In reality, the cycling of carbon, energy and water between the land and the atmosphere is much more complex. Considering all the interactions, large-scale increases in forest cover can actually make global warming worse.
Of course, this is counterintuitive. We all learn in school how trees effortlessly perform the marvel of photosynthesis: They take up carbon dioxide from the air and make oxygen. This process provides us with life, food, water, shelter, fiber and soil. The earth’s forests generously mop up about a quarter of the world’s fossil-fuel carbon emissions every year.
So it’s understandable that we’d expect trees to save us from rising temperatures, but climate science tells a different story. Besides the amount of greenhouse gases in the air, another important switch on the planetary thermostat is how much of the sun’s energy is taken up by the earth’s surface, compared to how much is reflected back to space. The dark color of trees means that they absorb more of the sun’s energy and raise the planet’s surface temperature.