Large quantities of organic carbon are stored in frozen soils (permafrost) within Arctic and sub-Arctic regions. A warming climate can induce environmental changes that accelerate the microbial breakdown of organic carbon and the release of the greenhouse gases carbon dioxide and methane. This feedback can accelerate climate change, but the magnitude and timing of greenhouse gas emission from these regions and their impact on climate change remain uncertain. Here we find that current evidence suggests a gradual and prolonged release of greenhouse gas emissions in a warming climate and present a research strategy with which to target poorly understood aspects of permafrost carbon dynamics.

Climate change and the permafrost carbon feedback

Changes in climate extremes and their impacts on the natural physical environment: An overview of the IPCC SREX report

Thawing permafrost and the resulting microbial decomposition of previously frozen organic carbon (C) is one of the most significant potential feedbacks from terrestrial ecosystems to the atmosphere in a changing climate. In this article we present an overview of the global permafrost C pool and of the processes that might transfer this C into the atmosphere, as well as the associated ecosystem changes that occur with thawing. We show that accounting for C stored deep in the permafrost more than doubles previous high-latitude inventory estimates, with this new estimate equivalent to twice the atmospheric C pool. The thawing of permafrost with warming occurs both gradually and catastrophically, exposing organic C to microbial decomposition. Other aspects of ecosystem dynamics can be altered by climate change along with thawing permafrost, such as growing season length, plant growth rates and species composition, and ecosystem energy exchange. However, these processes do not appear to be able to com...

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Vulnerability of Permafrost Carbon to Climate Change: Implications for the Global Carbon Cycle

Soils and other unconsolidated deposits in the northern circumpolar permafrost region store large amounts of soil organic carbon (SOC). This SOC is potentially vulnerable to remobilization following soil warming and permafrost thaw, but SOC stock estimates were poorly constrained and quantitative error estimates were lacking. This study presents revised estimates of permafrost SOC stocks, including quantitative uncertainty estimates, in the 0–3 m depth range in soils as well as for sediments deeper than 3 m in deltaic deposits of major rivers and in the Yedoma region of Siberia and Alaska. Revised estimates are based on significantly larger databases compared to previous studies. Despite this there is evidence of significant remaining regional data gaps. Estimates remain particularly poorly constrained for soils in the High Arctic region and physiographic regions with thin sedimentary overburden (mountains, highlands and plateaus) as well as for deposits below 3 m depth in deltas and the Yedoma region. While some components of the revised SOC stocks are similar in magnitude to those previously reported for this region, there are substantial differences in other components, including the fraction of perennially frozen SOC. Upscaled based on regional soil maps, estimated permafrost region SOC stocks are 217 ± 12 and 472 ± 27 Pg for the 0–0.3 and 0–1 m soil depths, respectively (±95% confidence intervals). Storage of SOC in 0–3 m of soils is estimated to 1035 ± 150 Pg. Of this, 34 ± 16 Pg C is stored in poorly developed soils of the High Arctic. Based on generalized calculations, storage of SOC below 3 m of surface soils in deltaic alluvium of major Arctic rivers is estimated as 91 ± 52 Pg. In the Yedoma region, estimated SOC stocks below 3 m depth are 181 ± 54 Pg, of which 74 ± 20 Pg is stored in intact Yedoma (late Pleistocene ice- and organic-rich silty sediments) with the remainder in refrozen thermokarst deposits. Total estimated SOC storage for the permafrost region is ∼1300 Pg with an uncertainty range of ∼1100 to 1500 Pg. Of this, ∼500 Pg is in non-permafrost soils, seasonally thawed in the active layer or in deeper taliks, while ∼800 Pg is perennially frozen. This represents a substantial ∼300 Pg lowering of the estimated perennially frozen SOC stock compared to previous estimates.

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Estimated stocks of circumpolar permafrost carbon with quantified uncertainty ranges and identified data gaps

Carbon stored in soils worldwide exceeds the amount of carbon stored in phytomass and the atmosphere. Despite the large quantity of carbon stored as soil organic carbon (SOC), consensus is lacking on the size of global SOC stocks, their spatial distribution, and the carbon emissions from soils due to changes in land use and land cover. This article summarizes published estimates of global SOC stocks through time and provides an overview of the likely impacts of management options on SOC stocks. We then discuss the implications of existing knowledge of SOC stocks, their geographical distribution and the emissions due to management regimes on policy decisions, and the need for better soil carbon science to mitigate losses and enhance soil carbon stocks.

Global soil carbon: understanding and managing the largest terrestrial carbon pool

