Forests Can’t Store Carbon If They’re Overheated

WORDS BY Yessenia Funes

A new study shows that our world’s carbon sinks may become carbon sources in only a few decades as rising temperatures threaten the ability of biomes to effectively sequester carbon through photosynthesis.

Photograph by Lino Mirgeler/picture alliance via Getty Images

As the planet warms, humans aren’t the only ones struggling with the heat. Every biological organism has an optimal temperature at which they function. That’s true for us, for polar bears, for plants, for all living things. In fact, global temperature rise is set to impact flora—and entire biomes—quite dramatically if we don’t get our collective shit together.


A new study, published in Science Advances Wednesday, finds that major carbon sinks may become carbon sources by 2050 if we don’t act fast enough to reduce greenhouse gas emissions. For those who may not be familiar, certain types of ecosystems—like tropical rainforests, vast grasslands, swampy wetlands—can suck carbon out of the air through photosynthesis. However, their ability to do this dwindles as temperatures soar.


Welcome to The Frontline, where scientists are breaking down the implications of this new research. I’m Yessenia Funes, climate editor of Atmos. This new study has potentially damning implications for humans. Earth risks losing 45 percent of its global land sink strength by midcentury, per the research paper. We need every carbon sink available—and to create even more by protecting our ecosystems—if we’re going to actually limit global warming to 1.5 degrees Celsius above pre-industrial levels, which is what the Paris Agreement suggests.






In 2003, Europe suffered a deadly heatwave. At least 30,000 people died. Turns out that this event also reversed the carbon sequestration of the region’s temperate ecosystems. When we talk about the loss of carbon sinks, we’re often talking about the destruction of essential ecosystems that store carbon. For instance, when the Amazon burned in 2019, headline after headline referred to the rainforest as Earth’s “lungs.” This name derives from its carbon sequestration ability: the sucking of carbon and release of oxygen. The Amazon stores some 200 billion tons of carbon.


However, deforestation and habitat loss aren’t the only threat to these critical ecosystems. As this new study highlights, temperature rise alone may be enough to potentially lose almost half of Earth’s ability to filter out all the carbon we emit. What’s more, this can happen in 30 years, according to the authors. The rainforests of South Asia and the Taiga forests of Russia and Canada may be among the first to see this change.The paper doesn’t account for deforestation or other direct elimination of carbon sinks, either, so the reality may be much worse.


“Based on the results, the study can be considered an early warning of major biological change,” says Ann Bartuska, a senior advisor at Resources for the Future, a nonprofit that examines environmental concerns, who wasn’t involved in the research. She calls the study “credible,” noting the important foundation it’s laid out for other scientists to build upon through its methodology and findings.


To assess the future of global carbon sinks, the researchers took more than two decades’ worth of data from FLUXNET, a global carbon monitoring network of towers that measure the exchange of carbon dioxide between ecosystems and the atmosphere, as well as other meteorological data such as air temperature and water exchange. The team then used statistical methods used in thermodynamics to calculate how this carbon exchange between ecosystems and the atmosphere shifts relative to temperature. This second part of the methodology is novel, says study author Katharyn Duffy, an earth systems scientist at Northern Arizona University’s School of Informatics, Computing, and Cyber Systems. It’s what allowed them to come up with temperature peaks for plants.


“We are really looking at the metabolism of the biosphere,” Duffy says. “That’s why I like to use the human body analogy. We have a natural base temperature, but when we get a fever and our body goes over, it starts to break down. Our body doesn’t work optimally anymore. That’s how you can think about this temperature versus photosynthesis link. There are temperatures where photosynthesis can operate maximally right in this little sweet spot, but if you warm it too much, processes start to break down within these plants.”


Because these methods rely on data from temperatures biomes have actually experienced, the study can only offer us estimates on how global warming will affect carbon sequestration abilities. These observations are also of vast ecosystems rather than a plant in an experimental setting, which may offer a different picture of these impacts.

“To ensure that natural climate investments are truly cost effective, we need to simultaneously and aggressively reduce fossil fuel-related emissions.”

Emily McGlynn
University of California at Davis

While this study certainly helps researchers better understand the impact warming has on photosynthesis, Stanford University earth system science professor Rob Jackson (who was not involved in the study) would’ve liked to see an exploration of specific biomes given the variation in heat tolerance. As the study found, however, even ecosystems most tolerant to heat have temperature limits that the climate crisis will challenge. While more heat-tolerant plants have a threshold of 28 degrees Celsius (82.4 degrees Fahrenheit), other plants begin to suffer, on average, at temperatures higher than 18 degrees Celsius (64.4 degrees Fahrenheit).


This study looks at our current emissions rate (Representative Concentration Pathway 8.5) where business as usual continues. Should leaders take another route—say RCP 2.6, where we finally take appropriate action—then we may be able to prevent about half of this damage, according to Duffy. Much of it is already baked in. Regardless, these findings show just how flawed our economic response to the climate crisis has been, says Emily McGlynn, a resource economist at the University of California at Davis and former White House Council of Environmental Quality deputy associate director for energy and climate change.


“This kind of analysis makes clear that focusing solely on ‘cheap’ emissions reduction opportunities—like afforestation, soil carbon sequestration, and other natural climate solutions—would be equivalent to flushing money down the drain,” McGlynn tells me via email. “That doesn’t mean that they are not worth pursuing, but to ensure that natural climate investments are truly cost effective, we need to simultaneously and aggressively reduce fossil fuel-related emissions.”


Protecting vital ecosystems isn’t enough on its own. We also need to dramatically reduce our consumption of fossil fuels. And these biomes aren’t only storing our carbon; they provide crops and timber to nearby communities, too. Reduced photosynthesis results in reduced productivity, which could lead to loss of plant cover and other ecological benefits these biomes provide, such as wildlife habitat and watershed protection.


And, of course, should these carbon sinks become carbon sources, irreversible feedback loops can result, as well, throwing us into a climate emergency like we cannot imagine. If things get that bad, Inuit communities in the Arctic will suffer first. As will the Indigenous peoples who call forests like the Amazon home.


This devastation will not be felt equally. That we know for certain.

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