The possibility of abrupt climate change and/or abrupt changes in the earth system triggered by climate change, with potentially catastrophic consequences, cannot be ruled out. (Meehl et al., 2007).
Disintegration of the West Antarctic Ice Sheet (See Meehl et al., 2007), if it occurred, could raise sea level by 4-6 meters over several centuries.
Deep ocean conveyor
A shutdown of the North Atlantic Thermohaline Circulation (See Meehl et al., 2007) could have far-reaching, adverse ecological and agricultural consequences,
Increases in the frequency of droughts (Salinger, 2005)
or a higher intensity of tropical cyclones (See Meehl et al., 2007) could occur.
Positive feedback from warming may cause the release of carbon or methane from the terrestrial biosphere and oceans (See Meehl et al., 2007), which would add to the mitigation required.