PNNL

The Science                                

How large are the carbon flows in the global carbon cycle? Satellites provide estimates of plant photosynthesis while researchers use ground measurements to understand respiration—the process by which living organisms send carbon dioxide, or CO₂, back to the atmosphere. These two quantities should be linked because photosynthesis is the ultimate source of all respired carbon. A new study calculated photosynthesis rates from respiration data and vice versa. The results show that estimates of these two processes differ widely, raising questions about current scientific understanding of the global carbon cycle.

The Impact

Large discrepancies between published estimates of global photosynthesis and respiration reflect uncertainties that hamper the scientific community’s capacity to understand and model how the global carbon cycle will evolve in response to climate change. This study documents that more recent estimation methods seem to be closing the gap between estimates of these two dominant land-based, or terrestrial, carbon fluxes. This is crucial as accurate estimates of the largest terrestrial carbon fluxes are necessary for correctly determining the land carbon sink—how strongly human emissions are being taken up by ecosystems worldwide.

Summary

The terrestrial carbon sink—the balance between photosynthesis and respiration—removes about a quarter of anthropogenic CO₂ emissions. The magnitude of global photosynthesis (GPP) is therefore one of the largest sources of uncertainty in predicting future trajectories of global temperature. Global GPP is roughly balanced by ecosystem-to-atmosphere respiratory fluxes and dominated by soil respiration (R_S). Although GPP and R_S are physiologically linked—since the former is the ultimate source of all respired carbon—there have been no attempts made to quantify how consistent GPP and R_S estimates are at the global scale. This study compares these two large carbon fluxes by using published estimates of one flux (either GPP or R_S) to compute the likeliest values of the other. Researchers found inconsistencies in the estimates that raise doubts about how robustly Earth system models can project changes in global carbon cycling. These results emphasize the importance of cross-comparing datasets and models to understand terrestrial carbon cycling as well as future climate change.

PNNL Contact

Ben Bond-Lamberty, Pacific Northwest National Laboratory, bondlamberty@pnnl.gov

Vanessa Bailey, Pacific Northwest National Laboratory, vanessa.bailey@pnnl.gov

Funding

This research was supported by the Department of Energy, Office of Science, Biological and Environmental Research, as part of the Terrestrial Ecosystem Sciences Program. The Pacific Northwest National Laboratory is operated for DOE by Battelle Memorial Institute.