Climate change and terrestrial biomass: what if trees do not migrate!

Climate changes induced by doubling atmospheric greenhouse gas (2XGHG) concentrations are expected to affect the distribution of global vegetation and thereby, the amount of carbon it stores. The role of the terrestrial biosphere as a source or sink for carbon during climate change is critical: if increased GHG concentration and warming enhances carbon storage, thereby reducing atmospheric concentrations, the climate changes would also be ameliorated. If instead, carbon storage is reduced, the warming could induce a positive feedback to further increase atmospheric concentrations already on the rise from burning of fossil fuels. Differing climate-defined static vegetation classifications have been used to project biome distributions for climates induced by 2XGHGs. These projections assume that species extirpation and invasion will track perfectly distributions of climate variables and they predict enhanced terrestrial carbon storage. However, theoretical calculations and palaeoecological evidence suggest an alternative, more realistic simplifying assumption: trees will be extirpated but will not invade new territory before 2XGHG climate is attained. We projected global terrestrial carbon under future climates using both assumptions. Simulated terrestrial carbon under delayed immigration decreasd 7 to 34 Pg from modern values in contrast to increases projected under instant migration in this and earlier model exercises.