Forests eat a lot of carbon dioxide — roughly a quarter of humanity's total yearly output. So when it comes to the future trajectory of climate change, the behavior of forests is one of the largest potential variables. If forests grow and thrive as the concentration of atmospheric carbon dioxide increases (thus providing them more potential food), then climate change could be strongly mitigated. On the other hand, if changing weather patterns fueled by global warming kill them off instead, they could release much of the carbon contained in their wood, as they die and rot.