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Title: Characterizing permafrost soil active layer dynamics and sensitivity to landscape spatial heterogeneity in Alaska

Abstract

An important feature of the Arctic is large spatial heterogeneity in active layer conditions, which is generally poorly represented by global models. In this study, we developed a spatially integrated modelling and analysis framework combining field observations, local scale (~ 50 m) active layer thickness (ALT) and soil moisture maps derived from airborne low frequency (L + P-band) radar measurements, and global satellite environmental observations to investigate the ALT sensitivity to recent climate trends and landscape heterogeneity in Alaska. Model simulated ALT results show good correspondence with in-situ measurements in higher permafrost probability (PP ≥ 70 %) areas (n = 33, R = 0.60, mean bias = 1.58 cm, RMSE = 20.32 cm). The model results also reveal widespread ALT deepening since 2001, with smaller ALT increases in northern Alaska (mean trend = 0.32 ± 1.18 cm yr -1) and much larger increases (> 3 cm yr -1) across interior and southern Alaska. The positive ALT trend coincides with regional warming and a longer snow-free season (R = 0.60 ± 0.32). Uncertainty in the spatial and vertical distribution of soil organic carbon (SOC) was found to be the most important factor affecting model ALT accuracy. Here, potential improvements in characterizingmore » SOC heterogeneity, including better spatial sampling of soil conditions and advances in remote sensing of SOC and soil moisture, will enable more accurate predictions of permafrost active layer conditions.« less

Authors:
 [1];  [1]; ORCiD logo [2];  [2];  [3];  [4]; ORCiD logo [5];  [5]
  1. The Univ. of Montana, Missoula, MT (United States)
  2. Univ. of Southern California, Los Angeles, CA (United States)
  3. NASA Goddard Space Flight Center (GSFC), Greenbelt, MD (United States)
  4. Argonne National Lab. (ANL), Argonne, IL (United States)
  5. San Diego State Univ., San Diego, CA (United States)
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
National Aeronautic and Space Administration (NASA); USDOE
OSTI Identifier:
1402081
Grant/Contract Number:
AC02-06CH11357
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
The Cryosphere Discussions
Additional Journal Information:
Journal Volume: 2017; Journal ID: ISSN 1994-0440
Publisher:
European Geosciences Union
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; 54 ENVIRONMENTAL SCIENCES; Active layer; Permafrost

Citation Formats

Yi, Yonghong, Kimball, John S., Chen, Richard, Moghaddam, Mahta, Reichle, Rolf H., Mishra, Umakant, Zona, Donatella, and Oechel, Walter C.. Characterizing permafrost soil active layer dynamics and sensitivity to landscape spatial heterogeneity in Alaska. United States: N. p., 2017. Web. doi:10.5194/tc-2017-87.
Yi, Yonghong, Kimball, John S., Chen, Richard, Moghaddam, Mahta, Reichle, Rolf H., Mishra, Umakant, Zona, Donatella, & Oechel, Walter C.. Characterizing permafrost soil active layer dynamics and sensitivity to landscape spatial heterogeneity in Alaska. United States. doi:10.5194/tc-2017-87.
Yi, Yonghong, Kimball, John S., Chen, Richard, Moghaddam, Mahta, Reichle, Rolf H., Mishra, Umakant, Zona, Donatella, and Oechel, Walter C.. Tue . "Characterizing permafrost soil active layer dynamics and sensitivity to landscape spatial heterogeneity in Alaska". United States. doi:10.5194/tc-2017-87. https://www.osti.gov/servlets/purl/1402081.
@article{osti_1402081,
title = {Characterizing permafrost soil active layer dynamics and sensitivity to landscape spatial heterogeneity in Alaska},
author = {Yi, Yonghong and Kimball, John S. and Chen, Richard and Moghaddam, Mahta and Reichle, Rolf H. and Mishra, Umakant and Zona, Donatella and Oechel, Walter C.},
abstractNote = {An important feature of the Arctic is large spatial heterogeneity in active layer conditions, which is generally poorly represented by global models. In this study, we developed a spatially integrated modelling and analysis framework combining field observations, local scale (~ 50 m) active layer thickness (ALT) and soil moisture maps derived from airborne low frequency (L + P-band) radar measurements, and global satellite environmental observations to investigate the ALT sensitivity to recent climate trends and landscape heterogeneity in Alaska. Model simulated ALT results show good correspondence with in-situ measurements in higher permafrost probability (PP ≥ 70 %) areas (n = 33, R = 0.60, mean bias = 1.58 cm, RMSE = 20.32 cm). The model results also reveal widespread ALT deepening since 2001, with smaller ALT increases in northern Alaska (mean trend = 0.32 ± 1.18 cm yr-1) and much larger increases (> 3 cm yr-1) across interior and southern Alaska. The positive ALT trend coincides with regional warming and a longer snow-free season (R = 0.60 ± 0.32). Uncertainty in the spatial and vertical distribution of soil organic carbon (SOC) was found to be the most important factor affecting model ALT accuracy. Here, potential improvements in characterizing SOC heterogeneity, including better spatial sampling of soil conditions and advances in remote sensing of SOC and soil moisture, will enable more accurate predictions of permafrost active layer conditions.},
doi = {10.5194/tc-2017-87},
journal = {The Cryosphere Discussions},
number = ,
volume = 2017,
place = {United States},
year = {Tue May 30 00:00:00 EDT 2017},
month = {Tue May 30 00:00:00 EDT 2017}
}

