September 26, 2016

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Soil Will Absorb Less Atmospheric Carbon Than Expected This Century

Analysis of carbon isotope data suggests Earth system models overestimate soil carbon sequestration potential.

The Science

Researchers used carbon-14 (14C) data from 157 globally distributed soil profiles to determine that current soil carbon is about 3,100 years old rather than the 450 years stipulated by many Earth system models (ESMs).  This analysis shows that the fifth Coupled Model Intercomparison Project (CMIP5), for example, underestimated the mean age of soil carbon by about a factor of six, resulting in an overestimate of soil carbon sequestration potential by a factor of nearly two. Consequently, a greater fraction of carbon dioxide (CO2) emissions than previously thought could remain in the atmosphere and contribute to global warming.

The Impact

These findings, which have important implications for future atmospheric CO2 levels, emphasize the need to incorporate better understanding of soil carbon cycling as well as 14C and other tracer diagnostics into ESMs to improve the quality of future climate projections. The work also illustrates the potential value of systematically exploiting available ecosystem measurements during model development to create more robust models.

Summary

Soil is the largest terrestrial carbon reservoir and may influence the sign and magnitude of carbon cycle–climate feedbacks. Many ESMs estimate a significant soil carbon sink by 2100, yet the underlying carbon dynamics determining this response have not been systematically tested against observations. Researchers from the University of California, Irvine; Max Planck Institute for Biogeochemistry; Lawrence Berkeley National Laboratory; Stanford University; and U.S. Geological Survey used 14C data from 157 globally distributed soil profiles sampled to 1-m depth to show that ESMs underestimated the mean age of soil carbon by a factor of more than six (430 ± 50 years versus 3100 ± 1800 years). Consequently, ESMs overestimated the carbon sequestration potential of soils by a factor of nearly two (40% ± 27%). This analysis shows that ESMs must better represent carbon stabilization processes and the turnover time of slow and passive soil carbon reservoirs when simulating future atmospheric CO2 dynamics.

Principal Investigator

James T. Randerson Randerson
University of California, Irvine
jranders@uci.edu

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This research was performed for the Biogeochemistry-Climate Feedbacks Scientific Focus Area (SFA) and the Berkeley Lab Terrestrial Ecosystem Science (TES) SFA, which are sponsored by the Regional and Global Climate Modeling (RGCM) and TES programs, respectively, in the Climate and Environmental Sciences Division of the Office of Biological and Environmental Research, within the U.S. Department of Energy Office of Science.

Related Links

References

He, Y., S. E. Trumbore, M. S. Torn, and J. W. Harden, et al. "Radiocarbon constraints imply reduced carbon uptake by soils during the 21st century". Science 353 (6306), 1419–1424  (2016). http://dx.doi.org/10.1126/science.aad4273.