The Role of High Latitude Ecosystems in the Global Carbon Cycle
ARCSS Land-Atmosphere-Ice Interactions (LAII) Home Page
PROJECT DESCRIPTION
The atmospheric concentrations of the major greenhouse gases continue to increase because of human activity (Watson et al., 1990, 1992; Schimel et al., 1995). Most general circulation models (GCMs) predict that the buildup of these gases is likely to lead to annual increases of 1.5 to 4.5 degrees C in global surface air temperature over the next century (Mitchell et al., 1990), with increases greater in high latitudes than in middle or low latitudes (Hansen and Lebedeff, 1987; Lashof and Ahuja, 1990). Changes of the magnitude estimated by GCMs for high latitudes have the potential to substantially affect both the function and structure of ecosystems in landscapes of arctic and subarctic regions. Functional responses represent changes in the biogeochemical cycling of carbon, nutrients and water in the ecosystems of a landscape (McGuire et al., 1995a). Structural responses represent changes in the species composition of organisms in a landscape, and may further modify the function of ecosystems (Melillo et al., 1996).
There is evidence that warming is occurring in some high-latitude areas (Lachenbruch and Marshall, 1986; Beltrami and Mareschai, 1991; Chapman and Walsh, 1993), and that the warming may be impacting both ecosystem function and structure (Oechel et al., 1993, 1995; Chapin et al., 1995). Temperature increase in high latitudes may influence responses of carbon storage in arctic and subarctic ecosystems. These ecosystems contain approximately 40% of the world's soil carbon inventory that is potentially reactive in the context of near-term climate change (McGuire et al., 1995a; Melillo et al., 1995; McGuire and Hobbie, 1996). a substantial amount of carbon and nitrogen could be released in inorganic forms from these soils in response to elevated temperature (Nadelhoffer et al., 1992). A large release of CO2 from these soils has the potential to influence the growth of atmospheric CO2, which may have consequences for the rate and magnitude of climate change. However, increased nitrogen availability associated with elevated temperature may result in increased net primary production (NPP). This response has the potential to buffer the carbon loss from the soil.
The degree of buffering will depend on the fate of "newly available" nitrogen that has resulted from warming (McGuire et al., 1992). Increased decomposition of soil organic matter because of warming will result in the release of CO2 to the atmosphere. If much of the nitrogen is taken up by the vegetation, the entire ecosystem will function as a net carbon sink because the vegetation has a much higher carbon to nitrogen ratio than the soil (Shaver et al., 1992). Alternatively, if "newly available" nitrogen is lost from the ecosystem, perhaps in the soil solution of a dropping water table, then the ecosystem will function as a net source of CO2 to the atmosphere.
Clearly, to achieve an understanding of the potential for high latitude ecosystems to function as a net source or a net sink for CO2 in response to climate change requires the integration of carbon, water and nitrogen dynamics. Furthermore, the degree to which high latitudes may stabilize or destabilize the atmospheric concentration of CO2 must be measured in the context of other terrestrial ecosystems of the biosphere. Thus, it is important to understand the role of arctic and subarctic ecosystems in the dynamics of the global carbon cycle. In this study, we seek to elucidate the role of high latitude ecosystems in the global carbon cycle with a global terrestrial ecosystem model that integrates carbon, water and nitrogen dynamics in a spatially explicit fashion. By clarifying this role, we will improve our ability to assess the sensitivity of carbon storage in arctic and subarctic ecosystems to potential climate change.
We propose to synthesize and integrate datafrom investigations of carbon cycling at local, regional and global scales in a study with a global terrestrial biogeochemical model, the Terrestrial Ecosystem Model (TEM; Raich et al., 1991; McGuire et al., 1992, 1993, 1995a, 1996a, 1996b; McGuire and Joyce, 1995; Melillo et al., 1993, 1995, 1996; Melillo, 1994; VEMAP Members, 1995; Kittel et al., 1995; Joyce et al., 1995; McGuire and Hobbie, 1996; Pan et al., 1996a, 1996b; Xiao et al., 1995, 1996a, 1996b; Schimel et al., 1996; Heimann et al., 1996; Kicklighter et al., 1994, 1996; Perez-Garcia, 1996). The goals of this research are: (1) to elucidate the role of high latitude ecosystems in the global carbon cycle and (2) to assess the sensitivity and uncertainty of terrestrial carbon storage responses in high latitudes to potential climate change. The synthesis and integration of data at local scales will include data from several efforts: (1) the LAII Project of the ARCSS NSF Program, (2) the BOREAS campaign of NASA, (3) the LTER efforts in the Arctic (toolik Lake) and subarctic (Bonanza Creek), and (4) process studies at Abisko, Sweden, which are funded as part of ITEX. Investigations at regional and global scales that will contirbute to the proposed study include: (1) the NASA Earth Observing System (EOS) Interdisciplinary Science Team at the Marine Biological Laboratory/University of New Hampshire (MBL/UNH), (2) the Carbon Cycle Model Linkage Project (CCMLP), (3) the NSF-funded study of contemporary water and constituent balances for the Pan-Arctic Drainage System (PADS), (4) the Vegetation/Ecosystem Modeling and Analysis Project (VEMAP), and (5) the MIT Joint Program on the Science and Policy of Global Change. Thus, the proposed research will involve synthesis and integration from an ARCSS component (LAII) with several non-ARCSS components to achieve a multidisciplinary understanding of Arctic System carbon cycling in the context of the global carbon cycle.
Our strategy for this study involves five tasks: