Climate change could expand forests. But will they cool the planet?

All around the Arctic Circle, trees are invading as the climate warms. In Norway, birch and pine are marching poleward, eclipsing the tundra. In Alaska, spruce are taking over from moss and lichen. Globally, recent research indicates forests are expanding along two-thirds of Earth’s 12,000-kilometer-long northern tree line—the point where forests give way to tundra—while receding along just 1%.

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