Ivar Berthling, Department of Geography, NTNU, Norway
There are a number of significant feedbacks and interactions between processes and materials at the earth surface and in the atmosphere. This is a crucial point when considering global warming and climate change effects because such feedbacks might amplify the ongoing warming. One such potential feedback that has received a significant amount of scientific and public attention is the fate of the organic carbon stored in the ground in Arctic permafrost areas. Permafrost is defined as the part of the ground that experience sub-zero temperatures for more than two years. Such temperature conditions essentially stop the decay of organic matter from microbiological activity. The effect is that organic carbon that becomes buried from geomorphic and biotic processes such as sedimentation on the surface, peat development and the churning of soils subjected to freezing and thawing tends to accumulate rather than be released as CO2 or methane. Over millennia, this condition has caused both the permafrost and the active layer of Arctic soils to become a sink for organic carbon. Berthling et al. 2, for instance, found from dating 17 samples within a 1.5 x 0.6 m section in the active layer of a sorted circle, that a large percentage of the organic carbon was old, dating back several thousand years before present.
The total amount of soil carbon in northern permafrost areas is now estimated to 1,330–1,580 Pg 3, which is equivalent to about twice the amount of carbon in the atmosphere. What is worrying is that accelerated warming in arctic regions will deepen the active layer and thaw permafrost, altering not only the temperature conditions but also the hydrology of the affected areas. The formerly sequestered organic carbon would then tend to decompose and to be released to the atmosphere, exacerbating the global increase in greenhouse gases. Global warming could thus alter the direction of the net carbon flux in the Arctic, transforming the area from a net sink to a source for greenhouse gases. The recent review by Schuur et al. 3 estimates that approximately 5-15% of the permafrost carbon stock is vulnerable to being released to the atmosphere during this century. They do not fear a catastrophic release of greenhouse gases, however, even when considering that some particularly ice-rich permafrost areas (Yedoma complex) may experience abrupt thaw due to collapse from thermokarst processes.
Modelling, in general, remains the only opportunity for scientists – and therefore for society at large – to predict future developments, as the models in principle can account for the many interacting processes involved. Currently, connections such as those between atmospheric temperature development and soil carbon dynamics are investigated by running so-called Earth System Models (ESMs), but soil organic carbon models for permafrost conditions has only recently been implemented into ESMs and were not part of the IPCC AR5 analysis. Models can, of course, never be better than the implemented equations that describe the processes, and Schuur et al. 3 lists a number of key issues that needs improvement within these permafrost carbon models.
Assessment of model performance against empirical data is also crucial, and such data are often lacking with respect to their spatial distribution – after all the Arctic encompasses vast areas with limited accessibility and often challenging logistics. It must, therefore, be realized that neither models nor the empirical data on which to validate them correspond directly to reality; they are only our best estimates so far. For instance, comparison between empirical data on actual soil carbon stocks in high latitudes and those simulated by the soil models implemented in an ESM show that the models perform reasonably well at large scale, but are not able to explain variation at the scale of the model grid size 4.
Geomorphic processes such as cryoturbation often take place on spatial scales much lower than that of the model grid sizes. Cryoturbation can thus not be explicitly modelled within an ESM and is so far approximated as a general diffusion process 5. The yearly sequestration of carbon that comes out of such a modelling experiment is lower than what is found from field studies in the Arctic but is still a significant improvement to gain agreement between modelled and empirical data of total soil carbon stocks. The process of solifluction has, to my knowledge, so far not been incorporated into models that try to simulate the build-up of soil carbon in permafrost environments. Both cryoturbation and solifluction may experience a transient increase in activity upon permafrost warming 6, 7, thus potentially increasing soil organic carbon burial and to some extent counteract the effects of increased decomposition. More research is therefore essential to further our understanding of the effects of global warming on the permafrost environment and to refine this knowledge in the Earth System Models.
1 Kääb, A., Girod, L., and Berthling, I. (2014): Surface kinematics of periglacial sorted circles using structure-from-motion technology: The Cryosphere, v. 8, p. 1041-1056.
2 Berthling, I., Hallet, B. & Sletten, R. (2014). Soil organic carbon content and age: case studies from Ny-Ålesund area, Svalbard. Book of Abstracts of EUCOP4 – 4 the European Conference on Permafrost 18-21 June 2014 – Évora, Portugal, p. 122.
3 Schuur, E., et al., Climate change and the permafrost carbon feedback. Nature, 2015. 520(7546): p. 171-179.
4 Todd-Brown, K., et al., Causes of variation in soil carbon simulations from CMIP5 Earth system models and comparison with observations. Biogeosciences, 2013. 10(3).
5 Koven, C., et al., On the formation of high?latitude soil carbon stocks: Effects of cryoturbation and insulation by organic matter in a land surface model. Geophysical Research Letters, 2009. 36(21).
6 Bockheim, J.G., Importance of cryoturbation in redistributing organic carbon in permafrost-affected soils. Soil Science Society of America Journal, 2007. 71(4): p. 1335-1342.
7 Harris, C., et al., The role of interannual climate variability in controlling solifluction processes, Endalen, Svalbard. Permafrost and Periglacial Processes, 2011. 22(3): p. 239-253.
Associate Professor Ivar Berthling
Department of Geography
Norwegian University of Science & Technology
Tel: +47 735 908 43