Earth’s terrestrial ecosystems will soon transition from climate-stabilizing carbon sinks to sources of greenhouse gases that will accelerate climate change. At the current rate of global warming, this tipping point could be reached within 20 to 30 years, according to an alarming new study led by Northern Arizona University.
Over the past few decades, the biosphere has absorbed more carbon than it has released, which prevents one-third of human-caused carbon emissions from entering the atmosphere.
The team has identified a temperature threshold beyond which plants will release more carbon than they absorb. The researchers noticed sharp declines in photosynthesis above this temperature threshold in nearly every biome across the globe, even after removing other factors such as water and sunlight.
“The Earth has a steadily growing fever, and much like the human body, we know every biological process has a range of temperatures at which it performs optimally, and ones above which function deteriorates,” said study lead author Katharyn Duffy. “So, we wanted to ask, how much can plants withstand?”
Based on data from the world’s largest continuous carbon monitoring network, the experts determined that up to half of land ecosystems will make the transition from carbon sinks to carbon sources by the end of the century.
However, the study also predicts that ecosystems which store the most carbon, such as rainforests and Taiga forests, could lose more than 45 percent of their carbon sink capabilities by the middle of the century.
The research is the first of its kind to establish a temperature threshold for photosynthesis based on observational data at a global scale. “We know that the temperature optima for humans lie around 37 degrees Celsius (98 degrees Fahrenheit), but we in the scientific community didn’t know what those optima were for the terrestrial biosphere,” explained Duffy.
To investigate, the scientists developed a new approach called the MacroMolecular Rate Theory (MMRT), which is based on the principles of thermodynamics. This method allowed the researchers to generate temperature curves for every major biome in the world.
“Different types of plants vary in the details of their temperature responses, but all show declines in photosynthesis when it gets too warm,” said study co-author George Koch.
Currently, less than 10 percent of all terrestrial ecosystems experience temperatures beyond the photosynthetic maximum. At the current rate of emissions, however, up to half the terrestrial biosphere could experience temperatures beyond that productivity threshold by mid-century.
Some of the most carbon-rich biomes in the world, including the Amazon, will be among the first to reach the tipping point.
“The most striking thing our analysis showed is that the temperature optima for photosynthesis in all ecosystems were so low,” said study co-author Vic Arcus.
“Combined with the increased rate of ecosystem respiration across the temperatures we observed, our findings suggest that any temperature increase above 18 degrees C is potentially detrimental to the terrestrial carbon sink.”
“Without curbing warming to remain at or below the levels established in the Paris Climate Accord, the land carbon sink will not continue to offset our emissions and buy us time.”
The study is published in the journal Science Advances.