Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 10 6 2006 10.5194/hess-10-903-2006 http://www.hydrol-earth-syst-sci.net/10/903/2006/ http://www.hydrol-earth-syst-sci.net/10/903/2006/hess-10-903-2006.html http://www.hydrol-earth-syst-sci.net/10/903/2006/hess-10-903-2006.pdf 903 912 2006-11-28 Abrupt change in climate and climate models A. J. Pitman apitman@els.mq.edu.au R. J. Stouffer Department of Physical Geography, Macquarie University, NSW, 2109 Australia Dynamics laboratory, Princeton, NJ, 08542, USA First, we review the evidence that abrupt climate changes have occurred in the past and then demonstrate that climate models have developing capacity to simulate many of these changes. In particular, the processes by which changes in the ocean circulation drive abrupt changes appear to be captured by climate models to a degree that is encouraging. The evidence that past changes in the ocean have driven abrupt change in terrestrial systems is also convincing, but these processes are only just beginning to be included in climate models. Second, we explore the likelihood that climate models can capture those abrupt changes in climate that may occur in the future due to the enhanced greenhouse effect. We note that existing evidence indicates that a major collapse of the thermohaline circulation seems unlikely in the 21st century, although very recent evidence suggests that a weakening may already be underway. We have confidence that current climate models can capture a weakening, but a collapse in the 21st century of the thermohaline circulation is not projected by climate models. Worrying evidence of instability in terrestrial carbon, from observations and modelling studies, is beginning to accumulate. 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