Plants Absorb 31% More Carbon Than Previously Thought, Prompting Updates to Climate Modeling

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.

Plant CO2 uptake predictions

Terrestrial Gross Primary Production, or GPP, is the quantity of carbon dioxide that plants extract from the atmosphere through photosynthesis.

This process, which is commonly estimated in petagrams of carbon per year (one pentagram is equivalent to one billion metric tons), is the biggest transfer of carbon between the atmosphere and the land. 

GPP was previously believed by scientists to be about 120 pentagram annually, a figure that was set 40 years ago.

This estimate has been updated to 157 petagram annually by the current study, which was conducted by Cornell University researchers with assistance from the Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL). This marks a significant change in our knowledge of how plants absorb carbon globally.

Conducting large-scale measurements of photosynthesis

The research team created a novel model to predict photosynthesis on a broad scale using carbonyl sulfide (OCS) as a stand-in. Like CO2, OCS travels through plant leaves and into chloroplasts, which are where photosynthesis occurs.

 OCS is a good indication of photosynthesis since it is simpler to monitor and quantify than CO2. The team demonstrated that OCS is a good stand-in for calculating global GPP since it is especially good at estimating photosynthesis over large areas and over extended periods of time.

Researchers used data on plants from a variety of sources, such as the LeafWeb database created by ORNL, which collects photosynthetic characteristic data from researchers all around the world.

The scientists used high-resolution data from environmental monitoring towers to corroborate its findings rather than depending only on satellite data, which can be less precise due to cloud interference, particularly in tropical locations.

Improving models with new insights

A team of scientists led by Cornell University, with support from the Department of Energy’s Oak Ridge National Laboratory, used new models and measurements to assess GPP from the land at 157 peta-grams of carbon per year, up from an estimate of 120 peta-grams established 40 years ago and currently used in most estimates of Earth’s carbon cycle.

The team used plant data from a variety of sources to inform model development. One of the sources was the Leaf Web database, established at ORNL in support of the DOE Terrestrial Ecosystem Science Scientific Focus Area, or TES-SFA. Leaf Web collects data about photosynthetic traits from scientists around the world to support carbon cycle modeling. The scientists verified the model results by comparing them with high-resolution data from environmental monitoring towers instead of satellite observations, which can be hindered by clouds, particularly in the tropics.

Key to the new estimate is a better representation of a process called mesophyll diffusion — how OCS and CO2 move from leaves into chloroplasts where carbon fixation occurs. Understanding mesophyll diffusion is essential to figuring out how efficiently plants conduct photosynthesis and even how they might adapt to changing environments.

Consequences for Tropical Rainforests and Forecasts of the Future Climate

Pan-tropical rainforests accounted for the biggest difference between previous estimates and the new figures, a finding that was corroborated by ground measurements, Gu said. The discovery suggests that rainforests are a more important natural carbon sink than previously estimated using satellite data.

Understanding how much carbon can be stored in land ecosystems, especially in forests with their large accumulations of biomass in wood, is essential to making predictions of future climate change.

“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, Corporate Fellow and lead for the Earth Systems Science Section at ORNL.

“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.”
“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.”