HR: 1340h
AN: B33A-0232    [Abstracts]
TI: The Influence of Biospheric Temperature Acclimation and Choice of Temperature Response Function on Feedbacks in Coupled Climate-Carbon Cycle Models
AU: * King, A W
EM: kingaw@ornl.gov
AF: Environmental Sciences, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6335 United States
AU: Post, W M
EM: postwmiii@ornl.gov
AF: Environmental Sciences, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6335 United States
AU: Tharp, M L
EM: tharpmp@ornl.gov
AF: Computational Sciences and Engineering, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6335 United States
AU: Erickson, D J
EM: ericksondj@ornl.gov
AF: Computer Science and Mathematics, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6016 United States
AU: Thompson, S L
EM: thompson59@llnl.gov
AF: Atmospheric Science, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550-9234 United States
AB: Future CO$_2$ concentrations and global mean temperatures are higher in general circulation models that include an interactive global carbon cycle than in those without the coupling. However, the magnitude of this positive climate-carbon cycle feedback varies among models. The differences can largely be attributed to terrestrial biosphere response, with higher ecosystem respiration at higher temperatures an important contributor. We are thus investigating the impact of different models of the temperature response of ecosystem respiration on simulated climate-carbon feedbacks. We focus on temperature acclimation of heterotrophic (soil) respiration (R$_h$), because acclimation of R$_h$ to warmer temperatures is hypothesized to reduce climate-carbon feedbacks. We have implemented alternative temperature functions for R$_h$, including those with temperature acclimation, in a site-scale ecosystem model (LoTEC), a global terrestrial biogeochemistry model (GTEC 2.0), a global integrated terrestrial biosphere model (IBIS 2.5), and a coupled climate-carbon cycle model (INCCA), which couples the Parallel Climate Model (PCM) with IBIS. Simulations with the ecosystem and uncoupled biosphere models show that the choice of temperature function strongly influences the accumulation of carbon in soil over time and space, and these differences in soil carbon stocks influence the release of CO$_2$ to the atmosphere via first order, mass-dependent decay dynamics. These indirect effects are more important in determining climate-carbon feedbacks at warmer temperatures than the primary effects of temperature on rates of respiration per unit mass of carbon. In some simulations, total respiratory flux of carbon to the atmosphere in projected warmer climates was virtually the same with or without acclimation because of compensating differences in carbon accumulation at earlier cooler temperatures. When the coupled climate-carbon model is forced by historical fossil-fuel emissions, total global soil carbon increases at 0.5 Gt C y$^{-1}$ when R$_h$ is modeled with a temperature response function exhibiting moderate acclimation to temperature but declines at 0.3 Gt C y$^{-1}$ when R$_h$ is modeled with a simple Q10 temperature function showing no acclimation. These results suggest that the choice of temperature functions for terrestrial R$_h$ can influence simulated climate-carbon feedbacks in coupled models. However, the influence is largely through indirect effects on soil carbon stocks, rather than acclimation or other differences in temperature dependent rates per se. Differences among temperature functions in the range of 10-35 $^0$C are likely more important than differences under warmer, but rarely experienced, conditions. Accordingly, when choosing a temperature response function, careful attention should be given to accurate simulation of soil respiration at historically prevailing temperatures.
DE: 4805 Biogeochemical cycles (1615)
DE: 1615 Biogeochemical processes (4805)
DE: 1620 Climate dynamics (3309)
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting