Climate change: Possible scenarios

A summary of a forecasting (sort of) report at RealClimate:

The group imagined three potential scenarios, labeled expected, severe, and catastrophic. These are not forecasts exactly, since forecasting society is even harder than forecasting climate, which is itself pretty dicey on a regional spatial scale, but rather a fleshing out of plausible possibilities, a story-telling, visualization-type exercise.

The "expected" scenario calls for 1.3 °C of warming globally above 1990 levels, by the year 2040. Changes in precipitation and sea level prompt migration at a scale sufficient to challenge the cohesion of nations. The potential responses to this scenario are broken down into specific regions with their particular historical and political settings. Just to pick a region at random, Nigeria in West Africa will suffer accelerated desertification with climate change, prompting intensified migration into the megacity of Lagos, which is itself threatened by sea level rise. Compounding Nigeria's misfortune, there is oil in the Niger Delta, and as global oil supplies dwindle, the strife and corruption that oil brings a weak nation will only intensify.

In the "severe" scenario, the globe warms by 2.6 °C by 2040 and sea level rises about a half a meter. Scientists in 2040 conclude that the eventual collapse of Greenland and the West Antarctic ice sheets has become inevitable in the centuries that follow. Agricultural production declines in the arid subtropics and in increasingly flooded river deltas. Again to pick a random example from the report: the river systems in the American Southwest collapse, leading to impoverishment of Northern Mexico and increased migration pressure in the U.S. Resource stress in Latin American leads to a tendency toward populist, Chavez-type governments, and more extensive regions of de facto anarchy such as found today in parts of Colombia.

The "catastrophic" scenario assumes positive feedbacks in the carbon cycle to warm the planet by 5.6 °C by the year 2100, and sea level has risen by 2 meters. I feel compelled to note that if this is supposed to be a worst-case scenario, I personally can imagine worse in terms of sea level rise. In the social realm the crystal ball gets murkier as the report progresses from expected to severe to catastrophic, but one important ingredient in the prognosis for the catastrophic scenario is the migration of millions of people, a scale unprecedented in human history, potentially enough to undermine the stability of civilized governance. One participant recommended that we check out the movie Mad Max, only imagine it hotter.