Forests Affected by Drought Store Less CO2 Than Assumed in Climate Models: Study

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Drought stressed forests, new research finds, store less carbon dioxide than predicted in climate models. Leander Anderegg

When scientists predict the future effects of climate change, they often rely on virtual models that assume forests will absorb massive amounts of CO2 from the atmosphere. A new study from researchers at the University of Utah and Princeton University has found that droughts will cause trees to absorb far less human-created CO2 than predicted in climate models.  

“Forests are more sensitive to drought than previously thought,” says study author William R.L. Anderegg, a biology professor at the University of Utah. “Drought also has a longer lasting effect than we previously thought.” The research analyzed 1,300 forests across the northern hemisphere, and found that on average, trees take two to four years to recover from droughts.

IMG_0555_lowerres Drought-stressed forests can be seen in mountainous regions of the western United States, where semi-arid conditions create a particular risk of drought. Leander Anderegg

Anderegg says that’s bad news for the planet. “The future of Earth’s forests is highly uncertain,” he says. “This is crucial to us because forests currently take up about a quarter of human CO2 emissions each year.”

Droughts are expected to increase in length and number as global warming worsens. They cause trees to die, but Anderegg’s research has found for the first time that they also cause trees to store less CO2 in their wood. Fewer trees with less carbon capacity would lead to more CO2 in the atmosphere, compounding the greenhouse effect. Drought is thus both cause and effect when it comes to global warming. The circularity of this problem is what climate scientists call “feedback,” Anderegg says.

“In most of our current models of ecosystems and climate, drought effects on forests switch on and off like a light,” he says in the study's press release. “When drought conditions go away, the models assume a forest's recovery is complete and close to immediate. That's not how the real world works.”

0730_drought_forest_01 Higher temperatures can kill off trees by contributing to disease, as seen in this pine forest in the western United States. Leander Anderegg

While the rates of recovery for individual tree species are not fully known, there is little doubt that drought poses a long-term threat to the survival of forests. Even tropical forests in the Amazon, Anderegg says, can take years to recover from severe droughts. The new findings are even more alarming for semiarid forested regions, such as the Western and Southwestern United States.

The new study also comes in the midst of growing national concern about the threat of climate change. On Thursday morning, the White House retweeted a video in which President Obama warned that climate change has the capacity to lead to political and social destabilization in many volatile regions.

Climate change is also rising to the level of a strategic security concern. The Pentagon, according to an official report submitted to Congress on July 23, now considers it a legitimate national security risk.

Anderegg says that saving our forests will take a combination of national and international efforts. At the local level, he says, forests need to be properly managed to reduce stress factors. This could include preventing overdensity (which contributes to catastrophic forest fires) and supporting the planting and growth of drought-resistant species. These are tried-and true techniques, but will require support from elected leaders, and funding from government agencies. The good news is that there’s still hope.

“As far as we can tell, we’ve haven’t crossed any tipping points,” he says. “We do control a lot of the fate of forests, with our decisions about climate change. The sooner we address climate change, the less risk that forests enter the vicious cycle.”