This chapter contrasted HadCM3LC simulations of Amazonian forest at the Last Glacial Maximum (21 kya) and a Younger Dryas-like period (13-12 kya) with predicted responses of future warming to provide estimates of the climatic limits under which the Amazon forest remains relatively stable. Simulations indicate that despite lower atmospheric CO2 concentrations and increased aridity during the LGM, Amazonia remains mostly forested, and that the cooler climate of the Younger Dryas-like period in fact causes a trend towards increased above-ground carbon balance relative to today. The vegetation feedbacks responsible for maintaining forest integrity in past climates (i.e. decreased evapotranspiration and reduced photorespiration) cannot be maintained in the future. Although elevated atmospheric CO2 contributes to a positive enhancement of plant carbon and water balance, decreased stomatal conductance and increased plant and soil respiration cause a positive feedback that amplifies localised drying and climate warming. The Amazonian forest appears to be presently near its critical resiliency threshold, and even minor climate warming may be sufficient to promote deleterious feedbacks on forest integrity.

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