|February 19, 2020|
The Arctic is getting greener. That's bad news for all of us
|Right now the Arctic is warming twice as fast as the rest of the planet, and transforming in massively consequential ways. Rapidly melting permafrost is gouging holes in the landscape. Thousands of years' worth of wet accumulated plant matter known as peat is drying out and burning in unprecedented wildfires. Lightning---a phenomenon more suited to places like Florida---is now striking within 100 miles of the North Pole.|
All the while, researchers are racing to quantify how the plant species of the Arctic are coping with a much, much warmer world. In a word, well. And probably: too well. Using satellite data, drones, and on-the-ground fieldwork, a team of dozens of scientists---ecologists, biologists, geographers, climate scientists, and more---is finding that vegetation like shrubs, grasses, and sedges are growing more abundant. The phenomenon is known as "Arctic greening," and with it comes a galaxy of strange and surprising knock-on effects with implications both for the Arctic landscape and the world's climate at large.
Despite its icy reputation, the Arctic isn't a lifeless place. Unlike Antarctica, which isn't home to trees or to many animals that you can see without a microscope, the Arctic is teeming with life, particularly plants. Its grasses and shrubs are beautifully adapted to survive winters in which their days are completely lightless, because the vegetation lies covered in a layer of snow, surviving mostly underground as roots. When the thaw comes, the plants have perhaps a month to do everything they need to survive and reproduce: make seeds, soak up nutrients, gather sunlight.
But as the world has warmed over the past few decades, satellites have been watching the Arctic get greener---with various levels of precision. One satellite might give you the resolution on the scale of a football field, another on the scale of Central Park. These days, the resolution of fancy modern cameras might be 10 by 10 meters. But even then, ecologists can't decipher exactly what these plant communities look like without being on the ground.
First, the Arctic is dark 24 hours a day in the winter. "That's a long-running challenge of using satellites in that part of the world," says Jeffrey Kerby, an ecologist and geographer formerly at Dartmouth College and now at the Aarhus Institute of Advanced Studies. He was one of the co-lead authors on a recent paper on Arctic greening published in Nature Climate Change by this international group of scientists, who received funding from the National Geographic Society and government agencies in the UK, North America, and Europe.
And even when you get 24 hours of light in the summer, it's a problematic kind of light. "Because the sun is so low, it can cast big shadows all over the place, and people generally aren't interested in studying shadows," Kerby says.
So with the help of small drones the team launches right from the field, researchers have been scouring landscapes to decode in fine detail how the Arctic is transforming, and marrying that with the data coming from the eyes in the sky. A drone can get close enough to the ground to tell them which plants might be benefiting in a particular landscape as it warms. The researchers can also quantify how an area is changing year over year by having the drones photograph the same regions, and by deploying, of all things, tea bags. "We stick tea bags in the ground, and over one year, two years, etc., and see how much of that gets decomposed across these different microclimates," says Isla Myers-Smith, a global change ecologist at the University of Edinburgh and co-lead author on the new paper.
They're finding that the change isn't driven by invasive species moving into the Arctic to exploit the warming climate. It's more that taller native species like shrubs are becoming more abundant. "It means that canopy heights are taller as a whole, and that has significant implications," says Myers-Smith. "It might be starting to influence the way the tundra plants protect the frozen soils and carbon below."
For instance, taller shrub canopies trap more snow in the winter, instead of allowing the stuff to blow around the tundra. This snow might build into an insulating layer that could prevent the cold from penetrating the soil. "So that accelerates---potentially---the thaw of permafrost," says Myers-Smith. "And you can also change the surface reflectance of the tundra when you have these taller plants, if they stick up above the snowpack." Vegetation is darker than snow, and therefore absorbs more heat, further exacerbating the thaw of the soil.
Thawing permafrost is one of the most dreaded climate feedback loops. Permafrost contains thousands of years of accumulated carbon in the form of plant material. A thaw---perhaps exacerbated by more abundant vegetation---threatens to release more CO2 and methane into the atmosphere. More carbon in the atmosphere means more warming, which means more permafrost thaw, ad infinitum---or at least until the permafrost is gone.
A permafrost melt also releases more water into the soil, leading to yet more knock-on effects for the vegetation. "When the ground is frozen, plants don't have any access to water," says Kerby. "So it's almost like being in a desert for part of the year."
Frozen ground limits when the plants can grow. But an earlier thaw could mean that plants kickstart their growth earlier in the year. As those soils thaw deeper and deeper, they will also release gobs of nutrients that have been trapped underground for perhaps thousands of years, supercharging the growth of these increasingly abundant Arctic plant species. This means the landscape could get even greener and even more hospitable to plants that can take advantage of warmer temperatures.
And really, underground is where so much of the Arctic mystery still lies: In these tundra ecosystems, up to 80 percent of the biomass is below ground. (Remember that in the deep chill of winter, roots survive underground.) "So when you see the green surface, that's just the tip of the iceberg, in terms of the biomass in these systems," says Myers-Smith. "So it could be that a lot of the climate change responses of these plants are actually all in the below-ground world that we have a very difficult time tracking and monitoring."
Another big unknown is how animal species---big and small---fit into a warmer, greener landscape. How might tiny herbivores like caterpillars take to an increasingly lush Arctic? How might large herbivores like caribou exploit the vegetation bounty, and might it even influence their migratory patterns, potentially threatening an important source of food for native people? And how might all these herbivores hoovering up the extra vegetation affect the carbon cycle? That is, the natural movement of carbon from soil to animals to the atmosphere.
For the scientists, the really worrying bit is the fact that there's twice as much carbon in permafrost as there is in the atmosphere. "That's a lot of carbon that has been sitting there for thousands of years, kind of locked up in ice," says Kerby. "And as that permafrost starts to thaw, microbes can start digesting all of the dead leaves and dead animals." The greening of the Arctic could already be exacerbating this thaw.
It might seem weird for humans to be rooting against plants. But sometimes greener pastures aren't a good thing.