- Lead Authors:
- Edward A. G. Schuur, Northern Arizona University
- A. David McGuire, U.S. Geological Survey and University of Alaska, Fairbanks
- Vladimir Romanovsky, University of Alaska, Fairbanks
- Contributing Authors:
- Christina Schädel, Northern Arizona University
- Michelle Mack, Northern Arizona University
<b>Schuur</b>, E. A. G., A. D. <b>McGuire</b>, V. <b>Romanovsky</b>, C. Schädel, and M. Mack, 2018: Chapter 11: Arctic and boreal carbon. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 428-468, https://doi.org/10.7930/ SOCCR2.2018.Ch11.
Arctic and Boreal Carbon
SUPPORTING EVIDENCE
KEY FINDINGS
Key Finding 1
Factors that control terrestrial carbon storage are changing. Surface air temperature change is amplified in high-latitude regions, as seen in the Arctic where temperature rise is about 2.5 times faster than that for the whole Earth. Permafrost temperatures have been increasing over the last 40 years. Disturbance by fire (particularly fire frequency and extreme fire years) is higher now than in the middle of the last century (very high confidence).
Description of evidence base
Key Finding 1 is supported by observational evidence from ground-based and remote-sensing measurements. Documented changes in surface air temperatures (data.giss.nasa.gov/gistemp/maps) at a rate higher than the global average are consistent with model projections (Overland et al., 2014) and theory (Pithan and Mauritsen 2014). Permafrost temperatures documented in borehole networks (Biskaborn et al., 2015) are increasing, with the largest absolute temperature increases in cold permafrost regions (Noetzli et al., 2016; Romanovsky et al., 2016). Decadal trends (Flannigan et al., 2009; Kasischke and Turetsky 2006) and paleoecological reconstructions (Kelly et al., 2013) show that area burned, fire frequency, and extreme fire years are higher now than in the first half of the last century and likely will last even longer.
Major uncertainties
Data are not collected uniformly across regions and often are limited by site access. High-latitude observation stations are limited as well. Boreholes often are not located at sites where abrupt permafrost change is evident (Biskaborn et al., 2015). Area burned and other metrics of fire severity can be quantified by remote sensing, but some metrics rely on more limited ground-truth information. Direct measurements of permafrost temperature and fire extend back only 50 to 60 years, but these factors can respond to drivers (e.g., past temperature fluctuations and fire cycles) over even longer time intervals.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
There is high confidence that drivers of carbon pool change are increasing in strength. In addition, there is very high confidence that surface air temperature change is amplified in high-latitude regions, as seen in the Arctic, where temperature rise is about 2.5 times faster than that for the entire planet. There is high confidence that permafrost temperatures have been rising and that fire disturbance is increasing, although the data records for the latter are shorter compared to temperature records.
Summary sentence or paragraph that integrates the above information
For Key Finding 1, there is very high confidence that drivers of carbon pool changes are increasing in strength. Key Finding 1 is supported by a large amount of observational evidence documented in the peer-reviewed literature. Similar statements previously have been made in assessments of Arctic climate change, including IPCC (2013) and Melillo et al. (2014). Key uncertainties are the length of the data records and the limited ground-based information for variables such as fire severity.
Key Finding 2
Soils in the northern circumpolar permafrost zone store 1,460 to 1,600 petagrams of organic carbon (Pg C), almost twice the amount contained in the atmosphere and about an order of magnitude more carbon than contained in plant biomass (55 Pg C), woody debris (16 Pg C), and litter (29 Pg C) in the boreal and tundra biomes combined. This large permafrost zone soil carbon pool has accumulated over hundreds to thousands of years. There are additional reservoirs in subsea permafrost and regions of deep sediments that are not added to this estimate because of data scarcity (very high confidence).
Description of evidence base
Key Finding 2 is supported by observational evidence from ground-based measurements of ecosystem carbon pools. Large surface soil carbon pools (to 1 m in depth) have been reported in the literature for decades (e.g., Gorham 1991), with new information on deeper permafrost carbon pools accumulating over the last decade (Hugelius et al., 2014; Schuur et al., 2015; Tarnocai et al., 2009; Zimov et al., 2006). Biomass pools have been synthesized from forest inventory data (Pan et al., 2011), and more recently using remote sensing (Neigh et al., 2013; Raynolds et al., 2012).
Major uncertainties
Soils data are not collected uniformly across regions and often are limited by site access (Johnson et al., 2011). Deep-soil inventories (>1 m in depth) are much more limited than surface soil information (Hugelius et al., 2014). Biomass inventories often exclude unmanaged forests, which are prevalent in this region (Pan et al., 2011). Aboveground plant biomass is best quantified, whereas root biomass most often is estimated (Saugier et al., 2001). Coarse wood and litter also are poorly known carbon pools, and, in some cases, large-scale estimates for these pools are model derived.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
There is very high confidence that permafrost soil carbon stocks are large and protected currently by waterlogged and frozen soil conditions across much of the region. There is also very high confidence that soil carbon stocks are more than 10 times larger than stocks of carbon in plant biomass, woody debris, and litter pools.
Summary sentence or paragraph that integrates the above information
In Key Finding 2, there is very high confidence that permafrost soil carbon stocks are large and protected currently by waterlogged and frozen soil conditions across much of the region. There is also very high confidence that soil carbon stocks are more than 10 times larger than stocks of carbon in plant biomass, woody debris, and litter pools. This Key Finding is supported by a large amount of observational evidence documented in the peer-reviewed literature. The key uncertainty is the scarcity of measurements for deep permafrost soil carbon relative to those for surface soils, biomass inventories in unmanaged forests, and belowground biomass.
Key Finding 3
Following the current trajectory of global and Arctic warming, 5% to 15% of the soil organic carbon stored in the northern circumpolar permafrost zone (mean 10% value equal to 146 to 160 Pg C) is considered vulnerable to release to the atmosphere by the year 2100. The potential carbon loss is likely to be up to an order of magnitude larger than the potential increase in carbon stored in plant biomass regionally under the same changing conditions (high confidence, very likely).
Description of evidence base
Key Finding 3 is supported by observational and modeling evidence from a range of literature sources and synthesized by Schuur et al. (2015). Observational data include soil incubation studies (Schädel et al., 2014, 2016) and synthesis of field observations (Belshe et al., 2013). Modeling evidence includes Burke et al. (2012), Burke et al. (2013), Koven et al. (2011), MacDougall et al. (2012), Schaefer et al. (2011), Schaphoff et al. (2013), Schneider von Deimling et al. (2012), and Zhuang et al. (2006).
Major uncertainties
This estimate is based largely on estimates of top-down permafrost thaw as a result of a warming climate and does not include abrupt permafrost thaw processes that can expose permafrost soils to higher temperature more rapidly than predicted by top-down thaw alone. Increasing evidence suggests that abrupt thaw processes are likely to be widespread across Arctic and boreal regions (Olefeldt et al., 2016). Waterlogging (oxygen limitation) is common in surface and subsurface soils because of limited infiltration as a result of permafrost. Oxygen limitation slows the decomposition of organic matter, but both wetter or drier soil conditions can result from degrading permafrost at the site scale. Whether high-latitude terrestrial ecosystems will be wetter or drier in the future at the landscape scale is unclear.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
There is high confidence that permafrost soil carbon stocks are vulnerable to loss with changing climate conditions. This is also true of changing plant biomass but with more uncertainty about the relative magnitude of change.
Estimated likelihood of impact or consequence, including short description of basis of estimate
Thawing permafrost has significant impacts on the global carbon cycle, serving as a source of carbon dioxide (CO2) and methane (CH4) emissions. The level of emissions projected here very likely will accelerate the rate of global climate change. Future emissions from the permafrost zone are expected to be a fraction of those from fossil fuels, but they may be similar to current estimates of land-use change emissions.
Summary sentence or paragraph that integrates the above information
For Key Finding 3, there is high confidence that permafrost soil carbon stocks are vulnerable to loss with changing climate conditions. Thawing permafrost has a significant impact on the global carbon cycle, serving as a source of CO2 and CH4 emissions. Permafrost-zone emissions levels are expected to be a fraction of those from fossil fuels, but they may be similar to current estimates of land-use change emissions. Key Finding 3 is supported by observational and modeling evidence documented in the peer-reviewed literature. Primary key uncertainties include the influence of abrupt thaw processes that can expose permafrost soil carbon much more rapidly than top-down thawing, which is the process represented by model projections. Also unclear is the degree to which soil waterlogging will increase or decrease as permafrost degrades, which influences the relative release of CO2 and CH4.
Key Finding 4
Some Earth System Models project that high-latitude carbon releases will be offset largely by increased plant uptake. However, these findings are not always supported by empirical measurements or other assessments, suggesting that structural features of many models are still limited in representing Arctic and boreal zone processes (very high confidence, very likely).
Description of evidence base
Key Finding 4 is supported by observational and modeling evidence from a range of literature sources. Modeling results are based on a permafrost carbon model intercomparison project that summarizes the results for 1960 to 2009 for 15 Earth System Models (McGuire et al., 2016) and on an earlier model intercomparison of dynamic global vegetation models for high latitudes (Qian et al., 2010). Observational data include tundra and boreal normalized difference vegetation index (NDVI) trend studies (Beck and Goetz 2011; Epstein et al., 2015) and expert assessment (Abbott et al., 2016).
Major uncertainties
NDVI trends represent changes in canopy and thus are not directly measuring carbon pools; observational datasets at regional to continental scales in the Arctic are scarce, making model evaluation difficult.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
There is high confidence that model projections are not always in agreement with observational constraints about plant carbon uptake offset.
Estimated likelihood of impact or consequence, including short description of basis of estimate
Thawing permafrost has significant impacts to the global carbon cycle, serving as a source of CO2 and CH4 emissions. Plant uptake may offset some of these releases, but the mismatch between models and observations may cause significant over- or underestimates of this offset, as well as shift the timing of significant net carbon change for this region.
Summary sentence or paragraph that integrates the above information
For Key Finding 4, there is high confidence that model projections are not always in agreement with observational constraints about plant carbon uptake offset. Thawing permafrost has significant impacts to the global carbon cycle, serving as a source of CO2 and CH4 emissions. Plant uptake may offset some of that release, but the mismatch between models and observations may cause significant over- or underestimates of this offset, as well as shift the timing of significant net carbon change for this region. Key Finding 4 is supported by observational and modeling evidence documented in the peer-reviewed literature. Primary key uncertainties include the response of plant growth to multiple global change factors, including primarily CO2 fertilization but also rising temperatures, changes in precipitation and growing season length, and changes in species distribution. Other uncertainties include deposition and storage of new carbon into surface soils.
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