For example, look at Europe where the study was focused. Once upon a time, its forests were largely leafy deciduous trees. Now, a majority of Europe's trees are managed by humans, and we've been planting trees like pines and spruces because they grow faster.
But by replacing older forests with ones that are newer and faster-growing, Europe has gone into "carbon debt." Harvesting those older trees and replacing them with conifers has released 3.1 billion metric tons of carbon.
Researchers say this change caused a temperature increase equal to 6 percent of warming attributed to fossil fuels. Which may not sound like much, but small changes in temperature can ripple out to larger changes in the environment.
It's not just about the carbon that's released, though. Conifer trees are darker and absorb more solar radiation. When less of that radiation is reflected into space, the planet can heat up.
Some countries are already planting trees to help combat climate change. China, for instance, has been planting a "green wall" in the Gobi Desert, a project that would eventually include over a million square miles of trees.