Plants absorb 31% more CO2 than previously estimated

A recent study has revealed that plants worldwide are absorbing significantly more carbon dioxide (CO2) than scientists had estimated before. 

The research shows that terrestrial plants absorb about 31% more CO2 than earlier calculations suggested. 

These findings are expected to enhance Earth system models used to predict climate changes and emphasize the crucial role of natural carbon sequestration in reducing greenhouse gas levels.

Estimates of plant CO2 uptake

The amount of carbon dioxide that plants remove from the atmosphere through photosynthesis is referred to as Terrestrial Gross Primary Production, or GPP. 

This process represents the largest transfer of carbon between land and the atmosphere and is typically measured in petagrams of carbon per year (one petagram equals one billion metric tons). 

Previously, scientists estimated that GPP was around 120 petagrams per year, a figure established 40 years ago. 

However, the new study, led by researchers at Cornell University with support from the Department of Energy (DOE)’s Oak Ridge National Laboratory (ORNL), has revised this estimate to 157 petagrams per year. This represents a major shift in our understanding of global carbon uptake by plants.

Measuring photosynthesis on a large scale

The research team developed an innovative model that uses carbonyl sulfide (OCS) as a proxy to estimate photosynthesis on a large scale. OCS follows a similar path as CO2 through plant leaves, moving into chloroplasts – the sites where photosynthesis takes place. 

Unlike CO2, OCS is easier to track and measure, making it a suitable indicator of photosynthesis. The team showed that OCS is particularly effective for estimating photosynthesis over extensive areas and long periods, making it a reliable proxy for determining global GPP.

Researchers utilized plant data from multiple sources, including the LeafWeb database established by ORNL, which gathers data on photosynthetic traits from researchers worldwide. 

Instead of relying solely on satellite data – which can be less accurate due to interference from clouds, especially in tropical regions – the team validated their findings using high-resolution data from environmental monitoring towers.

Improving models with new insights

A critical factor in updating the GPP estimate was a better understanding of mesophyll diffusion, the process by which OCS and CO2 move from the air through plant leaves into chloroplasts. 

This process determines how efficiently plants conduct photosynthesis and how they might respond to changing environmental conditions. 

Lianhong Gu, a co-author of the study and a photosynthesis expert at ORNL, played a key role in developing the mesophyll conductance model, which tracks the diffusion of OCS in leaves and its relationship to photosynthesis.

“Figuring out how much CO2 plants fix each year is a conundrum that scientists have been working on for a while,” Gu said. 

“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.”

The study’s improved understanding of mesophyll diffusion enables more accurate modeling of photosynthesis at a global scale, providing a clearer picture of how plants contribute to carbon sequestration.

Role of rainforests in carbon sequestration

The study revealed that pan-tropical rainforests played a significant role in the updated estimates, showing that these forests absorb far more carbon than previous estimates had indicated. 

This finding was corroborated by ground-based measurements, suggesting that rainforests serve as a more crucial natural carbon sink than previously thought when relying on satellite data alone. This discovery underscores the importance of rainforests in global carbon management and climate regulation.

“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.

Making accurate climate predictions

Understanding how much carbon can be stored in land ecosystems, especially in forests with their dense biomass, is vital for making accurate predictions about future climate change. 

By refining estimates of global GPP, this research will help reduce uncertainties in climate models, particularly those that forecast the response of tropical forests to changing climate conditions. 

These insights are especially valuable for initiatives like the DOE’s Next Generation Ecosystem Experiments in the Tropics, which aim to improve predictions about how tropical forests will respond to climate change.

“We have to make sure the fundamental processes in the carbon cycle are properly represented in our larger-scale models,” Gu explained. 

“For those Earth-scale simulations to work well, they need to represent the best understanding of the processes at work. This work represents a major step forward in terms of providing a definitive number.”

Climate impact of rising CO2 levels 

The study’s results emphasize the importance of including detailed processes like mesophyll conductance in models of photosynthesis, offering a more accurate understanding of how terrestrial ecosystems sequester carbon. 

With this refined knowledge, scientists can better predict the impact of rising CO2 levels on global climates and improve strategies for mitigating the effects of climate change through natural carbon sinks like forests.

The study is published in the journal Nature.

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