MC1 Model Simulations Suggest that Fire Will Determine whether the Biosphere Acts as a Carbon Source or Sink in the 21st Century

Modeling of net ecosystem production (NEP) and net biome production (NBP) using the MC1 dynamic global vegetation model suggests that fire will be a major determinant as to whether the biosphere acts as a carbon sink or source globally in the 21st century. For some future scenarios the biosphere acts as a carbon sink even with the presence of fire; for others fire losses exceed gains, such as those brought about by CO2 fertilization. Late 21st century fires in the Amazon basin produce most of the losses for those scenarios where they occur. Generally the biggest carbon gains happen in the northern hemisphere between the latitudes of 50 and 70 degrees N, but with occasional significant fire events limiting those gains. This analysis results from the examination of nine scenarios which are a combination of three AR4 GCMs (MIROC3.2 medres, HADCM3, and CSIRO Mk3.0) and three SRES scenarios (A2, A1B, and B1). Data from these GCM simulations were downscaled to a global half degree grid using an anomaly approach. The MC1 model was produced by combining physiologically based biogeographic rules, originally defined in the MAPSS model, with biogeochemical processes packaged in a modified version of CENTURY, and a fire disturbance model, MCFIRE. MC1 produces annual output of a wide range of variables from vegetation carbon and biomass consumed by fire, to deep flow runoff. For this analysis, decadal averages of NBP and biomass consumed by fire were mapped and plotted. The result is a series of maps and graphs showing where and when the big fires occur in the 21st century and the impacts of these fires on average cumulative NBP values temporally and across latitudes.