Confounding all projections of future climate is the possibility of abrupt changes in the climate system, other environmental systems, or human systems. Paleoclimate records indicate that the climate system can experience abrupt changes in as little as a decade. The Earth's temperature is now demonstrably higher than it has been for several hundred years, and GHG concentrations are now higher than they have been in at least 800,000 years. These sharp departures from historical climate regimes raise the possibility that "tipping points" or thresholds for stability might be crossed as the climate system warms, leading to rapid or abrupt changes in climate. Climate change may also lead to abrupt changes in human or ecological systems, especially systems that are also experiencing other environmental stresses. However, in general we have only a limited understanding of where the tipping points in the climate system, other environmental systems, or human systems might be, when they might be crossed, or what the consequences might be.
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