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Nearing the Climate Cliff: Study Warns Plants Are Decades Away From Absorbing Less Carbon

Forest land on Martha's Vineyard is perforated by houses.
David Foster
Harvard Forest

Plants absorb one-third of the planet’s greenhouse gas emissions, but if temperatures keep rising in the next 20 years, plants could become 50 percent less effective at absorbing carbon, according to a new study co-authored by a Woods Hole scientist. 


The study identifies a temperature tipping point in the coming decades at which plants' ability to absorb carbon would begin to significantly decrease, which in turn would trigger a rapid increase in global warming. 


“The main thing that comes out of this study is that the speed with which we will reach dangerous levels of climate change is most likely an underestimate,” said study co-author Christopher Schwalm, a senior scientist at Woodwell Climate Research Center. 


The study, published this week in the journal Science Advances, began with scientists asking how well plants will be able to absorb carbon as temperatures rise.


Answering that question required a closer look at two key processes that plants undergo: photosynthesis, which is described as “climate positive” because plants absorb carbon from the atmosphere during the process, and cellular respiration, described as “climate negative” because it releases carbon into the atmosphere. 


The team of scientists discovered that after temperatures exceed 64.4 degrees Fahrenheit, plants in most ecosystems photosynthesize less, meaning they absorb less carbon, while rates of respiration actually increase, meaning they release more into the atmosphere. 


“As things continue to warm, the photosynthesis will be overwhelmed by the amount of respiration,” Schwalm said. “The net balance of vegetation will shift from being climate positive —the slowing effect— to climate negative, which means it’ll accelerate climate change.”  


The big finding: if humans continue using fossil fuels at current rates, plants’ ability to absorb carbon dioxide will be reduced by almost 50 percent as soon as 2040. 


Schwalm explained what that would mean for emissions. 


“So [imagine] you have 10 units of emissions. Five stay in the atmosphere, 2.5 are absorbed by forests, 2.5 are absorbed by oceans. … This notion of by 2040 the land sink strength being halved means that … 6.25 units of carbon will stay in the atmosphere,” he said. 


Ultimately, Schwalm said, the takeaway is that countries, corporations, and individuals will have to work together to reduce greenhouse gas emissions as quickly as possible. 


“What this implies is that the carbon budget is significantly smaller than we thought it was,” he said. “We don’t have as much time as we thought we had.”  

Eve Zuckoff covers the environment and human impacts of climate change for CAI.