Matching Items (3)

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

Description

Soil temperature (T[subscript s]) change is a key indicator of the dynamics of permafrost. On seasonal and interannual timescales, the variability of T[subscript s] determines the active-layer depth, which regulates

Soil temperature (T[subscript s]) change is a key indicator of the dynamics of permafrost. On seasonal and interannual timescales, the variability of T[subscript s] determines the active-layer depth, which regulates hydrological soil properties and biogeochemical processes. On the multi-decadal scale, increasing T[subscript 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[subscript s] in permafrost regions. There is a large spread of T[subscript 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[superscript −1]. Most models show smaller increase in T[subscript s] with increasing depth. Air temperature (Tsub>a) and longwave downward radiation (LWDR) are the main drivers of T[subscript 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[subscript s] trends, while trends of T[subscript a] only explain 5 % of the differences in T[subscript s] trends. Uncertain climate forcing contributes a larger uncertainty in T[subscript s] trends (0.021 ± 0.008 °C yr[superscript −1], mean ± standard deviation) than the uncertainty of model structure (0.012 ± 0.001 °C yr[superscript −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[subscript 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[subscript s] at 1 m is estimated to be of −2.80 ± 0.67 million km[superscript 2 ]°C[superscript −1]. Finally, by using two long-term LWDR data sets and relationships between trends of LWDR and T[subscript s] across models, we infer an observation-constrained total boreal near-surface permafrost area decrease comprising between 39 ± 14  ×  10[superscript 3] and 75 ± 14  ×  10[superscript 3 ]km[superscript 2 ]yr[superscript −1] from 1960 to 2000. This corresponds to 9–18 % degradation of the current permafrost area.

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Date Created
  • 2016-01-20

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Model estimates of climate controls on pan-Arctic wetland methane emissions

Description

Climate factors including soil temperature and moisture, incident solar radiation, and atmospheric carbon dioxide concentration are important environmental controls on methane (CH[subscript 4]) emissions from northern wetlands. We investigated the

Climate factors including soil temperature and moisture, incident solar radiation, and atmospheric carbon dioxide concentration are important environmental controls on methane (CH[subscript 4]) emissions from northern wetlands. We investigated the spatiotemporal distributions of the influence of these factors on northern high-latitude wetland CH[subscript 4] emissions using an enhanced version of the Variable Infiltration Capacity (VIC) land surface model. We simulated CH[subscript 4] emissions from wetlands across the pan-Arctic domain over the period 1948–2006, yielding annual average emissions of 36.1 ± 6.7 Tg CH[subscript 4] yr[superscript −1] for the period 1997–2006. We characterized historical sensitivities of CH[subscript 4] emissions to air temperature, precipitation, incident long- and shortwave radiation, and atmospheric [CO[subscript 2]] as a function of average summer air temperature and precipitation. Emissions from relatively warm and dry wetlands in the southern (permafrost-free) portion of the domain were positively correlated with precipitation and negatively correlated with air temperature, while emissions from wetter and colder wetlands further north (permafrost) were positively correlated with air temperature. Over the entire period 1948–2006, our reconstructed CH[subscript 4] emissions increased by 20 %, the majority of which can be attributed to an increasing trend in summer air temperature. We estimated future emissions in response to 21st century warming as predicted by CMIP5 (Coupled Model Intercomparison Project Phase 5) model projections to result in end-of-century CH[subscript 4] emissions 38–53 % higher than our reconstructed 1997–2006 emissions, accompanied by the northward migration of warmer and drier than optimal conditions for CH[subscript 4] emissions, implying a reduced role for temperature in driving future increases in emissions.

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Date Created
  • 2015-11-02

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Assessment of model estimates of land-atmosphere CO2 exchange across Northern Eurasia

Description

A warming climate is altering land-atmosphere exchanges of carbon, with a potential for increased vegetation productivity as well as the mobilization of permafrost soil carbon stores. Here we investigate land-atmosphere

A warming climate is altering land-atmosphere exchanges of carbon, with a potential for increased vegetation productivity as well as the mobilization of permafrost soil carbon stores. Here we investigate land-atmosphere carbon dioxide (CO[superscript 2]) cycling through analysis of net ecosystem productivity (NEP) and its component fluxes of gross primary productivity (GPP) and ecosystem respiration (ER) and soil carbon residence time, simulated by a set of land surface models (LSMs) over a region spanning the drainage basin of Northern Eurasia. The retrospective simulations cover the period 1960–2009 at 0.5° resolution, which is a scale common among many global carbon and climate model simulations. Model performance benchmarks were drawn from comparisons against both observed CO[superscript 2] fluxes derived from site-based eddy covariance measurements as well as regional-scale GPP estimates based on satellite remote-sensing data. The site-based comparisons depict a tendency for overestimates in GPP and ER for several of the models, particularly at the two sites to the south. For several models the spatial pattern in GPP explains less than half the variance in the MODIS MOD17 GPP product. Across the models NEP increases by as little as 0.01 to as much as 0.79 g C m[superscript −2] yr[superscript −2], equivalent to 3 to 340 % of the respective model means, over the analysis period. For the multimodel average the increase is 135 % of the mean from the first to last 10 years of record (1960–1969 vs. 2000–2009), with a weakening CO[superscript 2] sink over the latter decades. Vegetation net primary productivity increased by 8 to 30 % from the first to last 10 years, contributing to soil carbon storage gains. The range in regional mean NEP among the group is twice the multimodel mean, indicative of the uncertainty in CO[superscript 2] sink strength. The models simulate that inputs to the soil carbon pool exceeded losses, resulting in a net soil carbon gain amid a decrease in residence time. Our analysis points to improvements in model elements controlling vegetation productivity and soil respiration as being needed for reducing uncertainty in land-atmosphere CO[superscript 2] exchange. These advances will require collection of new field data on vegetation and soil dynamics, the development of benchmarking data sets from measurements and remote-sensing observations, and investments in future model development and intercomparison studies.

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Date Created
  • 2015-07-28