Changing nature of Arctic tundra: Carbon sink to source

For thousands of years, the shrubs, sedges, mosses, and lichens of the Arctic have performed a vital task for the planet: gulping down carbon dioxide from the air and storing the carbon in their tissues.

- RAYMOND ZHONG

When the plants die, this carbon is entombed in the frigid soil, where it no longer helps warm Earth's surface.

But as fossil fuel emissions heat the planet, balmier air temperatures are thawing Arctic tundra, activating carbon-hungry microbes, and more vegetation is being burned up by wildfires.

The result, for the past two decades or so, is that the tundra has been adding more carbon dioxide to the atmosphere than it has removed, a reversal from the usual state of affairs since the peak of the last ice age.