Not quite “we’re all going to die” again, but close. But this time by James Hansen, and published in Proceedings of the Royal Society.There is an the Indescribably-over-hypeded write up of it. Featuring:
nothing short of a planetary rescue will save it from the environmental cataclysm of dangerous climate change. Those are not the words of eco-warriors but the considered opinion of a group of eminent scientists writing in a peer-reviewed scientific journal.
Diversion: the article has a side bar pointing to Earth in peril: Climate change brings early spring in the Arctic which is a prime example of the sort of thing that really winds me up. Early spring: yes. Peril? Much less clear. After documenting various critters emerging early, it ends with “Dr Hoye warned that the change in timing of emergence, egg-laying and flowering could disturb local food webs with some animals appearing ahead or behind of others on which they rely for food.” Well its possible… but its not certain. Maybe they will just all have a nice long summer? Much of the “danger” estimates from climate change come from ecosystem responses, hopefully there are studies which actually show some likelihood of problems rather than just maybes.
Anyway, back to Hansen. It looks to be mostly a sea level rise paper. Hansen isn’t happy with the std IPCC estimates, because he thinks that Greenland and Antarctica will contribute more that the IPCC think. Why does he think this? Thats never been terribly clear to me; this is glacio stuff; he isn’t a glacio, and I don’t think many glacios agree with him. He says
The IPCC analyses and projections do not well account for the nonlinear physics of wet ice sheet disintegration, ice streams and eroding ice shelves, nor are they consistent with the palaeoclimate evidence we have presented for the absence of discernible lag between ice sheet forcing and sea-level rise
and thats probably true for the first half: we don’t have a good understanding of that. Hansen seems to translate it into “and therefore they are going to fall apart quickly”. The new thing appears to be the palaeo evidence:
Their study looked back over more than 400,000 years of climate records from deep ice cores and found evidence to suggest that rapid climate change over a period of centuries, or even decades, have in the past occurred once the world began to heat up and ice sheets started melting
Hmm, not really sure what this is referring to. Maybe its necessary to actually read the paper. Well there is quite a lot of it. p1936 sez Sea level following Termination II may have reached 4+/-2 m higher than today (Overpeck et al. 2006), which would already qualify as dangerous change. OK, I’m happy with the idea that 4m of sea level change would be dangerous, if it occurred within a century. But will it… Global warming of approximately 3oC is predicted by practically all climate models for ‘business-as-usual’ (BAU) growth of GHGs (IPCC 2001, 2007). Yet IPCC (2001, 2007) foresees twenty-first century sea-level rise of only a fraction of a metre with BAU global warming. Their analysis assumes an inertia for ice sheets that, we argue, is incompatible with palaeoclimate data and inconsistent with observations of current ice sheet behaviour. This seems less reasonable: there is nothing in the palaeo data I can see that tells you sea level *will* rise within a century.
Continuing, …As a result, large portions of West Antarctica and Greenland would be bathed in melt water. Already areas of summer melt have increased rapidly on Greenland (Steffen et al. 2004), the melt season is beginning earlier and lasting longer, and summer melt is being observed on parts of West Antarctica which doesn’t seem reasonable at all. Even in January most (or all, according to most of the GCMs I looked at) don’t have any areas above 0 oC by 2090-2100 over West Antarctica (assuming he doesn’t include the Peninsula in W Antarctica… most people don’t). My pic shows the fraction of AR4 models with sfc T above 0 oC in 2010-20 (left) and 2090-2100 (right), using a quick and dirty interpolation so there may be some dubious areas. So I think the models think there is precious little today and not much more by 2090 (incidentally GISS E, which was used in H06, seems to be one of the more enthusiastically warm over Antarctica).
OMiaC also doesn’t quite know what to make of Hansen.
Its very hard (from the paper) to work out what kind of probability Hansen thinks there is for large SLR within a century. But the paper ends with We have presented evidence (Hansen et al. 2006b) that the dangerous level of CO2 can be no more than approximately 450 ppm. Our present discussion, including the conclusion that slow feedbacks (ice, vegetation and GHG) can come into play on century time-scales or sooner, makes it probable that the dangerous level is even lower. OK, so the previous stuff says that 450 is “dangerous” (CO2 or CO2e?) and this stuff says that 450 is too high, “probably”. I don’t know about the previous papers, but if the present one is supposed to demonstrate that lower levels are dangerous then they become irrelevant. But I don’t think that the present one *does* establish that 450 or lower is “probably dangerous”.
There is more inconclusive discussion on the Global Change group.