Major study reveals plants now absorbing 30% more CO2 worldwide

Terrestrial ecosystems play a critical role in regulating Earth’s climate by removing carbon dioxide (CO2) from the atmosphere through photosynthesis. This process, known as gross primary production (GPP), represents the largest carbon flux on Earth, driving the terrestrial carbon cycle. However, for decades, scientists have struggled to accurately quantify GPP due to uncertainties in its global magnitude, spatial distribution, and temporal variability.

Traditional estimates pegged global GPP at around 120 petagrams of carbon (PgC) per year since the 1980s. This figure, derived from remote sensing and other indirect methods, has been widely used to model Earth’s carbon cycle. However, recent research challenges this estimate, suggesting that plants globally absorb as much as 157 PgC annually—a 31% increase over previous calculations.

This finding, published in Nature, has significant implications for predicting future climate scenarios and understanding the role of terrestrial ecosystems in mitigating greenhouse gas emissions.

A team led by Cornell University, with support from Oak Ridge National Laboratory (ORNL), introduced a novel approach to estimating GPP using carbonyl sulfide (OCS), a trace gas closely linked to photosynthetic activity. Unlike CO2, which is released back into the atmosphere during plant respiration, OCS uptake is irreversible, making it a reliable proxy for measuring photosynthesis.

The researchers developed a model that traces the movement of OCS from the atmosphere into plant chloroplasts—the cellular structures where photosynthesis occurs. This approach leverages OCS’s unique properties to overcome limitations of traditional methods, such as satellite-based measurements, which can be obstructed by cloud cover, particularly over tropical rainforests.

“Figuring out how much CO2 plants fix each year is a conundrum that scientists have been working on for a while,” said Lianhong Gu, a distinguished staff scientist at ORNL and co-author of the study. “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.”

Key to the new GPP estimate is a better representation of mesophyll diffusion—the process by which OCS and CO2 move from leaves into chloroplasts. Mesophyll layers act as significant barriers to gas movement, influencing how efficiently plants conduct photosynthesis. Previous models often neglected this process or used simplified assumptions, leading to underestimates of GPP.

To address this gap, the team incorporated detailed mesophyll conductance models into the National Center for Atmospheric Research’s Community Land Model version 5 (CLM5). By explicitly accounting for mesophyll diffusion, the researchers improved the accuracy of their simulations, aligning them closely with field measurements from monitoring sites like Harvard Forest in the United States and Hyytiälä Forest in Finland.

The study revealed that mesophyll conductance significantly impacts seasonal and spatial patterns of OCS uptake and GPP. For instance, during dormant and shoulder seasons, previous models underestimated plant activity because they failed to capture the subtle dynamics of gas exchange within leaves.

The updated model also highlighted that nighttime plant OCS uptake—accounting for 20-30% of daily totals—plays a more prominent role than previously thought.

This finding emphasizes the complexity of photosynthetic processes and the importance of incorporating detailed physiological mechanisms into global carbon cycle models. The results also suggest that mesophyll conductance can vary significantly with environmental conditions and phenological stages, further influencing GPP estimates.

The revised GPP estimate has far-reaching implications for climate science. As a primary determinant of terrestrial carbon sinks, GPP shapes how ecosystems respond to rising CO2 levels and climate change.

Pan-tropical rainforests, in particular, emerged as a more productive carbon sink than satellite data had indicated. This discovery underscores the importance of ground-based measurements in validating and refining global models.

“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,” said Peter Thornton, lead for the Earth Systems Science Section at ORNL.

Accurate GPP measurements also enhance Earth system models, which are essential for simulating future climate scenarios. By incorporating mesophyll conductance and OCS-based methodologies, scientists can reduce uncertainties in predictions of tropical forest carbon dynamics and global climate trajectories.

This progress is particularly relevant for initiatives like the Department of Energy’s Next Generation Ecosystem Experiments in the Tropics, which aim to refine models of tropical forest responses to climate change.

Moreover, the findings highlight the limitations of relying solely on satellite observations for estimating global GPP. Ground-based methods, such as OCS tracking, provide a more detailed and accurate picture of photosynthetic activity. This is especially critical in regions like tropical rainforests, where dense cloud cover can obscure satellite data.

The study’s findings highlight the importance of integrating new scientific insights into global carbon cycle models. Researchers verified their results against high-resolution data from environmental monitoring towers, avoiding the limitations of satellite observations. The inclusion of diverse data sources, such as the LeafWeb database, further strengthened the model’s reliability.

LeafWeb, an initiative supported by ORNL’s Terrestrial Ecosystem Science Scientific Focus Area, collects data on photosynthetic traits from scientists worldwide. This wealth of information was instrumental in refining the mesophyll conductance models and ensuring the accuracy of GPP estimates. By leveraging these data, the researchers were able to capture the intricate dynamics of photosynthesis at both local and global scales.

Support for this groundbreaking research came from institutions including Cornell University, Wageningen University in the Netherlands, and the Department of Energy’s Biological and Environmental Research program. Collaborators emphasized that advancements in fundamental carbon cycle processes are crucial for addressing some of the most pressing challenges in climate science.

“We have to make sure the fundamental processes in the carbon cycle are properly represented in our larger-scale models,” Gu explained. “This work represents a major step forward in terms of providing a definitive number.”

As scientists continue to refine methods for estimating GPP, this research provides a robust framework for understanding the intricate dynamics of terrestrial photosynthesis. By revealing the true magnitude of carbon uptake by plants, the study underscores the vital role of natural ecosystems in mitigating climate change.

In addition to its implications for climate modeling, the study opens new avenues for research into plant responses to environmental stressors. Understanding how mesophyll diffusion and other physiological processes adapt to changing conditions could provide valuable insights into ecosystem resilience and sustainability. These findings could inform conservation strategies and help policymakers develop more effective approaches to managing natural carbon sinks.

The integration of innovative methodologies, such as OCS tracking, represents a significant advancement in the field of carbon cycle science. By bridging the gap between small-scale physiological processes and large-scale ecosystem dynamics, this research sets a new standard for studying global photosynthesis. As the planet faces unprecedented environmental challenges, such breakthroughs are essential for building a sustainable future.

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