Slide 31: Examples of non-linear changes
Most GCMs show a slowing of the Atlantic Thermohaline Circulation as the world heats up. However, some show the circulation stopping entirely as heating reaches a threshold value. (Manabe and Stouffer, 1988). The shutdown does not occur abruptly, but the speed with which it stops (decades to centuries) is affected by the rate of heating.
Sea ice in the northern seas is set to reduce, and this process may be accelerated by feedback processes associated with the concurrent drop in salinity and reduced albedo.
Sea level rise may destabilise large polar ice masses, ice sheets, or even entire ice shelves, accelerating sea level rise.
Observed variability of ENSO indicate a transition to increased occurrence of ENSO in 1976, although not enough is know to say whether this is an anthropogenic effect, or even if it is a long-term transition.
Large-scale (possibly irreversible) transformations in the biosphere such as the growth of the Sahara desert (Claussen et al., 1999), have occurred even with minimal anthropogenic interaction. These can be seen as non-linear changes triggered by slow changes in forcing factors, and it seems highly possible that this could occur given the current level of anthropogenic disturbance. However, not enough is know about this incredibly complex system to say this with any degree of certainty.
Claussen, M., C. Kubatzki, V. Brovkin, A. Ganopolski, P. Hoelzmann and H.-J. Pachur, 1999: Simulation of an abrupt change in Saharan vegetation in the mid-Holocene. Geophys. Res. Lett., 26, 2037-2040.
Manabe, S. and R.J. Stouffer, 1988: Two stable equilibria of a coupled ocean atmosphere model. J. Clim., 1, 841-866