A novel method for estimating the rate of photosynthesis from land plants reveals that satellite observations – the current gold standard for quantifying terrestrial carbon uptake – underestimate this important metric, a result that could inform and improve the accuracy of model predictions of future climate change.

A paper describing the new method, “Terrestrial Photosynthesis Inferred from Plant Carbonyl Sulfide Uptake,” published Oct. 16 in Nature, revealed that most of the underestimation from satellites is largely due to inadequate coverage of ground data in the tropics.

The new method relies on tracking a trace gas called carbonyl sulfide, which enters the atmosphere from oceans, volcanoes and deep sea vents. Carbonyl sulfide is absorbed in parallel with carbon dioxide into plants, yet provides a straightforward measurement of photosynthesis rates than the standard process of quantifying carbon dioxide uptake by leaves. 

Scientists use the term gross primary production (GPP) synonymously with photosynthesis rates to describe the total amount of carbon that plants take in during photosynthesis to grow leaves, stems and roots in an ecosystem over a given period of time. Calculating GPP informs scientists about how much carbon dioxide plants pull from the atmosphere, but confounding factors, such as respiration where plants also release carbon dioxide, make it impossible to get an accurate measurement. 

“This paper made a significant step towards a more constrained GPP estimation,” said first author Jiameng Lai, a doctoral student in the lab of senior author Ying Sun, associate professor of geospatial sciences in the School of Integrative Plant Science Soil and Crop Sciences Section in the College of Agriculture and Life Sciences. “Our approach is capable of resolving spatial and temporal GPP distributions – not just informing us about global magnitude but also pointing to where and when GPP is over or under-estimated,” Lai said. 

GPP values based on carbonyl sulfide revealed that plants use (and remove from the atmosphere) roughly 160 petagrams (Pg) carbon per year, an estimate that is much higher than existing GPP estimates derived from satellite observations, which are around 120 to 130 Pgs carbon per year. A petagram is a unit of mass that is equal to 1 billion metric tons.

“That means our estimate is much higher than the current golden standard for the global GPP that is used to benchmark climate model simulations,” Sun said. “This large underestimation of current GPP estimates comes from the tropics.” 

The researchers identified the tropics as a key area requiring more ground and satellite data, since orbiting satellites lack continuous observations as they pass over areas in intervals and are hindered by cloud cover. Also, climate models simulate the tropical GPP as if the forest structures and functions were homogenous throughout. In reality, the Amazon, for example, has huge spatial variability in terms of its carbon sink capacity. When droughts occur, impacts vary for different areas of the Amazon region.

“No satellite observations or current climate model simulations can really characterize such spatial gradients or variability within the tropical rainforest,” Sun said. 

In the paper, the researchers developed a mechanistic model to simulate carbonyl sulfide absorption by plants and then translate those estimates to GPP. Though scientists have already developed models to calculate carbonyl sulfide fluxes, there is a key mechanism – called mesophyll diffusion – missing in them. Until now, models only accounted for diffusion of carbon dioxide from the atmosphere into leaf pores. But there is an additional interior pathway – mesophyll diffusion – where the gas diffuses deeper in the leaf into chloroplasts where photosynthesis occurs.

“Right now, climate and land surface modelers ignore this process, but without accounting for it dynamically in models, we cannot simulate carbonyl sulfide fluxes or global photosynthesis accurately,” Sun said.

“The new estimate of GPP from carbonyl sulfide will substantially alter future climate predictions given everything else is unchanged,” said co-author Yiqi Luo, Liberty Hyde Bailey Professor in the School of Integrative Plant Science, Soil and Crop Sciences Section in CALS. “It means the land biosphere has a higher capacity to take up more carbon dioxide from the atmosphere.” The results also have the potential to alter how carbon dioxide removal strategies to mitigate climate change are designed, Luo said. 

While the study reveals that plants may be taking up more CO2 than previously thought, it is also likely that the current estimate of global respiration – where plants release CO2 – is also underestimated, the authors noted. 

Co-authors include researchers from Wageningen University and Research, Netherlands; Stanford University; Colorado State University; University of California, Santa Cruz; Oak Ridge National Laboratory and the Jet Propulsion Laboratory. 

The study was funded by the Soil and Crop Section at Cornell University; the National Science Foundation; NASA; the U.S. Department of Energy; and the Oak Ridge National Laboratory.