A new international study is challenging a long-held belief about how ecosystems continue absorbing carbon under global warming. Scientists found that carbon uptake increased during recent decades, but not primarily due to plants adapting to higher temperatures, as previously thought. Instead, the increase is largely driven by plants using water more efficiently while at the same time developing larger tree and crop canopies giving plants more leaf area to absorb light and fix carbon. These factors appear to matter much more than temperature adaptation to determine how much carbon dioxide ecosystems can absorb in a warming world.

An international team of scientists has discovered that plants are not responding to global warming in the way researchers long assumed. Scientists have expected that ecosystems would keep pace with warming by rising the temperature at which photosynthesis works best. A new study published in the Cell Press journal One Earth is challenging that theory. The study reveals that land ecosystems absorbed more carbon dioxide in recent decades, but that the increase in carbon uptake was caused primarily by improved water use and greater canopies, and less by increasing the temperature optimum for photosynthesis. The findings mark a fundamental shift in how researchers view the carbon cycle. They suggest that future climate models must look beyond temperature alone to forecast how much carbon dioxide ecosystems can remove from the atmosphere to reduce climate change.

The research was led by Prof. José M. Grünzweig and Dr. Chongyang Xu of the Robert H. Smith Institute of Plant Sciences and Genetics in Agriculture at the Hebrew University of Jerusalem, in collaboration with Prof. Hongyan Liu of Peking University and Prof. Dan Yakir of the Weizmann Institute of Science and other colleagues.

Land ecosystems are one of Earth's most important natural defenses against climate change, absorbing roughly a quarter of the carbon dioxide released by human activities each year. By removing carbon dioxide from the atmosphere, plants help slow global warming. Understanding what controls this natural carbon storage and how it continues in a warming world is therefore essential for predicting future atmospheric carbon dioxide levels and the pace of climate change.

For years, scientists believed that persisting increase in carbon uptake by ecosystems at the same time as the globe is warming would be possible only when plants adapt to rising temperatures by shifting the temperature at which photosynthesis works best. If this were the main way plants responded to climate change, the optimal temperature of photosynthesis would be expected to rise as the planet warms.

To investigate the ecosystem carbon uptake and the optimal temperature, the researchers combined two decades of global observations from ground-based carbon measurements and satellite observations between 2000 and 2019.

What the researchers found was a striking mismatch. While ecosystems around the world increased their maximum rates of carbon uptake by photosynthesis over the past two decades, the temperature at which photosynthesis works best remained largely unchanged, particularly in arid and cold regions.

The study found that the rise in the optimal temperature of photosynthesis accounted for less than 20% of the global increase in maximum carbon uptake. Instead, plants grew more leaves and used water more efficiently by absorbing more carbon dioxide for every drop of water they spent. The higher efficiency of water use was the dominant factor even in humid environments, such as tropical, temperate and cold regions. The researchers also found that drier air and soils further reduced the importance of temperature adaptation, highlighting canopy growth as a key factor shaping how ecosystems respond to climate change.

"These findings make it clear that we cannot predict future carbon storage by looking at temperature responses alone," said Prof. Grünzweig. “Our study shows that water plays a much bigger role than previously recognized. Understanding how plants use water and build their canopy will be critical for predicting the future of Earth's natural carbon dioxide absorption.”

In addition, understanding why arid regions continued to increase their carbon uptake under global warming is critical for evaluating the global carbon storage potential. “Our findings were surprising,” Dr. Xu said. “Although arid regions showed little evidence of adapting to rising temperatures, their carbon uptake continued to increase over the past two decades. We found that this increase was mainly due to canopy expansion, with ecological restoration programs contributing substantially in some areas.”

As temperatures rise and droughts become more common, the study points to a new direction for climate science: understanding how plants manage water and expand their canopies will be critical to predicting the future of Earth's natural carbon uptake. The findings could help scientists build more accurate climate models and better forecast the planet's carbon balance in a warming world.