Those interested in the fine print of the numbers used in these calculations can consult this quite technical study.
Two degrees would still bring all kinds of very undesirable consequences. It would be likely to mean virtually no summer sea ice in the Arctic, the loss of most coral reefs around the world, potentially dramatic declines in total ocean productivity (at least as far as fish are concerned; jellyfish may do quite well), the eventual extinction of hundreds of thousands or even millions of species, significant suppression of total global crop yields (when total food demands are likely to double by 2050), sea level rises of 50-100 cm by 2100 and of many metres over the coming centuries, changes in precipitation patterns leading to both worse droughts and floods, a more fragile Amazon and already the possibility of passing thresholds that could precipitate sudden and irreversible changes. Two degrees is no walk in the park.
While the world agreed that 2ºC ought to be treated as an upper limit (except low-lying island nations, for whom 2ºC would already likely be a death-sentence), the pledges made as a result of these negotiations put us on track for a world that is more likely to be around 4ºC warmer by 2100, and more than 6ºC warmer during the following century. Note that these pledges are in some cases aspirational and lack any legislative framework to accompany them. In Australia's case, our pledge (lying quite firmly at the less ambitious end of the scale) is dependent upon the implementation and success of the Gillard government's proposed scheme to put a price on carbon. So even were we (and all other nations) to implement successfully our plans, we are still far more likely to be at 4ºC by 2100 than anywhere near 2ºC.
If a 2ºC world sees us suffering from a wide range of very difficult and worsening challenges that will stretch our ability to cope, a 4ºC world would be unrecognisable. A conference this week looking at the likely impacts on Australia of a four degrees rise suggested that Australia, the world's sixth largest food exporter, may no longer be able to feed itself. The difficulty of understanding just how different such a world would be is illustrated by the following quote from Professor Hans Joachim Schellnhuber, Chair of the German Scientific Advisory Council, advisor to the German Chancellor Angela Merkel and Director of the Potsdam Institute for Climate Impact Research (PIK). In March 2009, Schellnhuber said that on a four degree world the planet’s “carrying capacity estimates [are] below one billion people.”*
Just let that sink in.
Or find ways to avoid thinking about it.
*Carrying capacity is a complex and contested notion and obviously depends on a range of assumptions about average standard of living. The point is not to suggest that one billion is a fixed limit, but simply to highlight how severely compromised the systems on which we rely for a world of seven billion people may be in a four degrees warmer world.
UPDATE: Kevin Anderson, until recently the director of the U.K.’s leading climate research institution, the Tyndall Energy Program, had this to say about four degrees: “a 4 degrees C future is incompatible with an organized global community, is likely to be beyond ‘adaptation’, is devastating to the majority of ecosystems, and has a high probability of not being stable.”
Incompatible with an organised global community. Parse that how you will, it ain't pretty.