the summer. "It's pretty extreme. It's breaking records---but it's not that much higher than the maximum temperatures that would probably have been experienced at some point in the region," says Myers-Smith. That is, the plants are likely already adapted to such swings in the north. Many are quite short, so they stay insulated in the snowpack in the winter, and out of the parching wind when it's warmer. Deciduous plants in this region drop their leaves in the winter to avoid damage, while evergreen plants utilize tough, fleshy leaves that resist both cold and heat.
But plants stand little chance against another consequence of a warming Arctic: peat wildfires. Peat is a goopy kind of soil made of layers of slowly-decomposing plant matter. When peat dries out, as it is increasingly doing up north, it turns into carbon-rich fuel. All it takes is a single lightning strike to spark a smoldering blaze, which penetrates deeper and deeper into the peat layers, spreading slowly across a landscape and igniting the vegetation above. For every hectare of burning peat, 200 tons of carbon might spew into the atmosphere. (For comparison, a typical car emits 5 tons a year.) With the Arctic warming so rapidly, thunderstorms---which form when warm, moist air rises to meet cold air above---are moving ever farther north. That means lightning is now striking just a few hundred miles from the North Pole.
Weirdly, these smoldering peat fires can overwinter, turning into "zombie" fires. "They continue to burn within the soil profile over the winter, even if there is snow and other winter processes going on," says Wilkinson, the wildfire scientist at McMaster University. "And then when the surface of the soil dries out again, they have the ability to basically reemerge, which is where the 'zombie' definition comes from. And then basically, you're starting on a back foot there, because you'll be dealing with last year's fires before you even have the new ignitions of this year."
And so a troubling portrait of a new Arctic is emerging. Its protective ice is receding. Ever fiercer heatwaves are drying out more vegetation, which fuels more massive wildfires. When peat fires ignite by lightning during the summer, they can survive underground over the winter, reemerging the next year. Animal species are on the move. The Arctic is getting greener, and that underscores a sad reality: Earth's northern lands are suffering massive change.
"It really is quite an unprecedented time," says Wilkinson. "Every time we think there has been a big event or a big anomaly, there tends to be something that follows and overshadows it the next year."