Plants Prove More Efficient at CO2 Absorption, Changing Climate Calculations

A groundbreaking study reveals that plants across the globe are absorbing 31% more carbon dioxide than previous estimates suggested. This discovery, published in the journal Nature, is expected to significantly enhance the accuracy of Earth system models used to forecast future climate scenarios. It also emphasizes the crucial role natural carbon sequestration plays in mitigating the effects of greenhouse gases, highlighting the importance of plants as a natural defense against climate change.

The process by which land plants remove CO2 from the atmosphere through photosynthesis is referred to as Terrestrial Gross Primary Production (GPP). As the largest carbon exchange between land and atmosphere, GPP is pivotal for understanding the Earth's carbon cycle. The scale of this removal is measured in petagrams, with one petagram representing one billion metric tons—equivalent to the emissions from about 238 million gas-powered vehicles annually.

A team of researchers, led by Cornell University and supported by the Department of Energy's Oak Ridge National Laboratory (ORNL), utilized new methodologies to revise the GPP estimate from 120 petagrams per year to 157 petagrams. This updated figure, a substantial increase from the previous estimate made 40 years ago, is documented in their paper "Terrestrial Photosynthesis Inferred from Plant Carbonyl Sulfide Uptake." This finding offers a more accurate picture of how plants contribute to carbon sequestration on a global scale.

To reach these conclusions, scientists developed a cutting-edge model that tracks carbonyl sulfide (OCS), a chemical compound, as it moves from the atmosphere into plant leaves. This compound follows a similar path as CO2 and is easier to trace, making it a valuable proxy for studying photosynthesis. By monitoring OCS movement, researchers were able to better quantify photosynthetic activity, revealing OCS as a reliable indicator of GPP at both large scales and over extended timeframes.

The research team incorporated plant data from a range of sources, including the LeafWeb database, which was established by ORNL to support the Department of Energy’s Terrestrial Ecosystem Science initiative. The model’s accuracy was verified by comparing the results with high-resolution data collected from environmental monitoring towers, a more reliable method than satellite observations, particularly in cloud-heavy regions like the tropics.

Central to the improved GPP estimate was the team's enhanced understanding of a process known as mesophyll diffusion, which explains how OCS and CO2 move into plant leaves to facilitate photosynthesis. Accurately representing mesophyll diffusion is critical for determining how efficiently plants carry out photosynthesis and how they might adapt to environmental changes in the future.

Lianhong Gu, a co-author of the study and a distinguished scientist at ORNL, was instrumental in developing the model for mesophyll conductance. He explained, “Figuring out how much CO2 plants fix each year is a conundrum that scientists have been working on for a while. The original estimate of 120 petagrams per year was established in the 1980s, and it stuck as we tried to figure out a new approach. It's important that we get a good handle on global GPP since that initial land carbon uptake affects the rest of our representations of Earth's carbon cycle.”

Gu emphasized the importance of accurate carbon cycle modeling, adding, "We have to make sure the fundamental processes in the carbon cycle are properly represented in our larger-scale models. This work represents a major step forward in terms of providing a definitive number."

The research identified the greatest disparity between previous estimates and the new findings in pan-tropical rainforests, confirming that these ecosystems are more significant carbon sinks than earlier satellite data suggested. This underscores the vital role rainforests play in absorbing CO2 and reducing atmospheric carbon levels.

Understanding the capacity of land ecosystems, particularly forests, to store carbon is essential for forecasting future climate change. Peter Thornton, Corporate Fellow and lead for the Earth Systems Science Section at ORNL, noted, “Nailing down our estimates of GPP with reliable global-scale observations is a critical step in improving our predictions of future CO2 in the atmosphere, and the consequences for global climate.”