Tropical forests remove and absorb carbon from the atmosphere as they grow, and researchers estimate that despite current deforestation levels, they still hold more carbon than civilization has generated by burning coal, oil and natural gas over the past 30 years. But as trees stressed by heat and drought due to global warming die and release their carbon, their ability to act as reservoirs will diminish. A global team of more than 200 researchers led by tropical ecologist Oliver Phillips, of Leeds University, measured more than half a million trees in 813 forests in 24 countries to calculate how much carbon the different forests now store, based on the height, diameter and species of each tree. Their research, published in Science, also looked at how carbon storage varied from place to place using data from 590 long-term monitoring plots. If warming reaches 2° C above preindustrial levels, the study found huge swaths of the world’s tropical forests will begin to lose more carbon than they accumulate. Already, the hottest forests in South America have reached that point.