Stocks of soil organic carbon represent a large component of the carbon cycle that may participate in climate change feedbacks, particularly on decadal and centennial timescales. For Earth system models (ESMs), the ability to accurately represent the global distribution of existing soil carbon stocks is a prerequisite for accurately predicting future carbon–climate feedbacks. We compared soil carbon simulations from 11 model centers to empirical data from the Harmonized World Soil Database (HWSD) and the Northern Circumpolar Soil Carbon Database (NCSCD). Model estimates of global soil carbon stocks ranged from 510 to 3040 Pg C, compared to an estimate of 1260 Pg C (with a 95% confidence interval of 890–1660 Pg C) from the HWSD. Model simulations for the high northern latitudes fell between 60 and 820 Pg C, compared to 500 Pg C (with a 95% confidence interval of 380–620 Pg C) for the NCSCD and 290 Pg C for the HWSD. Global soil carbon varied 5.9 fold across models in response to a 2.6-fold variation in global net primary productivity (NPP) and a 3.6-fold variation in global soil carbon turnover times. Model–data agreement was moderate at the biome level (R2 values ranged from 0.38 to 0.97 with a mean of 0.75); however, the spatial distribution of soil carbon simulated by the ESMs at the 1° scale was not well correlated with the HWSD (Pearson correlation coefficients less than 0.4 and root mean square errors from 9.4 to 20.8 kg C m−2). In northern latitudes where the two data sets overlapped, agreement between the HWSD and the NCSCD was poor (Pearson correlation coefficient 0.33), indicating uncertainty in empirical estimates of soil carbon. We found that a reduced complexity model dependent on NPP and soil temperature explained much of the 1° spatial variation in soil carbon within most ESMs (R2 values between 0.62 and 0.93 for 9 of 11 model centers). However, the same reduced complexity model only explained 10% of the spatial variation in HWSD soil carbon when driven by observations of NPP and temperature, implying that other drivers or processes may be more important in explaining observed soil carbon distributions. The reduced complexity model also showed that differences in simulated soil carbon across ESMs were driven by differences in simulated NPP and the parameterization of soil heterotrophic respiration (inter-model R2 = 0.93), not by structural differences between the models. Overall, our results suggest that despite fair global-scale agreement with observational data and moderate agreement at the biome scale, most ESMs cannot reproduce grid-scale variation in soil carbon and may be missing key processes. Future work should focus on improving the simulation of driving variables for soil carbon stocks and modifying model structures to include additional processes.

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Causes of variation in soil carbon simulations from CMIP5 Earth system models and comparison with observations

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 principles of cryostratigraphy.

Deep Yedoma permafrost: A synthesis of depositional characteristics and carbon vulnerability

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

Syngenetic permafrost forms when alluvial, aeolian and/or colluvial sediment accumulates under cold-climate conditions. Observations from within the CRREL permafrost tunnel near Fairbanks, Alaska, indicate that layered, lenticular-layered and micro-lenticular cryogenic structures are characteristic of this type of permafrost. In contrast, reticulate cryogenic structures indicate local thaw modification. During the growth of syngenetic permafrost, episodes of thermokarst erosion may operate preferentially along ice wedges leading to the development of gullies and tunnels in the near-surface sediments. The local thaw unconformities that result are inferred by the recognition of thermokarst-cave ice (‘pool’ ice), and various soil and ice pseudomorphs. These may be regarded as further characteristics of syngenetic permafrost growth. Copyright © 2004 John Wiley & Sons, Ltd.

Syngenetic permafrost growth: cryostratigraphic observations from the CRREL tunnel near Fairbanks, Alaska

Cryostratigraphicmapping inthe main shaft of the CRREL tunnelindicates secondary modiﬁcationoforiginal Pleistocene-age syngenetic permafrost. Layered, lenticular-layered and micro-lenticularcryostructures within undisturbed silt characterise the syngenetic permafrost. Thawed and refrozendeposits are characterised by massive and reticulate-chaotic cryostructures. The micro-morphology oftypical cryostructures, as observed through an environmental scanning electron microscope (ESEM),is described. It is inferred that preferential cryogenic modiﬁcation has occurred due to ﬂuvio-thermalerosion operating along ice wedges. Soil and ice pseudomorphs are the manifestation of this process.Gravel, silt, ice and mixed (silt/ice) pseudomorphs occur within silt. Ice pseudomorphs are formed bythermokarst-cave ice (‘pool ice’) which ﬁlled thermokarst pits, channels and gullies. Copyright #2006 John Wiley & Sons, Ltd. KEY WORDS : permafrost; ground ice; cryostructures; thermokarst; pseudomorphs

Further Cryostratigraphic Observations in the CRREL Permafrost Tunnel, Fox, Alaska

Partially eroded ice wedges and lenticularly shaped bodies of massive thermokarst-cave ice in ice-rich syngenetic permafrost (yedoma) are exposed in the CRREL tunnel near Fairbanks, Alaska The ice wedges, which formed 25 000 − 40 000 years ago, were subsequently affected by localised thermal erosion, resulting in underground cavities that filled with surface water infiltrating through a network of conduits This water froze inward from the walls of the cavity We report the biogeochemical characteristics of one of these thermokarst-cave ice features and four nearby ice wedges The thermokarst-cave ice has 30 times the dissolved organic carbon concentration, 20 times the total dissolved nitrogen concentration and five to 20 times the inorganic solute concentrations of the surrounding (original) ice wedge material Based on these results we present a schematic model to describe how the thermokarst-cave ice was formed and preserved and what processes led to its current biogeochemical characteristics Current estimates of soluble solutes stored in permafrost may underestimate the total carbon and nutrient load where wedge material has been extensively replaced by surface water rich in organic carbon, nutrients or inorganic solutes Published in 2011 by John Wiley & Sons, Ltd

Biogeochemical and geocryological characteristics of wedge and thermokarst-cave ice in the CRREL permafrost tunnel, Alaska†

A large scale 105-m-long, 0.9-m-diam chilled pipeline experiment was conducted to assess the response of a production pipeline that transits between unfrozen (seasonally frozen) and permafrost soil...

Matthew T. Bray

Papers

Syngenetic permafrost growth: cryostratigraphic observations from the CRREL tunnel near Fairbanks, Alaska

Further Cryostratigraphic Observations in the CRREL Permafrost Tunnel, Fox, Alaska

Biogeochemical and geocryological characteristics of wedge and thermokarst-cave ice in the CRREL permafrost tunnel, Alaska†

Field Investigation of Soil Heave by a Large Diameter Chilled Gas Pipeline Experiment, Fairbanks, Alaska

The influence of cryostructure on the creep behavior of ice-rich permafrost