Abstract
Abstract. Under climate change thawing permafrost will cause old carbon which is currently frozen and inert to become vulnerable to decomposition and release into the climate system. This paper develops a simple framework for estimating the impact of this permafrost carbon release on the global mean temperature (P-GMT). The analysis is based on simulations made with the Hadley Centre climate model (HadGEM2-ES) for a range of representative CO2 concentration pathways. Results using the high concentration pathway (RCP 8.5) suggest that by 2100 the annual methane (CH4) emission rate is 2–59 Tg CH4 yr−1 and 50–270 Pg C has been released as CO2 with an associated P-GMT of 0.08–0.36 °C (all 5th–95th percentile ranges). P-GMT is considerably lower – between 0.02 and 0.11 °C – for the low concentration pathway (RCP2.6). The uncertainty in climate model scenario causes about 50% of the spread in P-GMT by the end of the 21st century. The distribution of soil carbon, in particular how it varies with depth, contributes to about half of the remaining spread, with quality of soil carbon and decomposition processes contributing a further quarter each. These latter uncertainties could be reduced through additional observations. Over the next 20–30 yr, whilst scenario uncertainty is small, improving our knowledge of the quality of soil carbon will contribute significantly to reducing the spread in the, albeit relatively small, P-GMT.
Highlights
Permafrost soil – soil which is below 0 ◦C for 2 yr or more – underlies approximately a quarter of the exposed land surface of the Northern Hemisphere (Zhang et al, 1999)
Future climate change projections suggest a marked warming at northern high latitudes of between 2.8 and 7.8 degrees (A1B scenario) by the end of the century (Solomon et al, 2007), which will result in further degradation of permafrost
Interpolation of output from this latter model is assumed to reproduce the active layer more accurately. These model simulations are pan Arctic simulations from the Joint UK Land Environment Simulator (JULES: Best et al, 2011; Clark et al, 2011), which is comparable to the land surface component of the Hadley Centre climate model
Summary
Permafrost soil – soil which is below 0 ◦C for 2 yr or more – underlies approximately a quarter of the exposed land surface of the Northern Hemisphere (Zhang et al, 1999). Model projections of permafrost degradation over the 21st century are highly uncertain, ranging from a 7 to 88 % loss of permafrost area and a 40 to 100 % increase in active layer thickness (Schaefer et al, 2011) These uncertainties depend on study region, future emissions scenario and model (Schaefer et al, 2011; Koven et al, 2011; Schneider von Deimling et al, 2012). Koven et al (2011) included a vertically discretised soil carbon module and CH4 emissions from wetlands and permafrost in their land surface scheme They used surface weather data calculated under the SRES A2 emissions scenario and showed a cumulative release of 62 ± 7 Pg C due to the partial decomposition of the old permafrost carbon pool over the 21st century. It will inform the development of appropriate schemes to quantify the permafrost climate feedback in global earth system models
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