Increased Arctic Shrubbery May Destroy Permafrost
It’s hard to be 100% certain of the exact consequences of climate change. The cascade effect, where changes in one system affect others, which affect more in turn, makes it very difficult to predict the future of Earth’s complexly inter-related systems without a lot of real-life data.
Case in point: last year, an article was published in Global Change Biology, a scientific journal, about the effects of increased shrubbery on the Arctic tundra. The tundra is a desert-like ecosystem in the far North. Below a certain depth, the ground is frozen and never, even during the brief summers, thaws. This is the permafrost. In the thin layer of ground that lies above the permafrost layer, tough grasses grow and not much else.
At least that’s how it was 20 years ago, when I took elementary school geography. But the Arctic is changing more rapidly due to climate change than most parts of the world. There’s been a significant increase in the amount of shrubbery in the tundra, and what this will mean ecologically and geographically is not yet clear.
The writers of this original paper (which I’m afraid I can’t find online) did some controlled field studies, measuring temperatures of exposed ground versus shaded ground under shrubbery. They determined that the shrubbery might actually help to keep the ground, more particularly the permafrost, cool, by shading it from the sun. Thus the shrubbery that has been springing up in the wake of warmer Arctic temperatures may actually protect the permafrost from melting, at least for a little while.
But a new paper released in the latest volume of Environmental Review Letters blows that prediction out of the water. What the last study failed to take into account were the effects of albedo and transpiration from the new shrubbery. Albedo is just a measure of how good something is at reflecting away the sun’s rays and, therefore, heat. The almost pure-white Arctic tundra normally has a very high albedo. The highly reflective surface area is one reason the Inuit long ago invented slitted glasses to protect their eyes when out on the tundra on a bright day.
The sparse grasses don’t make much of a difference, but the darker, thicker shrubbery has an obvious effect, absorbing much more heat from the rays of the sun, and releasing this heat into the ground and air.
Additionally, transpiration, the absorption and release of moisture which is part of all plants’ metabolisms, is greatly increased with large, leafy shrubbery. A potential increase in humidity may also have an effect on the ability of the ground to cool itself through evaporation. The earlier paper didn’t take either of these effects, albedo or transpiration, into account.
All of which means that shrubbery, according to the more detailed and thorough study just published, may be another of those positive feedback loops, accelerating warming at the Arctic rather than slowing it. As a result, the melting of the permafrost may occur sooner than previously expected.
The tundra being a fairly precarious ecosystem to begin with, a major change like the melting of the permafrost might have any number of unforeseen effects. We’ll have to wait and see exactly what they are.
Photo credit: Dentren