First, we review the evidence that abrupt climate changes have occurred inthe past and then demonstrate that climate models have developing capacityto simulate many of these changes. In particular, the processes by whichchanges in the ocean circulation drive abrupt changes appear to be capturedby climate models to a degree that is encouraging. The evidence that pastchanges in the ocean have driven abrupt change in terrestrial systems isalso convincing, but these processes are only just beginning to be includedin climate models. Second, we explore the likelihood that climate models cancapture those abrupt changes in climate that may occur in the future due tothe enhanced greenhouse effect. We note that existing evidence indicatesthat a major collapse of the thermohaline circulation seems unlikely in the21st century, although very recent evidence suggests that a weakeningmay already be underway. We have confidence that current climate models cancapture a weakening, but a collapse in the 21st century of the thermohalinecirculation is not projected by climate models. Worrying evidence ofinstability in terrestrial carbon, from observations and modelling studies,is beginning to accumulate. Current climate models used by theIntergovernmental Panel on Climate Change for the 4th Assessment Reportdo not include these terrestrial carbon processes. We therefore can not makestatements with any confidence regarding these changes. At present, thescale of the terrestrial carbon feedback is believed to be small enough thatit does not significantly affect projections of warming during the firsthalf of the 21st century. However, the uncertainties in how biologicalsystems will respond to warming are sufficiently large to undermineconfidence in this belief and point us to areas requiring significantadditional work.
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