The most widely used assumptions in climate forecasting may be missing a key driver of how ecosystems absorb carbon dioxide, with implications for predicting how much natural vegetation can slow global warming, scientists warned.
Plants have absorbed more carbon dioxide in recent decades, but not mainly because they adapted photosynthesis to higher temperatures, as scientists long expected.
An Israeli scientist found that climate models may need to place greater emphasis on how plants manage water use and expand their leaf cover, rather than relying primarily on temperature responses to estimate how much carbon dioxide ecosystems can remove from the atmosphere.
“We found that there is an increase in absorption of carbon dioxide, but not because the plants adapted themselves and raised the temperature, but because of other factors,” lead researcher Professor José M. Grünzweig of the Institute of Plant Sciences and Genetics in Agriculture at the Hebrew University of Jerusalem reported. He led a team of researchers from China, Switzerland, Italy, Germany, Australia, Finland and Russia. The findings were recently published in the peer-reviewed journal One Earth.
“This is important because all the global models that examine how much carbon dioxide plants around the world absorb, and how that then affects the climate, need to know how the plants are doing it,” Grünzweig added.
Plants absorb roughly a quarter of the carbon dioxide released by human activity each year, making them one of the planet’s most important natural buffers against global warming and climate change. Understanding what allows that carbon uptake to continue as the world warms is central to forecasting future climate conditions, Grünzweig said.
For years, researchers assumed that plants would keep pace with warming by shifting the temperature at which photosynthesis works best. If that were the main mechanism, the optimal temperature for photosynthesis would be expected to rise alongside global temperatures.
But the study found that this explanation accounts for only a small share of the increase in carbon uptake observed over the past two decades.
To examine the issue, the researchers analyzed global ground-based carbon measurements together with satellite observations from 2000 to 2019.
They found that ecosystems increased their maximum photosynthetic carbon uptake during that period, but the temperature at which photosynthesis performed best changed little, especially in dry and cold regions.
According to the study, rising optimal photosynthesis temperature accounted for less than 20% of the global increase in maximum carbon uptake.
Instead, the main drivers were greater canopy growth and improved water-use efficiency. Plants absorbed more carbon dioxide per unit of water used, while expanded leaf cover allowed ecosystems to take in more carbon overall.
Grünzweig said the findings could help improve climate models, particularly as rising temperatures and more frequent droughts make it harder to predict how land ecosystems will behave.
Moreover, many carbon-offset projects assume forests and ecosystems will continue absorbing carbon at certain rates. But if water availability turns out to be a major limiting factor, project developers may need to account more carefully for drought risk, long-term water availability and vegetation health.
“We know that plants are perhaps still the most effective factor in reducing carbon dioxide in the atmosphere, and therefore in reducing global warming and climate change. The question is to what extent plants can continue doing this,” Grünzweig said.
“This does not spare us the urgent need to stop emitting so much carbon dioxide into the atmosphere. But it can help. We showed that plants will be able to continue helping because they have different mechanisms for doing so.”
