Drought Is Making Climate Change Even Worse

Bill Anderegg grew up camping and fishing in the Rocky Mountains, but when he returned to the forests of his youth as an adult he discovered something heart-wrenching. Even though the area was not experiencing drought during his visit, many of the once vibrant trees were still brown, dying, or dead.

“I was blown away by how much drought had affected these trees during my lifetime,” he said.

Anderegg, a postdoctoral fellow at Princeton University, and a team of researchers set out to figure out what was going on. Their research, published Thursday in the journal Science, found that drought affects trees long after dry periods have ended. On average, they found, it takes trees two to four years to recover from drought and resume previous growth rates. Some trees take longer—as much as seven years.

Trees, according to the paper, play an important role in the global carbon cycle by absorbing carbon dioxide and “fixing” it inside their long-lived trunks. If trees grow more slowly, they absorb less carbon. Meanwhile, they put more effort into short-term leaves and roots, which then decay and release their carbon content faster than woody trunks. Current climate-change models count on trees absorbing and fixing carbon, so if forests can’t do that, climate change could accelerate more quickly than previously predicted.

Climate-change models have already estimated that droughts will become more frequent and more severe. That presents an uncertain future for the trees we all depend on. “Our forests around the globe are vulnerable to drought, and we need to address climate change now to protect them,” Anderegg said.

In addition to the carbon angle, trees that experience drought stress become more vulnerable to diseases and insect infestations, as well as to fires. All of those situations could further increase the death rate of the world’s forests.

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Anderegg said it was known that drought damage is hard for trees to repair, but the scientists were not prepared for what their research uncovered. “I was surprised by how widespread and pervasive these ‘legacy’ effects were,” he said.

The researchers looked at tree-ring data from more than 1,300 forest sites around the world. The long-term effects of drought, they found, varied by region and tree species. For instance, they found that the trees in the American Southwest experienced some of the worst long-term damage from droughts. Data from California, meanwhile, suggested that the trees there were more likely to recover, although Anderegg thinks that could be a statistical anomaly, as drought and fires may have killed off many younger trees, leaving only the large trees left for scientists to examine.

Experts agreed that the paper presents an important new idea that bears further study.

“In areas where droughts are expected to become longer and more severe, such as we may already be seeing in the American West, this paper suggests that forests in these places might not be the best places to try to store extra carbon,” said Jens Stevens, a forest ecologist with the John Muir Institute of the Environment at the University of California, Davis, who was not affiliated with the study. “To me, this really highlights the importance of distinguishing between different forest types as we try to manage for ecosystem resilience in the future.”

Although the research provided scary conclusions, Anderegg said he is in a state of both worry and optimism. “The future of forests lies in our hands,” he said. “That’s an opportunity. The next decade matters.”