Ahem. So previously there was a lot of hype and confusion and not much paper. Now that has changed, with Reconstructing sea level from paleo and projected temperatures 200 to 2100AD by Aslak Grinsted, John C. Moore & Svetlana Jevrejeva.
We use a physically plausible 4 parameter linear response equation to relate 2000 years of global temperatures and sea level. We estimate likelihood distributions of equation parameters using Monte Carlo inversion, which then allows visualization of past and future sea level scenarios. The model has good predictive power when calibrated on the pre-1990 period and validated against the high rates of sea level rise from the satellite altimetry. Future sea level is projected from IPCC temperature scenarios and past sea level from established multi-proxy reconstructions assuming that the established relationship between temperature and sea level holds from 200-2100 A.D. Over the last 2000 years minimum sea level (-19 to -26 cm) occurred around 1730 AD, maximum sea level (12 to 21 cm) around 1150 AD. Sea level 2090-2099 is projected to be 0.9 to 1.3 m for the A1B scenario, with low probability of the rise being within Intergovernmental Panel on Climate Change (IPCC) confidence limits.
(This abstract is a bit confusing. We use a physically plausible 4 parameter linear response equation to relate 2000 years of global temperatures and sea level. doesn’t mean what it appears to say. They only have 150 (or 300) years of sea level and T together; the 2000 years comes from using the resulting relation to reconstruct sea level over 2000 years. And the results (of the sea level reconstruction) must be dubious, because they earlier say We will, therefore, restrict the use of equation 2 to a relatively short period dominated by sea level rise, and (according to their results) this isn’t true of the last 2 kyr).
OK, so this a now comprehensible: we have some trust in the IPCC model projections of temperature, but we reckon their SLR estimates are too low because they don’t take into account ice sheet melt (see-also RC). So rather than try to model it, which we can’t, we’ll just look at the historical T-SLR relationship and project it into the future. This is a reasonable idea, and Rahmstorf did it in Science in 2007 and defended it. He only did 1880-2001; I’m not sure why he picked that period, it can’t be because he doesn’t trust he HS, perhaps he doesn’t trust the early sea level records.
Grinsted et al. believe that doubt has been cast on the assumptions of Rahmstorf, thought I’m not quite sure why. They do 1700-2007, by use Moberg or the Jones and Mann T reconstruction, and the Amsterdam SL record. Assuming I’m reading their table 2 right, when using only the historical data (1850 onwards) they get basically the same answer (0.32m – 1.34m) as Rahmstorf (0.5m – 1.4m) for SLR to 2100. Which isn’t too surprising, as its a very similar method. Using the longer record from 1700, they get 0.91m – 1.32m from the Moberg reconstruction, or 1.21 – 1.79 from J&M; but the J&M fit isn’t good (fig 7) so they prefer the Moberg version.
Using the 1850-2007 data only, the response time comes out at ~1 kyr. Which doesn’t sound right: I doubt you can determine such a long timescale from a short dataset. As indeed they notice: The simple conclusion is that the calibration time series is too short relative to the response time. Inclusion of the additional pre-1850 data clearly favors faster response and a higher sensitivity (aÏ-1) than instrumental observations alone. For Moberg, the response time is much shorter: ~200 years. I wonder if that interacts with Hansens stuff at all? OTOH if you took the response time from the whole Holocene dataset (~2.5 kyr) then SLR at 2100 comes down to ~0.6m.
So where do we end up? Hard to say. The answers are compatible with Rahmstorf, but they effectively reject the Rahmstorf stuff because they don’t believe the response times when restricted to the shorter period. “Don’t believe a word of it” is no longer a fair response; its a reasonable piece of work, though I’ve no idea if its right or not. Boiled down, it amounts to “we’ll probably get more SLR from the ice sheets, but we don’t know how much yet”.
ps: Daniel says, play The Codex of Alchemical Engineering. So do I.
pps: To warm the cockles of Broons heart: when told that VAT was down to 15% from 17.5% D’s first response (having first checked what VAT was) was “oh cool, so I can buy more stuff!”
[Update: Of course, We’re all going to die -W]
[See-also: Aslak Grinsted. Recent global sea level acceleration started over 200 years ago? may also be of interest -W]