Says the World Bank [*]. I could see why an imminently 4 oC world would be problematic. But the problem, as Timmy rather bluntly pointed out, is the report saying it spells out what the world would be like if it warmed by 4 degrees Celsius, which is what scientists are nearly unanimously predicting by the end of the century, without serious policy changes (my bold) which is, errm, bollocks. Note that they really are talking about global average temperature here, so it isn’t possible to fix this up by saying they are talking about land, or NH land, or somesuch.
That’s from the foreword, which is presumably designed to gain attention at the expense of accuracy. The Exec summary is a bit better, reporting Even with the current mitigation commitments and pledges fully implemented, there is roughly a 20 percent likelihood of exceeding 4°C by 2100. That’s starting to become something believable. There’s rather a degree of vagueness about whose estimates and scenarios are being used at that point; we’ll need to read further to find out, into chapter 3, 21st Century Projections.
Even there things are a touch vague, but The nonmitigation IPCC Special Report on Emissions Scenarios (SRES) (Nakicenovic and Swart 2000), assessed in the IPCC AR4, gave a warming range for 2100 of 1.6–6.9°C above preindustrial temperatures. In these projections, about half the uncertainty range is due to the uncertainties in the climate system response to greenhouse gas emissions. Assuming a “best guess” climate response, the warming response was projected at 2.3–4.5°C by 2100, the remaining uncertainty being due to different assumptions about how the world population, economy, and technology will develop during the 21st century. No central, or most likely, estimate was provided of future emissions for the SRES scenarios, as it was not possible to choose one emissions pathway over another as more likely (Nakicenovic and Swart 2000). The range from the SRES scenarios, nevertheless, indicates that there are many nonmitigation scenarios that could lead to warming in excess of 4°C indicates, I think, that we’re in IPCC land: these are std climate model projections. And However, RCP6, one of the RCP mitigation scenarios that assumes only a limited degree of climate policy intervention, already projects warming exceeding 4°C by 2100 with a probability of more than 15 percent is, I think, where they are getting their 4 oC from.
So, OK, it is possible to find 4 oC from the std projections. However it isn’t possible to say what they do say, viz “warmed by 4 degrees Celsius, which is what scientists are nearly unanimously predicting by the end of the century”. Its only a possibility. Now you could say, as mt does and I largely agree, that you need to weight these less-probable-but-not-clearly-assessable probabilities carefully – but that is a different matter. As is all the response and impacts stuff in this document, which I haven’t read.
[*] Or do they? Actually This work is a product of the staff of The World Bank with external contributions. The findings, interpretations, and conclusions expressed in this work do not necessarily reflect the views of The World Bank, its Board of Executive Directors, or the governments they represent. So maybe not.
Update: AA makes a good point in the comments, which I should have said upfront: that focussing on 2100, as though the warming will stop then, isn’t good. The graph here makes that clear. Nonetheless it doesn’t get the Bank out of their textual hole, since they very clearly said “by the end of the century”.
* Climate Scientists Applaud Dire World Bank Report – LiveScience (that’s “Dire” as in “worring”, I think, not as in “hopeless”).