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Article | Argonne National Laboratory

Challenges for improving estimates of soil organic carbon stored in permafrost regions

One of the greatest environmental challenges of the 21st century lies in predicting the impacts of anthropogenic activities on Earth’s carbon cycle.

Soil is a significant component of the carbon cycle, because it contains at least two-thirds of the world’s terrestrial carbon and more than twice as much carbon as the atmosphere. Although soil organic carbon (SOC) stocks were built over millennial time scales, they are susceptible to a far more rapid release back to the atmosphere due to climatic and land use change.

If environmental perturbations negatively impact the processes regulating the storage of SOC, significant amounts of this carbon could be decomposed by soil microbes or combusted and thereby transferred to the atmosphere, as carbon dioxide or methane, where it could accelerate the rate of future climatic change.

In the northern circumpolar region, permafrost (frozen ground that remains at or below 0°C for at least two consecutive years) underlies 16% of the exposed global land surface. The low temperatures of the high latitudes coupled with several unique regional processes — including peat accumulation, intermittent burial of windblown and waterborne sediments, and cryoturbation (the churning of surface soil together with lower layers that is caused by freeze-thaw cycles) — have led to long-term buildup of carbon in circumpolar soils that exceeds expectations based on current environmental and ecological conditions. Furthermore, because of the unique processes and cold conditions creating and preserving this buildup, much of the carbon in permafrost-region soils is stored in a state that would be highly susceptible to relatively rapid decomposition and mineralization to carbon dioxide or methane upon thawing.

Recent research efforts have revealed that the amount of SOC stored in the northern circumpolar permafrost region is far greater than earlier estimates and have called attention to the potential vulnerability of this carbon for release to the atmosphere due to climatic change. However, these new estimates of the quantity, decomposability, and combustibility of permafrost-region SOC stocks are poorly constrained and, thus, contribute to the large uncertainties in current model predictions of carbon-climate feedbacks under future warming. Improving the reliability of model predictions depends on both the incorporation of process/mechanistic understanding and accurate initializations of baseline carbon stocks. Thus, better empirical data for the northern circumpolar region are required to constrain these baseline estimates and reduce modeling uncertainties.

As a result of two recent workshops at Argonne National Laboratory, the current differences between empirical and model estimates of the size and distribution of permafrost-region SOC stocks were determined, and research efforts that will reduce this discrepancy were identified. Five research challenges for improving empirical assessments of the distribution and potential mineralization of SOC stocks in the northern permafrost region were highlighted through subsequent discussions and synthesis by workshop participants and other collaborating researchers. These challenges include (1) improving the number and robustness of observations, (2) predicting the spatial and vertical distributions of SOC stocks, (3) characterizing existing carbon forms to better predict their fate, (4) using improved observation-based SOC estimates to inform model development, and (5) quantifying uncertainties in observations and predictions. These research challenges are logically interlinked and suggest opportunities to organize, prioritize, and coordinate future research to substantially reduce the uncertainties associated with the northern circumpolar permafrost region in the efforts to predict future global carbon-climate feedbacks and change.

Inconsistencies in SOC stocks (to 1 m depth) between observation-based (Northern Circumpolar Soil Carbon Database) and baseline ESM estimates calculated from the mean values in four CMIP5 models (BCC-CSM1.1, CanESM2, MIROC-ESM, and GFDL-ESM2M). Brown (+ sign) identifies overpredictions by models compared to observation-based data; white shows tentative agreement; and blue (– sign) shows model under-predictions.

Reference: Mishra U., J.D. Jastrow, R. Matamala, G. Hugelius, C.D. Koven, J.W. Harden, C.L. Ping, G.J. Michaelson, Z. Fan, R.M. Miller, A.D. McGuire, C. Tarnocai, P. Kuhry, W.J. Riley, K. Schaefer, E.A.G. Schuur, M.T. Jorgenson, and L.D. Hinzman. 2013. Empirical estimates to reduce modeling uncertainties of soil organic carbon in permafrost regions: a review of recent progress and remaining challenges. Environmental Research Letters 8:035020. doi:10.1088/1748-9326/8/3/035020 .