Stability analysis of the climate-vegetation system in the northern high latitudes

The stability of the climate-vegetation system in the northern high latitudes is analysed with three climate system models of different complexity: A comprehensive 3-diniensional model of the climate system, GENESIS-IBIS, and two Earth system models of intermediate complexity (EMICs), CLIMBER-2 and MoBidiC. The biogeophysical feedback in the latitudinal belt 60-70degrees N, although positive, is not strong enough to support multiple steady states: A unique equilibrium in the climate-vegetation system is simulated by all the models on a zonal scale for present-day climate and doubled CO2 climate. EMIC simulations with decreased insolation also reveal a unique steady state. However, the climate sensitivity to tree cover, DeltaT(F), exhibits non-linear behaviour within the models. For GENESIS-IBIS and CLIMBER-2, DeltaT(F) is lower for doubled CO2 climate than for present-day climate due to a shorter snow season and increased relative significance of the hydrological effect of forest cover. For the EMICs, DeltaT(F) is higher for low tree fraction than for high tree fraction, mainly due to a time shift in spring snow melt in response to changes in tree cover. The climate sensitivity to tree cover is reduced when thermohaline circulation feedbacks are accounted for in the EMIC simulations. Simpler parameterizations of oceanic processes have opposite effects on DeltaT(F): DeltaT(F) is lower in simulations with fixed SSTs and higher in simulations with mixed layer oceans. Experiments with transient CO2 forcing show climate and vegetation not in equilibrium in the northern high latitudes at the end of the 20th century. The delayed response of vegetation and accelerated global warming lead to rather abrupt changes in northern vegetation cover in the first half of the 21st century, when vegetation cover changes at double the present day rate.