In the Greenhouse: Forests Get More Water Efficient as Carbon Dioxide Levels Rise

Nature may have its own way of adapting to climate change, as a new study shows that forests get more economic with water as greenhouse gases levels increase

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Forests may benefit from higher levels of CO2—at least for awhile

The response to climate change has two sides: mitigation and adaptation. Mitigation means reducing greenhouse gases in an effort to slow the pace of warming and lessen its effects. Adaptation means responding to those effects, working to blunt climate change as it unfolds. It’s offense versus defense.

Mitigation gets most of the attention because it mostly involves changing the way we use energy, something that we spend trillions of dollars on. Such shifts can have a huge impact on the economy—for better and for worse—which is why the political battle over climate policy tends to be so heated. But as extreme weather events like Sandy have given us coming attractions of what life in a warmer world could well be like—insert caveat that you can’t directly tie any single weather event to warming—adaptation has moved up the political agenda. This is as it should be—whatever the impacts of pumping billions and billions of tons of carbon into the atmosphere turn out to be, we know for sure that a more crowded and richer world is one that will be increasingly vulnerable to all sorts of natural shocks: droughts, floods, storms. It makes sense to prepare.

But human beings aren’t the only species that will need to adjust to a changing planet. Plants and wildlife will have to adapt as well—adapt or die. And that’s what makes a new paper in Nature on how forests have been responding to increased levels of carbon dioxide so interesting.

Plants absorb CO2 to fuel photosynthesis, opening their stomata to admit the gas, and every time they do they lose water through evaporation. As CO2 levels rise, forests are able to take in more of the gas and simultaneously reduce the amount of water they need to use to do so, because the stomata don’t need to open as wide to take in CO2. Computer climate models have suggested that as CO2 increases and the world warms, water-use efficiency and plant growth should improve—at least to a point—as an unintentional side benefit of climate change.

The American and German researchers who worked on the Nature study wanted to test out those models in the real world. Using data collected from forests in the northeastern U.S., they found that as carbon concentrations increased by about 5% per decade over the past 20 years, the rates of water-use efficiency increased by about 3% a year. That’s much faster than computer models would have suggested—it means the improvement in water-use efficiency is about six times as large as the corresponding increase in carbon concentrations. As Trevor Keenan of Harvard University, a lead author on the paper, put it in a statement:

This could be considered a beneficial effect of increased atmospheric carbon dioxide. What’s surprising is we didn’t expect the effect to be this big. A large proportion of the ecosystems in the world are limited by water–they don’t have enough water during the year to reach their maximum potential growth. If they become more efficient at using water, they should be able to take more carbon out of the atmosphere due to higher growth rates.

We’re hardly off the hook for global warming—as temperatures climb thanks to climate change, crippling heat waves will likely overwhelm the beneficial effects of more carbon dioxide in the air. But the Nature study shows that even as human beings belatedly begin to prepare for a hotter and more violent planet—and make efforts to mitigate the worst of the damage—nature may have its own adaptation plan.