Supplementary material to “A Scientific Synthesis and Assessment of the Arctic Carbon Cycle”

Daniel J. Hayes, Department of Biology and Wildlife, University of Alaska Fairbanks; Laodong Guo, Department of Marine Science, University of Southern Mississippi, Hattiesburg; A. David McGuire, Department of Biology and Wildlife, University of Alaska Fairbanks

Citation:
Hayes, D. J., L. Guo, and A. D. McGuire (2007), A scientific synthesis and assessment of the Arctic carbon cycle, Eos Trans. AGU, 88(26), 270. [Full Article (pdf)]

The release of large stores of carbon in land and ocean systems of the Arctic in response to climate change have the potential to substantially increase the concentration of the radiatively active gases carbon dioxide (CO2) and methane (CH4), which would act as a positive feedback to climate change and have consequences both within and outside the Arctic. However, the biogeochemical consequences of arctic warming, carbon inventory of different reservoirs, and fluxes across land/ocean/atmosphere interfaces are poorly quantified. A synthesis of arctic system carbon cycle science was proposed by the Arctic Monitoring and Assessment Program (AMAP) as an arctic climate impact assessment action. The product of this assessment will be a scientific report on the current state of carbon stocks and flux in the Arctic, and their potential sensitivities to change.

AMAP, along with the Climate and Cryosphere (CliC) Project and the International Arctic Science Committee (IASC), sponsored the Arctic Carbon Cycle Assessment Workshop at the Red Lion Hotel in Seattle, Washington, USA between 27 February and 1 March 2007. The workshop was held in a general effort toward the scientific synthesis and assessment of the Arctic System Carbon Cycle, as well as to generate feedback on the working draft of an assessment document. The initial assessment was prepared by the Arctic Carbon Cycle Assessment writing team, which is lead by A. David McGuire (University of Alaska Fairbanks, USA) and includes Leif Anderson (Göteborg University, Sweden), Torben Christensen (Lund University, Sweden), Scott Dallimore (Natural Resources Canada), Laodong Guo (The University of Southern Mississippi, USA), Martin Heimann (Max Planck Institute, Germany), Robie MacDonald (Department of Fisheries and Oceans, Canada), and Nigel Roulet (McGill University, Canada).

The specific objectives of the workshop were to

  1. present the initial assessment to a medium size group of experts, and
  2. identify how the assessment can be improved.

The workshop brought together leading researchers in the fields of terrestrial, marine and atmospheric science to report on and discuss the current state of knowledge on contemporary carbon stocks and fluxes in the Artic and their potential responses to a changing climate. The workshop was attended by 35 scientists representing institutions from 10 countries in addition to two representatives of the sponsor agencies (John Calder for AMAP and Diane Verseghy for CliC).

The Assessment of Carbon Stocks and Flux in the Arctic

The first day of the workshop was designed to introduce the topic of the Arctic Carbon Cycle Assessment and provide the framework for subsequent discussion by beginning with a series of presentations by the writing team on the current state of knowledge regarding the stocks and flux in the system. The foundation for the presentations and discussions to follow was laid with a background presentation defining the Arctic Carbon Cycle both in terms of the geographic extent as well as the major stocks and fluxes that are being considered in the assessment. In the development of these definitions, the assessment team considered the Arctic Carbon Cycle from atmospheric, terrestrial, hydrologic, cryospheric and marine perspectives. The assessment intends to use these somewhat flexible guidelines in synthesizing information generated by studies from these various perspectives.

The major carbon pools in the Arctic were described and estimates of the current stocks in terrestrial and marine systems were presented by members of the writing team. Synthesizing data and results from various “bottom-up” sources, including inventories and process model simulations, the important terrestrial pools identified included soil organic matter, vegetation organic carbon, “soil gases” (CO2 and CH4), gas hydrates at depth, limestone and fossil fuels. In addition, the important carbon pools in Arctic marine systems and the current estimates of these stocks were presented. The marine assessment considered carbon pools of the continental shelf, the deep arctic basin and the Nordic seas in the form of dissolved inorganic and organic carbon (DIC and DOC), CH4, new primary production (PP) and particulate carbon (POC) in sediment.

