Publication Date:
2015-04-10
Description:
Simulated high-latitude soil thermal dynamics during the past four decades The Cryosphere Discussions, 9, 2301-2337, 2015 Author(s): S. Peng, P. Ciais, G. Krinner, T. Wang, I. Gouttevin, A. D. McGuire, D. Lawrence, E. Burke, X. Chen, C. Delire, C. Koven, A. MacDougall, A. Rinke, K. Saito, W. Zhang, R. Alkama, T. J. Bohn, B. Decharme, T. Hajima, D. Ji, D. P. Lettenmaier, P. A. Miller, J. C. Moore, B. Smith, and T. Sueyoshi Soil temperature ( T s ) change is a key indicator of the dynamics of permafrost. On seasonal and inter-annual time scales, the variability of T s determines the active layer depth, which regulates hydrological soil properties and biogeochemical processes. On the multi-decadal scale, increasing T s not only drives permafrost thaw/retreat, but can also trigger and accelerate the decomposition of soil organic carbon. The magnitude of permafrost carbon feedbacks is thus closely linked to the rate of change of soil thermal regimes. In this study, we used nine process-based ecosystem models with permafrost processes, all forced by different observation-based climate forcing during the period 1960–2000, to characterize the warming rate of T s in permafrost regions. There is a large spread of T s trends at 20 cm depth across the models, with trend values ranging from 0.010 ± 0.003 to 0.031 ± 0.005 °C yr −1 . Most models show smaller increase in T s with increasing depth. Air temperature ( T a ) and longwave downward radiation (LWDR) are the main drivers of T s trends, but their relative contributions differ amongst the models. Different trends of LWDR used in the forcing of models can explain 61% of their differences in T s trends, while trends of T a only explain 5% of the differences in T s trends. Uncertain climate forcing contributes a larger uncertainty in T s trends (0.021 ± 0.008 °C yr −1 , mean ± SD) than the uncertainty of model structure (0.012 ± 0.001 °C yr −1 ), diagnosed from the range of response between different models, normalized to the same forcing. In addition, the loss rate of near-surface permafrost area, defined as total area where the maximum seasonal active layer thickness (ALT) is less than 3 m loss rate is found to be significantly correlated with the magnitude of the trends of T s at 1 m depth across the models ( R = −0.85, P = 0.003), but not with the initial total near-surface permafrost area ( R = −0.30, P = 0.438). The sensitivity of the total boreal near-surface permafrost area to T s at 1 m, is estimated to be of −2.80 ± 0.67 million km 2 °C −1 . Finally, by using two long-term LWDR datasets and relationships between trends of LWDR and T s across models, we infer an observation-constrained total boreal near-surface permafrost area decrease comprised between 39 ± 14 × 10 3 and 75 ± 14 × 10 3 km 2 yr −1 from 1960 to 2000. This corresponds to 9–18% degradation of the current permafrost area.
Print ISSN:
1994-0432
Electronic ISSN:
1994-0440
Topics:
Geography
,
Geosciences
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