Soil Will Absorb Less Atmospheric Carbon than Expected This Century

An analysis suggests models overestimate soil’s potential for carbon sequestration.

Atmospheric Carbon
A recent analysis finds that soil integrates carbon far slower than thought. This means the amount that soils can absorb from the atmosphere this century is far less than what current Earth system models predict. Image courtesy of University of California, Irvine

The Science

Researchers used radiocarbon dating to determine that current soil carbon is about 3,100 years old rather than the 450 years stipulated by many Earth system models. Most global models that participated in the latest Coupled Model Intercomparion Project (CMIP5) underestimated the mean age of soil carbon. This underestimation results in an overestimation of soil’s carbon sequestration potential. Consequently, more carbon dioxide emissions than previously thought could remain in the atmosphere and contribute to global warming.

The Impact

These findings have important implications for future atmospheric carbon dioxide levels. The results emphasize the need to incorporate better insights into soil carbon cycling into Earth system models. The results also highlight the need for more insights into carbon-14 and other tracer diagnostics. These insights would 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 models that are more robust.

Summary

Soil is the largest terrestrial carbon reservoir and may influence the sign and magnitude of carbon cycle–climate feedbacks. Many Earth system models 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 carbon-14 data from 157 globally distributed soil profiles sampled to 1-meter depth to show that Earth system models underestimated the mean age of soil carbon by a factor of more than six (430 ± 50 years versus 3100 ± 1800 years). Consequently, Earth system models overestimated the carbon sequestration potential of soils by a factor of nearly two (40 ± 27%). This analysis shows that Earth system models must better represent carbon stabilization processes and the turnover time of slow and passive soil carbon reservoirs when simulating future atmospheric carbon dioxide dynamics.