As we talk to people around the world on some of the key findings and views that we're building into the next World Development Report, we encounter some heated debates. One of these much-argued points is our view that the world must aim to keep mean global warming below 2oC, but as one of our advisors says, "be prepared for 4oC".
Here are the reactions. Some (mostly, but not just, in Europe) find it shocking that we can even consider a world with warming above 2oC or with concentrations of CO2 at or above 550 ppm. Others worry that we are setting 2oC as a target, which is very sensitive in the context of the upcoming negotiations. We are not. We are simply agreeing in the light of mounting evidence that the world should try very hard to stay below 2oC, since losses will likely begin to rise rapidly above that temperature and irreversibe impacts may occur - particularly in developing countries. A new version of the "burning ember chart"  makes this painfully obvious.
But we also note that achieving this temperature change limit would likely require stabilizing the atmospheric concentration of greenhouse gases at around 450 ppm (in “CO2-equivalents”); and that this is a daunting, maybe even Herculean, task requiring extremely rapid and costly “decarbonization” of the world’s energy systems. (Right now the atmospheric concentration is around 387 ppm; the pre-industrial level is estimated to be around 270 ppm, and most of the increase from that level occurred at an accelerating rate through the 20th century.) A 550 ppm concentration target is still challenging, but certainly easier than 450 ppm. However, 550 ppm creates a significant probability of longer-term temperature increases seen by scientists as dangerous.
The trade-off across these options is very difficult, but must be confronted by the global community now, since the option of 450 ppm is already close to becoming unachievable .