The world’s tropical rainforests will be able to absorb “high levels” of CO2 even as the planet warms, provided that global temperatures do not rise more than 2°C from pre-industrial levels and that these verdant ecosystems are not further fragmented by deforestation, predicts a study just published in Science magazine.

But that 2°C threshold is not to be trifled with, writes Carbon Brief. Go beyond it, and the rainforests will rapidly lose their capacity to store carbon.

Involving an international team of more than 200 researchers measuring carbon stocks in more than half a million trees in forests across South America, Africa, Asia, and Australia, the study sought to better understand how climatic drivers like temperature and precipitation affect the ability of rainforests to store carbon over the long term—and how such drivers interact with each other to alter those affects. Such a wide geographic spread helped the researchers see how climate change might affect tropical forest carbon storage “on the scale of hundreds of years,” said lead author Martin Sullivan, a lecturer in statistical ecology at Manchester Metropolitan University.

“Even with decades of monitoring, we can only just start to see changes in carbon storage emerging. This is because trees are really long-lived,” he told Carbon Brief. In lieu of being able to see into the far, far past, the researchers decided to use “variation in space as a proxy for time.”

Using wide spatial analysis of hundreds of plots around the world, the researchers determined that “the most influential negative factor on forest carbon stocks in the long term is increases in maximum daily temperature.” 

In its own coverage of the study, Science noted that the finding departs from earlier research that tied long-term carbon storage capacity to how cold a forest could become at night. The new conclusion may be registering the fact that, “on hot days, trees slow their carbon dioxide intake to reduce water loss.”

Carbon Brief notes a second finding of the Science study, namely that a phenomenon known as the CO2 fertilization effect—a feedback loop in which CO2-dependent photosynthesis is accelerated by higher concentrations of atmospheric CO2—“will partially or wholly offset the effect of this temperature increase” on the capacity of tropical rainforests to store carbon—but, critically, this will only happen if the ecosystems remain intact.

“Trees that can cope with these new warmer climates will need to be able to move to new areas. As we’re fragmenting forests, we’re impeding that movement,” said Sullivan. To allow the trees to do their work, humans will have to put an end to rampant tropical deforestation.

Still, some are urging caution about the findings. “There are several reasons why this CO2 effect may not be as strong as models expect in the future,” said Anja Ramming, a land-surface interaction expert with the Technical University of Munich. For example, if forest soils are depleted of nutrients vital to the photosynthesis process, like phosphorus, the carbon losses could be larger, she told Carbon Brief. 

Lara Kueppers, an ecosystem ecologist at the University of California, Berkeley, told Science magazine she was concerned the study was “too optimistic in forecasting that cooler forests, especially in Asia and Africa, will continue to accumulate large quantities of carbon as they warm.”

Despite the need for further investigation, the study is “a wake-up call to action,” writes Science. “Even though tropical forest sinks will weaken, conserving them is still better than not having them at all and turning them into carbon sources,” said Richard Betts, a global carbon cycle modeller at the University of Exeter. “It is not too late to avoid the most severe impacts.”