Plants Absorb 31% More CO2 Than Previously Estimated: A Boost for Climate Predictions

Recent research has revealed that plants are taking in about 31% more carbon dioxide (CO2) than scientists previously believed. This new finding, published in the journal Nature, could significantly improve climate prediction models and highlights the vital role of natural carbon absorption in managing greenhouse gas levels.

The Role of Terrestrial Gross Primary Production (GPP)

The amount of CO2 absorbed by land plants through photosynthesis is known as Terrestrial Gross Primary Production, or GPP. This process is the largest exchange of carbon between the land and the atmosphere, measured in petagrams of carbon per year. To give some perspective, one petagram is equal to one billion metric tons of carbon — about as much CO2 as 238 million gasoline-powered cars emit annually.

For decades, GPP estimates hovered around 120 petagrams per year, a number established back in the 1980s. But a team led by Cornell University, with support from the Department of Energy's Oak Ridge National Laboratory (ORNL), has now recalculated this figure to be a significant 157 petagrams of carbon annually. That's a considerable increase, and it suggests that plants are working even harder than we thought to soak up carbon from the atmosphere.

A Fresh Approach Using Carbonyl Sulfide (OCS)

So, what’s behind this updated estimate? The research team used an innovative approach that tracks a chemical compound called carbonyl sulfide (OCS). This compound travels through a leaf in much the same way as CO2 and is closely related to the photosynthesis process. However, OCS is easier to measure than CO2, making it a useful stand-in for estimating global photosynthetic activity.

The scientists developed an integrated model that traces OCS from the atmosphere into the chloroplasts inside plant cells — the green "factories" where photosynthesis happens. By tracking OCS, they could reliably estimate the amount of photosynthesis occurring on a global scale and over extended time periods. This method provided a clearer picture of GPP than traditional approaches relying solely on satellite data, which can struggle with cloud cover, especially in tropical regions.

The Importance of Mesophyll Diffusion

A key factor in this new estimate is a better understanding of a process called mesophyll diffusion. Essentially, this refers to how gases like CO2 and OCS move from the leaf's surface into the chloroplasts where carbon fixation occurs. Understanding mesophyll diffusion helps researchers determine how efficiently plants are conducting photosynthesis, offering insights into how they might adapt to changing environmental conditions.

The mesophyll conductance model was developed with the expertise of Lianhong Gu, a distinguished staff scientist at ORNL. He explained that the longstanding estimate of 120 petagrams had been used as a reference for years, despite ongoing efforts to refine it. "It's crucial that we get an accurate understanding of global GPP because it influences our entire view of the Earth's carbon cycle," Gu noted.

A Closer Look at Tropical Rainforests

Interestingly, the most significant differences between the old and new estimates were found in tropical rainforests. Ground measurements showed that these ecosystems absorb far more carbon than previous estimates suggested, making them an even more important natural carbon sink than initially thought. This finding underscores the need to include accurate representations of processes like mesophyll diffusion in climate models to improve the predictions of tropical forest responses to climate change.

Improving Climate Predictions

The results of this study are not just about updating a number; they’re about enhancing our ability to predict future CO2 levels and, consequently, global climate change. "Refining our estimates of GPP with reliable, global-scale observations is a critical step," said Peter Thornton, an ORNL Corporate Fellow and lead for the Earth Systems Science Section. He emphasized that including key processes like mesophyll conductance in models could significantly reduce uncertainties in predictions about tropical forest carbon dynamics.

A Collaborative Effort

This groundbreaking research brought together experts from various institutions, including Cornell's School of Integrative Plant Sciences, Wageningen University and Research in The Netherlands, Carnegie Institution for Science, Colorado State University, University of California Santa Cruz, and NASA's Jet Propulsion Laboratory. Funding support came from multiple sources, such as Cornell, the National Science Foundation, and the DOE’s Office of Science Biological and Environmental Research program.

What’s Next?

These findings are expected to inform new models that can more accurately predict how tropical forests — and the global ecosystem as a whole — will respond to changing climate conditions. Initiatives like the Department of Energy’s Next Generation Ecosystem Experiments in the Tropics aim to refine our understanding of tropical carbon cycles further, providing critical insights for future climate mitigation strategies.