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  • There is a substantial amount of carbon stored in the permafrost soils of boreal forest ecosystems, where it is currently protected from decomposition. The surface organic horizons insulate the deeper soil from variations in atmospheric temperature. The removal of these insulating horizons through consumption by fire increases the vulnerability of permafrost to thaw, and the carbon stored in permafrost to decomposition. In this study we ask how warming and fire regime may influence spatial and temporal changes in active layer and carbon dynamics across a boreal forest landscape in interior Alaska. To address this question, we (1) developed and testedmore » a predictive model of the effect of fire severity on soil organic horizons that depends on landscape-level conditions and (2) used this model to evaluate the long-term consequences of warming and changes in fire regime on active layer and soil carbon dynamics of black spruce forests across interior Alaska. The predictive model of fire severity, designed from the analysis of field observations, reproduces the effect of local topography (landform category, the slope angle and aspect and flow accumulation), weather conditions (drought index, soil moisture) and fire characteristics (day of year and size of the fire) on the reduction of the organic layercaused by fire. The integration of the fire severity model into an ecosystem process-based model allowed us to document the relative importance and interactions among local topography, fire regime and climate warming on active layer and soil carbon dynamics. Lowlands were more resistant to severe fires and climate warming, showing smaller increases in active layer thickness and soil carbon loss compared to drier flat uplands and slopes. In simulations that included the effects of both warming and fire at the regional scale, fire was primarily responsible for a reduction in organic layer thickness of 0.06 m on average by 2100 that led to an increase in active layer thickness of 1.1 m on average by 2100. The combination of warming and fire led to a simulated cumulative loss of 9.6 kgC m 2 on average by 2100. Our analysis suggests that ecosystem carbon storage in boreal forests in interior Alaska is particularly vulnerable, primarily due to the combustion of organic layer thickness in fire and the related increase in active layer thickness that exposes previously protected permafrost soil carbon to decomposition.« less
  • Over 20% of Earth’s terrestrial surface is underlain by permafrost with vast stores of carbon that, if thawed may represent the largest future transfer of C from the biosphere to the atmosphere 1. This process is largely dependent on microbial responses, but we know little about microbial activity in intact, let alone in thawing permafrost. Molecular approaches have recently revealed the identities and functional gene composition of microorganisms in some permafrost soils 2-4 and a rapid shift in functional gene composition during short-term thaw experiments 3. However, the fate of permafrost C depends on climatic, hydrologic, and microbial responses tomore » thaw at decadal scales 5, 6. Here the combination of several molecular “omics” approaches enabled us to determine the phylogenetic composition of the microbial community, including several draft genomes of novel species, their functional potential and activity in soils representing different states of thaw: intact permafrost, seasonally thawed active layer and thermokarst bog. The multi-omics strategy revealed a good correlation of process rates to omics data for dominant processes, such as methanogenesis in the bog, as well as novel survival strategies for potentially active microbes in permafrost.« less
  • A significant portion of the large amount of carbon (C) currently stored in soils of the permafrost region in the Northern Hemisphere has the potential to be emitted as the greenhouse gases CO 2 and CH 4 under a warmer climate. In this study we evaluated the variability in the sensitivity of permafrost and C in recent decades among land surface model simulations over the permafrost region between 1960 and 2009. The 15 model simulations all predict a loss of near-surface permafrost (within 3 m) area over the region, but there are large differences in the magnitude of the simulatedmore » rates of loss among the models (0.2 to 58.8 × 10 3 km 2 yr –1). Sensitivity simulations indicated that changes in air temperature largely explained changes in permafrost area, although interactions among changes in other environmental variables also played a role. All of the models indicate that both vegetation and soil C storage together have increased by 156 to 954 Tg C yr –1 between 1960 and 2009 over the permafrost region even though model analyses indicate that warming alone would decrease soil C storage. Increases in gross primary production (GPP) largely explain the simulated increases in vegetation and soil C. The sensitivity of GPP to increases in atmospheric CO 2 was the dominant cause of increases in GPP across the models, but comparison of simulated GPP trends across the 1982–2009 period with that of a global GPP data set indicates that all of the models overestimate the trend in GPP. Disturbance also appears to be an important factor affecting C storage, as models that consider disturbance had lower increases in C storage than models that did not consider disturbance. Furthermore, to improve the modeling of C in the permafrost region, there is the need for the modeling community to standardize structural representation of permafrost and carbon dynamics among models that are used to evaluate the permafrost C feedback and for the modeling and observational communities to jointly develop data sets and methodologies to more effectively benchmark models.« less
  • Permafrost soils contain enormous amounts of organic carbon whose stability is contingent on remaining frozen. With future warming, these soils may release carbon to the atmosphere and act as a positive feedback to climate change. Significant uncertainty remains on the postthaw carbon dynamics of permafrost-affected ecosystems, in particular since most of the carbon resides at depth where decomposition dynamics may differ from surface soils, and since nitrogen mineralized by decomposition may enhance plant growth. Here we show, using a carbon–nitrogen model that includes permafrost processes forced in an unmitigated warming scenario, that the future carbon balance of the permafrost regionmore » is highly sensitive to the decomposability of deeper carbon, with the net balance ranging from 21 Pg C to 164 Pg C losses by 2300. Increased soil nitrogen mineralization reduces nutrient limitations, but the impact of deep nitrogen on the carbon budget is small due to enhanced nitrogen availability from warming surface soils and seasonal asynchrony between deeper nitrogen availability and plant nitrogen demands. The future carbon balance of this region is projected to hinge more on the rate and extent of permafrost thaw and soil decomposition than on enhanced nitrogen availability for vegetation growth resulting from permafrost thaw.« less