科学美国人60秒:Forests Suck Up Less Carbon after Drought

Climate scientists forecast sea levels to rise anywhere from one to four feet by the end of the century. That's a pretty big range. And there's a good reason for that: there's a lot of uncertainty baked into climate models. 

Take, for example, the way climate models predict how trees respond to drought. "Drought in these models is treated as a light switch"—either on or off—“but in the real world we know that drought damages trees, and it can take a while for trees to repair this damage and recover." 

William Anderegg, an ecologist at Princeton University. He and his colleagues examined tree ring data from more than 1,300 sites around the world. And by comparing the rings with known drought records they found that trees don't simply kick back into gear as soon as rains return. Drought actually puts the trees' water transport systems under a huge amount of tension, he says, causing air bubbles to leak in, which damages or blocks those pipes. "I often compare this to a sort of a heart attack for a tree. That in some cases it can be lethal and in some cases they can repair that blockage." 

That drought 'hangover' causes tree growth to lag five to ten percent below normal for several years following the dry spell. "This is a problem because forests currently take up about 25 percent of human emissions of CO2, which is an incredible break on climate change.” And the less CO2 the trees are able to take up—the warmer it gets. The findings appears in the journal Science. [W. R. L. Anderegg et al, Pervasive drought legacies in forest ecosystems and their implications for carbon cycle models]

The thing this study makes clear, is that predicting climate change… is hard. "It's really hard. These models have an incredibly challenging task of representing processes that occur from a leaf scale to a continent scale in space. And from several seconds to hundreds of years or at least a hundred years in time." But maybe a better understanding of how much carbon trees soak up—and how much they don't—will make climate forecasting just a little bit easier.

—Christopher Intagliata