Future Climate Change Scenarios
The models we use to simulate ecosystem responses to future climate change require estimates of future climates. To create input climate datasets for our models, we use coupled ocean-atmosphere general circulation model (OAGCM) data from the World Climate Research Programme's (WCRP's) Coupled Model Intercomparison Project phase 3 (CMIP3) multi-model dataset
OAGCMs are typically run at very coarse spatial resolutions with model grid cells spanning hundreds of kilometers in latitude and longitude. To create input climate data at finer spatial resolutions we downscale the climate data using various interpolation techniques. The figure below displays downscaled annual temperature anomalies as simulated by ten different OAGCMs for the period 2071-2100 (30-year mean). Although the models simulate different regional patterns of future temperature changes, they all simulate warmer future conditions. By using climate data from multiple OAGCMs produced under multiple emissions scenarios we are able to evaluate simulated ecosystem responses to the range of potential future climate changes these models currently forecast.
Simulated Vegetation Change
Vegetation will be significantly affected by future climate change. In addition to responding to changes in temperature and precipitation, plants will respond to changes in atmospheric carbon dioxide (CO2) concentrations. To simulate vegetation responses to future climate change we use mechanistic vegetation models, such as BIOME4 (Kaplan et al. 2003) and LPJ (Lund-Potsdam-Jena, Sitch et al. 2003). These models simulate the distribution of plant functional types (e.g., grass, evergreen needle-leaved trees), which can be combined to represent biomes and habitat types. These same vegetation models that we use to explore future climate change are also used to investigate paleovegetation change as part of the USGS Climate Change and Land Use Sensitivity (CLUES) Project.
Future vegetation simulations for the western United States (see figure below) reveal the effects of changes in temperature, precipitation, and atmospheric CO2 concentrations on the distribution of simplified habitat types. These simulations were created using a modified version of the equilibrium vegetation model BIOME4, run with simulated climate data for 2071-2100 (30-year mean) and a future atmospheric CO2 concentration of 540 ppm. Under all three future simulations, woody vegetation (e.g., forest, shrubland) is simulated to expand and areas of subalpine forest, tundra, and grassland are simulated to contract.
These simulated future vegetation changes are relatively large, and there are important caveats that must be considered when interpreting these results. BIOME4 is an equilibrium vegetation model, which means that it simulates the vegetation that would occur if the 30-year mean climate for, in this case, 2071-2100, was the long-term mean climate. Future climate change, however, is projected to change throughout the coming century. These vegetation simulations also do not include the potential effects of many ecological processes that will be important in determining plant responses to climate change, including changes in disturbance regimes (e.g., fire frequency, insect outbreaks), plant migration rates, changing human land use activities, etc. Thus, the future vegetation simulations displayed in the figure above should not be interpreted as predictions of future vegetation change at any particular location in the western United States, but instead as experimental results that provide us with information about the potential magnitude, rate and spatial pattern of future changes.
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