The subsequent discussions on the current state of knowledge regarding Arctic carbon fluxes were initiated with a presentation on atmospheric perspectives on CO2 and CH4 exchange. Here, results from atmospheric inversion models were compared with other estimates of terrestrial and marine exchange with the atmosphere based on flask sampling, land-based inventories, process model simulations and remotely sensed data. Land–atmosphere exchange monitoring studies are still only few and sporadic in the circumpolar North and there is a documented large inter-annual and inter-site variability, which makes it difficult to generalize about the current status of the Arctic as a whole. However, terrestrial ecosystems of the Arctic are most likely currently contributing to global warming because of large emissions of CH4.

The current data and uncertainty pertaining to the horizontal exchanges of DOC, DIC, and particulate organic carbon (POC) from arctic terrestrial ecosystems to arctic marine ecosystems, including Arctic riverine export fluxes and coastal erosion, were presented. Riverine export fluxes of DOC are likely underestimated while DIC fluxes are overestimated, largely due to under sampling during ice opening season where river discharge and DOC concentrations are at the highest for arctic rivers. Lateral carbon transport through coastal erosion is equivalent to riverine POC fluxes but much smaller than riverine DOC fluxes. The fate of terrestrial organic carbon during their transport and once they are delivered to the Arctic Ocean was also discussed but remains uncertain.

The current exchanges of CO2 and CH4 between the atmosphere and marine systems of the Arctic were also considered in the assessment. The exchanges of CO2 and CH4 between the atmosphere and marine systems of the Arctic depend heavily on sea ice dynamics, brine formation and biological productivity. The air–sea flux of these gases is small on a global scale but could be significantly altered in a climate change scenario. The exchange of carbon with surrounding oceans is significant, but the net transport is small.

The writing team closed the first day’s talks with a summary and synthesis of the state of knowledge on the Arctic Carbon Cycle, illustrated by a diagram labeled with the current carbon stock and CO2/CH4 flux estimates for each component. The issues of sensitivity and uncertainty in the Arctic Carbon Cycle, to be discussed by the group in the following day’s meetings and break-out sessions, were introduced.

Key Leverage Points in the Arctic Carbon Cycle

The second day’s session was focused around an organization of three break-out groups:

  1. a marine processes group,
  2. a terrestrial and terrestrial to marine processes group and
  3. a sensitivity group.

All three groups were asked to consider the key leverage points of the system and evaluate these potential sensitivities. The complete group reconvened following the break-out sessions and each break-out group provided a summary presentation on their respective discussions to the rest of the group.

The first group pinpointed issues such as marine particulate organic carbon content, ocean sediment turnover rates and CH4 estimates for the water column as important considerations for inclusion in the assessment. The key leverage points in the ocean system were identified by this group as marine gas hydrates, sea bed permafrost, and how changes in sea ice affect the solubility and biological pumps. The second group identified important gaps and uncertainties in available data on current terrestrial carbon stocks, including estimates from varying soil depths, carbon stocks in coarse woody debris, and gaseous carbon forms in the soil and permafrost and as gas hydrates at depth. The group also identified river DOC and particulate inorganic carbon (PIC) flux estimates as important areas of uncertainty in the assessment. The key leverage point with respect to terrestrial sensitivity is associated with changes in landscape wetness/dryness. The third group discussed the potential sensitivity of the Arctic Carbon Cycle on a 50 to 100 year time scale and in the context of the impact of Arctic carbon cycle feedbacks on the global carbon cycle. Issues related to disturbance (e.g. fire regime) and land cover change (e.g. variability in wetland expansion and drying) were identified as key driving forces that will determine the future state of carbon stores and CO2 and CH4 fluxes in the Arctic.

Improving the Assessment

In general, the scientists attending the workshop were very positive and enthusiastic about the assessment. The scientists offered some advice for technically improving the representation of the current state of the Arctic. The break-out sessions were very useful for identifying which parts of the carbon cycle in the Arctic were most sensitive to climate change in the next 50 to 100 years and how the understanding of the carbon cycle in the Arctic can be improved. A final discussion followed on steps to be taken to improve our understanding of responses of the Arctic Carbon Cycle. First, the assessment needs to identify key processes that need to be better studied. The assessment document and future research should focus on the most sensitive components of the system. From here, key data daps can be identified and become the focus of further research.

Authors:

Daniel J. Hayes, Department of Biology and Wildlife, University of Alaska Fairbanks; E-mail: ffdjh1@uaf.edu; Laodong Guo, Department of Marine Science, University of Southern Mississippi, Hattiesburg; and A. David McGuire, Department of Biology and Wildlife, University of Alaska Fairbanks