I have been waiting for a couple of months for this to appear in Atmospheric Chemistry and Physics, after I first spotted it on the Discussions pages back whenever. ‘ Dangerous human-made interference with climate: a GISS Model E study.’
The team looks at the effects of varying degrees of emissions developments to 2100 and beyond, in relation to average temperature rise (iow: Global Warming). Then they compare the IPCC scenarios to their own ‘alternative’ scenario. (The paper uses the TAR materials).
The main argument that is being pushed here is that a global warming of ~1C from year 2000 values would constitute dangerous climate change by the UNFCC definition. They consider the issues of sea-level rise, ice sheet stability, methane production as a feedback, and hurricane intensity, in separate sections, coming to the conclusion that the only feasible way to prevent ‘dangerous’ changes from human interference is to hold atmospheric CO2 levels at or below 450ppm, along with other controls on WHGHGs.
The paper considers that a Business As Usual scenario will cause a doubling of atmospheric CO2 levels from the pre-industrial period in around 40 years [my own calculation makes it 42-3 years]. This is then linked through the model to an estimated temperature increase of 2-3C, by 2100, with further warming beyond.
The principle argument for claiming that a rise of 1C would be dangerous appears to be that it is likely to cause sufficient changes, in the cryosphere in particular, to increase sea levels by some metres in a period of centuries, with no possibility of reversing or stopping the trend once it gets started (ifit hasn’t already done so). The argument goes on to relate this to the vulnerability of coastal-dwellers to storm surges and inundations from extreme weather.
What makes the paper worrying is that these levels are a long way below those used as a baseline by the IPCC, or most international governmental discussions,of 550ppm of CO2. Hansen et. al. conclude that, unless the effective 2ppm/year rise in CO2 emissions is halted within ten years, the odds are that dangerous consequences are then more or less inevitable.
The paper also contains an unusual component, in that it advocates for scientists to contribute actively to discussions of climate change, in order to improve public understanding of the issues and the problems.
It is, to my eyes, very much a ‘what if…’ paper. It’s most obvious limiting factor being that it is based on output from only one GCM. It does contain some assumptions about the forcing roles of aerosols, CO2 and WMGHGs, but these are within the IPCC constraints, so whether or not you accept them will relate to whether you agree with the IPCC’s WG1 report [though I repeat here that the team actually work with the TAR numbers, not the AR4’s].
I would also suggest that much of the discussion of feedbacks of the physical system is largely conjectural, at least to my untrained eye, and, whilst they are in general supported by the existing literature, inasmuch as they have been assessed, likelihood numbers for any one of these response to GW are, currently, at the low end of the scale.
The logical appearance of the argument used in the paper, then, looks a bit like: ‘If X, and if x>y, then Y, and if XY>Z, then Z…’ and so on. Too many ifs for my taste.
This is not to say I am not concerned by Hansen et. al.s’ work: some time spent checking the journals (see blogroll links) will reveal that much of what is speculated on here is well within the bounds of possibility, and evidence already exists of some of the consequences which they discuss [ in the sense of ‘early warning signs’, like sea-ice loss].
One of the reasons why I am featuring this paper is that it is not from the IPCC AR4, that it differs in its conclusions from the AR4 in important ways, and because it paints an even more negative picture of the effect of not controlling CO2 emissions than the AR4 suggests. So if you don’t like the IPCC reports, for whatever reason, take a look and see whether you find this a compelling argument for pushing for action on emissions now, rather than later.