Simulated high-latitude soil thermal dynamics during the past 4 decades

Shushi Peng ,Paul A Miller ,Andrew H Macdougall ,Bertrand Decharme ,Wenxing Zhang ,Xiaodong Chen ,Ramdane Alkama ,Philippe Ciais ,Dennis P Lettenmaier ,A David Mcguire ,Eleanor J Burke ,Tomohiro Hajima ,Duoying Ji ,Christine Delire ,David M Lawrence ,Kazuyuki Saitô ,Isabelle Gouttevin ,John C Moore ,T J Bohn ,C Koven ,Annette Rinke ,Gerhard Krinner ,Tao Wang ,Benjamin Smith ,Tetsuo Sueyoshi

doi:10.5194/tc-10-179-2016

Abstract

Abstract. Soil temperature (Ts) change is a key indicator of the dynamics of permafrost. On seasonal and interannual timescales, the variability of Ts determines the active-layer depth, which regulates hydrological soil properties and biogeochemical processes. On the multi-decadal scale, increasing Ts 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 Ts in permafrost regions. There is a large spread of Ts 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 Ts with increasing depth. Air temperature (Tsub>a) and longwave downward radiation (LWDR) are the main drivers of Ts 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 Ts trends, while trends of Ta only explain 5 % of the differences in Ts trends. Uncertain climate forcing contributes a larger uncertainty in Ts trends (0.021 ± 0.008 °C yr−1, mean ± standard deviation) 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 Ts 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 Ts at 1 m is estimated to be of −2.80 ± 0.67 million km2 °C−1. Finally, by using two long-term LWDR data sets and relationships between trends of LWDR and Ts across models, we infer an observation-constrained total boreal near-surface permafrost area decrease comprising between 39 ± 14 × 103 and 75 ± 14 × 103 km2 yr−1 from 1960 to 2000. This corresponds to 9–18 % degradation of the current permafrost area.

Highlights

Arctic permafrost regions store ∼ 1300 Pg carbon (C) in the soil, including ∼ 1100 Pg C in frozen soil and deposits (Hugelius et al, 2014)
Among the six models with smaller Ts at 20 cm in boreal Europe (BOEU), we found that Ts at 20 cm in BOEU is significantly lower than in boreal Asia (BOAS) and in boreal North America (BONA) (P < 0.001, two-sample t test)
All models show an increase of Ts at 20 cm in northern BONA, but this increase is of different magnitude between models (Fig. 4)

Summary

Introduction

Arctic permafrost regions store ∼ 1300 Pg carbon (C) in the soil, including ∼ 1100 Pg C in frozen soil and deposits (Hugelius et al, 2014) Decomposition of these large carbon pools in response to permafrost thawing from projected future warming is expected to be a positive feedback on climate warming through increased emissions of CO2 and CH4 (Khvorostyanov et al, 2008; Schuur et al, 2008; McGuire et al, 2009; Koven et al, 2011; Schaefer et al, 2011). The borehole record of Alert in Canada (82◦30 N, 62◦25 W) shows that soil temperature at 9, 15, and 24 m increased at rates of 0.6, 0.4, and 0.2 ◦C decade−1 from 1978 to 2007, respectively (Smith et al, 2012) These observations provide long-term local monitoring of changes in active-layer thickness and soil temperature, but the measurement sites are sparse, and their temporal sampling frequency is often low (Romanovsky et al, 2010). Because site measurements cannot document permafrost area loss on a large scale, land surface models including “cold processes”, such as soil freeze– thaw and the thermal and radiative properties of snow, are important tools for quantifying the rate of permafrost degradation on a large scale, and its evolution in response to climate change scenarios

Methods

Results

Conclusion