Tropical Forests Soak Up Much More Carbon than We Thought

Another reason to be grateful for Earth's tropical forests: Not only do they r​elease massive quantities of oxygen, creating a pleasantly breathable atmosphere, not only do they harbor over half​ the planet's biodiversity, they're also doing a bang-up job mopping up all that extra carbon we've been pouring into the atmosphere.

That's the conclusion of a new​ study led by researchers at NASA and the National Center for Atmospheric Research, which finds tropical forests may be absorbing far more human-emitted carbon dioxide than we thought. To wit, some 1.4 billion metric tons of CO2 annually—roughly the same amount of carbon that's emitted every year as we slash and burn our way through them.

"This is good news, because uptake in northern forests may already be slowing, while tropical forests may continue to take up carbon for many years," said lead study author David Schimel in a press re​lease.

By pumping carbon into the atmosphere, we're not just warming the planet directly, we're setting in motion a number of different "feedback" cycles. Some of these feedbacks—methane release due to permafrost melting, for instance—accelerate climate change. But our CO2 emissions also stimulate plants to grow and suck down more carbon, a negative feedback known as the "fertilization" effect.

The CO2 fertilization effect has been known for decades, but actual data on the effect is spotty, and comes from a range of sources that aren't necessarily comparable: ecosystem and atmospheric models, satellite images, experimental plots and so forth. And while in theory, the effect should be greater in warmer climates—plant growth depends on temperature as well as CO2—most atmospheric models have observed stronger CO2 fertilization at high latitudes.

The present study is the first to devise a way to make apples-to-apples comparisons of all data sources, from the leaf scale to the global scale, and to come up with a robust global estimate. The authors find that altogether, Earth's forests remove some 30 percent of our annual CO2 emissions via photosynthesis, and that the tropics are soaking up the lion's share.

"What we've had up till this paper was a theory of carbon dioxide fertilization based on phenomena at the microscopic scale and observations at the global scale that appeared to contradict those phenomena," Schimel said. "Here, at least, is a hypothesis that provides a consistent explanation that includes both how we know photosynthesis works and what's happening at the planetary scale."

The authors warn, however, that while the CO2 fertilization effect dominates over positive climate feedbacks right now, in the future, accelerated ecosystem carbon losses due to droughts—which will slow forest growth and increase the frequency of wildfires—are likely to come out on top.

And, of course, this sink will only continue to work in our favor so long as we protect our planet's tropical forests from decimation. For now, we can thank our forests for helping buffer us against the consequences of our fossil fuel appetite, but at the end of the day, we've still got to get our act together.