Flavour of the week appears to be ice sheets and sea level rise. This is an area where I have tried to keep up with developments. To start with, following the TAR in 2001, the ACIA and ICARP have produced substantial reports, between 2004-2006, on the state of the Arctic and Antarctic.

It has been intimated on Climate Science, that such reports are flawed in that their use of scientific evidence is biased towards a politically-originated ‘proof’ of anthropogenic climate change. As such, they do not necessarily take into account all of the available evidence.

Roger Pielke Sr. may, arguably, have a point when it comes to the IPCC AR4, given the open editorial written by Pachauri (the ‘boss’) in one of the EGU publications, which seems to have an implicit agenda embedded in the process surrounding the production of, if not the technical section, at least the Summary for Policy Makers. But I see little evidence that this is the case for the other two major summaries mentioned above. What can be seen is an extensive summary of research focussed on Polar climate and systems.

RealClimate (probably thanks to the inputs of Rasmus Benestad and William Connolley, both specialists), frequently updates its regular readers about recent findings. These have become more significant since the SPM came out in February, as its calculations of sea level rise did not include ‘dynamical’ processes, e.g. ice sheet melt calculations or estimates.

There are many bloggers who are up-to-date on the current state of knowledge, including Hank Roberts and Eli Rabett, as well as a number of other familiar sites which have more emphasis on ecological issues than climate ones. Given that, collectively, these represent a substantial body of knowledge and understanding of the science, it would be foolish to try and copy or improve upon what is already available.

Uncle Eli very generously gave the Old Man a name-check yesterday in repeating a question I asked on his blog: what does it mean to talk of a ‘collapse’ of an ice sheet? Having read Hansen et. al. in ACPD on ‘dangerous climate change’, I can confirm that Eli’s summary appears to be broadly correct; what is implied is not a ‘Larsen’-type event, where the entire WAIS (or even a chunk of it) suddenly cracks off &  floats around the hemisphere, slowly melting, but a more generic disintegration, which reaches a point beyond which continued ablation/loss (and therefore continued sea level rises) becomes irreversible.

Two questions/worries now cross the Old Man’s mind: how rapid is a process of ‘disintegration’ likely to be, and Has the process already started?

The answer to the first question is very much open to debate at the moment. Though there is increasing evidence of mass loss on both the GIS and the WAIS, and increasing evidence of acceleration of the processes, the relative contributions to sea level have as yet been quite small. As Rasmus Benestad points out, current rates of ablation would lead to a net contribution of perhaps 20cm (or more) by 2100 to sea level; hardly a headline-grabbing amount. The Hansen paper mentioned above suggests that a sea level rise of 1-1.5 metres in response to a further increase in global average surface temperature of less than 1C is plausible.

If we take at face value the IPCC figure of ~50cm rise by 2095, not including ice sheet changes, we would then need to see a comparable contribution from glaciers and ice sheets to raise sea level by 1 metre. Is this possible? Is it likely?

Hopefully, this being International Polar Year,  we will see a flurry of papers and research findings which will help to answer these questions. There will still be substantial uncertainties; the databases are short, the data collection extremely difficult, and the natural variability much greater than  the normal, non-Polar spread.  But I am going to stick a  neck out here. I can do this because I am not a scientists and won’t lose my job if I’m wrong.

My prediction is that new research published in 2007 is very likely to establish that the processes leading towards ‘collapse’ of ice sheets (as understood above) are already well under way. The likelihood is also very high that new estimates of mass loss, calving rates, ablation, coastal warming, and various other metrics, will all show a similar pattern of acceleration that has been observed in the past few years.

As a ballpark indicator of what might happen this year, the Cryosphere Today website contains much that is useful. A glance at the current rapid decline in ice cover may be a ‘blip’, but does fit with recently observed patterns. A glance at the anomalies will show that the current anomaly, of ~1.5 Million Km2, has only ever been equalled once, in September 1995, and is arguably exceptional. NOAA’s summary climate report for the Winter just past showed it to be the warmest ever, globally, with warmth particularly noticeable on the continental landmasses abutting the Arctic. With the exception of the developing La Nina, none of the teleconnections indicates the likelihood of a cooler than average Spring-Autumn 2007. It will be important to monitor the AO index, though, as there may be a swing here this year.

It may be that the point at which ice sheet disintegration becomes ‘critical’ is still some years off, but the balance of evidence, to my eyes, would suggest that such a point can be measured in years, or at most, decades, rather than the centuries which have up to now been suggested. If this is indeed the case, we may find, in two or three years’ time, that the need for mitigative policy/action becomes more or less self-evident.

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