We develop a permafrost classification system to describe the complex interaction of climatic and ecological processes in permafrost formation and degradation that differentiates five patterns of formation: ‘climate-driven’; ‘climate-driven, ecosystem-modified’; ‘climate-driven, ecosystem-protected’; ‘ecosystem-driven’; and ‘ecosystem-protected’ permafrost. Climate-driven permafrost develops in the continuous permafrost zone, where permafrost forms immediately after the surface is exposed to the atmosphere and even under shallow water. Climate-driven, ecosystem-modified permafrost occurs in the continuous permafrost zone when vegetation succession and organic-matter accumulation lead to development of an ice-rich layer at the top of the permafrost. During warming climates, permafrost that has formed as climate-driven can occur in the discontinuous permafrost zone, where it can persist for a long time as ecosystem-protected. Climate-driven, ecosystem protected permafrost, and its associated ground ice, cannot re-establish in the discontinuous zone once degraded, although the near surface can recover as ecosystem-driven permafrost. Ecosystem-driven permafrost forms in the discontinuous permafrost zone in poorly drained, low-lying and north-facing landscape conditions, and under strong ecosystem influence. Finally, ecosystem-protected permafrost persists as sporadic patches under warmer climates, but cannot be re-established after disturbance. These distinctions are important because the various types react differently to climate change and surface disturbances. For example, climate-driven, ecosystem-modified permafrost can experience thermokarst even under cold conditions because of its ice-rich layer formed during ecosystem development, and ecosystem-driven permafrost is unlikely to recover after disturbance, such as fire, if there is sufficient climate warming. Copyright © 2007 John Wiley & Sons, Ltd.

Patterns of permafrost formation and degradation in relation to climate and ecosystems

The Arctic soil organic-carbon pool is a significant, but poorly constrained, carbon store. The most cited pool size estimates are based on a study that severely undersamples Arctic soils, with only five out of the 48 soils examined actually from the Arctic region. Furthermore, previous measurements have been confined to the top 40 cm of soil. Here, we present 1-m-deep measurements of soil organic carbon obtained at 117 locations in the North American Arctic region. To this dataset we add previously published measurements to generate a total sample size of 139 North American Arctic soils. We show that soil organic-carbon stores are highly dependent on landscape type, being highest in lowland and hilly upland soils, where values average 55.1 and 40.6 kg soil organic carbon m−2 respectively, and lowest in rubbleland and mountain soils, where values average 3.4 and 3.8 kg soil organic carbon m−2 respectively. Extrapolating our measurements using known distributions of landscape types we estimate that the total organic carbon pool in North American Arctic soils, together with the average amount of carbon per unit area, is considerably higher than previously thought. Our estimates of the depth distribution and total amount of organic carbon in North American Arctic soils will form an important basis for studies examining the impact of climate warming on CO2 release in the region. The Arctic soil organic-carbon pool is poorly constrained. Measurements of soil organic carbon in the North American Arctic reveal that the carbon store in this region is larger than previous estimates suggest, and highly dependent on landscape type.

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High stocks of soil organic carbon in the North American Arctic region

. Knowledge of soils in the permafrost region has advanced immensely in recent decades, despite the remoteness and inaccessibility of most of the region and the sampling limitations posed by the severe environment. These efforts significantly increased estimates of the amount of organic carbon stored in permafrost-region soils and improved understanding of how pedogenic processes unique to permafrost environments built enormous organic carbon stocks during the Quaternary. This knowledge has also called attention to the importance of permafrost-affected soils to the global carbon cycle and the potential vulnerability of the region's soil organic carbon (SOC) stocks to changing climatic conditions. In this review, we briefly introduce the permafrost characteristics, ice structures, and cryopedogenic processes that shape the development of permafrost-affected soils, and discuss their effects on soil structures and on organic matter distributions within the soil profile. We then examine the quantity of organic carbon stored in permafrost-region soils, as well as the characteristics, intrinsic decomposability, and potential vulnerability of this organic carbon to permafrost thaw under a warming climate. Overall, frozen conditions and cryopedogenic processes, such as cryoturbation, have slowed decomposition and enhanced the sequestration of organic carbon in permafrost-affected soils over millennial timescales. Due to the low temperatures, the organic matter in permafrost soils is often less humified than in more temperate soils, making some portion of this stored organic carbon relatively vulnerable to mineralization upon thawing of permafrost.

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Permafrost soils and carbon cycling

Observations and measurements were made of physical and ecological changes that have occurred since 1985 at a tundra site near Healy, Alaska. Air temperatures decreased (1985 through 1999) while permafrost warmed and thawed creating thermokarst terrain, probably as a result of increased snow depths. Permafrost, active layer and ground-ice conditions at the Healy site are the result of the interaction of climatic, ecologic and other factors. The slow accumulation of ground ice in an intermediate permafrost layer formed by upward freezing from the permafrost surface leads to long-term differential frost heave and microrelief. When ground ice in the permafrost melts, the ground surface settles differentially resulting in thermokarst terrain (pits, gullies). Windblown snow fills the thermokarst depressions causing further warming and thawing of the underlying permafrost — a positive feedback effect that enhances permafrost degradation. Thermokarst-induced changes in relief alter the near-surface hydrology and ecological processes. Changes in vegetation included differential tussock growth and mortality and a shift in moss species abundance and relative productivity, depending on microtopographic position created by the thermokarst terrain. Water redistribution towards thermokarst depressions caused adjacent higher areas to become drier and resulted in increased moss mortality and shrub abundance. Copyright # 2009 John Wiley & Sons, Ltd.

https://www.fs.fed.us/pnw/pubs/journals/pnw_2009_osterkamp001.pdf

M. T. Jorgenson

Papers

Patterns of permafrost formation and degradation in relation to climate and ecosystems

High stocks of soil organic carbon in the North American Arctic region

Permafrost soils and carbon cycling

Physical and ecological changes associated with warming permafrost and thermokarst in Interior Alaska

Cryostratigraphy of late Pleistocene syngenetic permafrost (yedoma) in northern Alaska, Itkillik River exposure