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Abstract

Future climate change and increasing atmospheric CO2 are expected
to cause major changes in vegetation structure and function
over large fractions of the global land surface. Seven global
vegetation models are used to analyze possible responses to future
climate simulated by a range of general circulation models run
under all four representative concentration pathway scenarios of
changing concentrations of greenhouse gases. All 110 simulations
predict an increase in global vegetation carbon to 2100, but with
substantial variation between vegetation models. For example, at
4 °C of global land surface warming (510–758 ppm of CO2), vegetation
carbon increases by 52–477 Pg C (224 Pg C mean), mainly
due to CO2 fertilization of photosynthesis. Simulations agree on
large regional increases across much of the boreal forest, western
Amazonia, central Africa, western China, and southeast Asia, with
reductions across southwestern North America, central South America,
southern Mediterranean areas, southwestern Africa, and southwestern
Australia. Four vegetation models display discontinuities
across 4 °C of warming, indicating global thresholds in the balance
of positive and negative influences on productivity and biomass. In
contrast to previous global vegetation model studies, we emphasize
the importance of uncertainties in projected changes in carbon residence
times. We find, when all seven models are considered for one
representative concentration pathway × general circulation model
combination, such uncertainties explain 30% more variation in modeled
vegetation carbon change than responses of net primary productivity
alone, increasing to 151% for non-HYBRID4 models. A
change in research priorities away from production and toward structural
dynamics and demographic processes is recommended