2 degrees over 200 years might be manageable. Alas, that's now at the very low end of what we're likely to see -- more probably, 2-4 degrees over 100 years. On

2 degrees over 200 years might be manageable. Alas, that's now at the very low end of what we're likely to see -- more probably, 2-4 degrees over 100 years. One problem with a single headline number is that this is an average temperature rise. What we're actually seeing is a mixture of things: more energy accumulating in the hydrosphere and atmosphere means more energetic weather events (e.g. powerful hurricanes/typhoons). Some areas warming lead to other areas cooling (e.g. melting Arctic ice cap and meltwater outflow from the Greenland ice cap may lead to temporary shutdown of the North Atlantic thermohaline circulation and a Gulf Stream stall, freezing the British Isles and Western Europe with a -5 to -10 celsius temperature drop in the short term -- meaning, decades to centuries). So there'll be a rise in the incidence of weather/climate related disasters. Another problem is that we can't simply shift agriculture to new, thawed territories. The Canadian Shield and Siberian Tundra are pretty poor soil, unsuited to intensive wheat/corn growing: it might be possible to condition them, but it'd take decades to centuries of hard work, during which other agricultural territory (the US mid-west and south, much of Asia and sub-Saharan Africa) will be subject to lethal heat emergencies. Lethal not so much for humans -- we know how to build solar-powered air conditioning -- but for crops and livestock which will die in the fields. Suggestions that we're one supermarket supply chain collapse away from the end of civilization are obviously bogus. But if we start having large-scale crop failures 3-5 times a decade, food prices will soar globally, leading eventually to civil unrest and war. (For example, the Arab Spring was to some extend a side-effect of the 2007-08 global financial crisis, as capital sought refuge in commodity futures and the price of bread throughout the Middle East doubled-to-tripled in twelve months: what will 2-4 consecutive wheat harvest failures accomplish?) Infrastructure damage (including agricultural infrastructure) due to strife and mass migration will in turn increase our dependence on surviving infrastructure, reducing our overall resilience. And there may eventually come a tipping point at which we end up in a positive-feedback loop of mutually reinforcing failures.