ANTARCTICA: In my previous message I stated that Antarctica would detract 6-8cm from sea level rise along the 21st century. To be precise, it would detract from 6±4 to 7.5±4.5 cm in most scenarios, and 8.5±5.5 cm in the worst-case (AIF1) scenario (the mid points across scenarios are about 6-8cm).

ENHANCED MELTING: Enhanced melting, if it happens at all, would add from 4.5±4.5 to 7±6 cm on most scenarios (9±8 in A1F1): the mid points across scenarios are about 4.5 cm, or about 9 cm in A1F1. See Meehl 2007 p.820.

Notice that “most scenarios” includes also A2, the business-as-usual scenario with explosive population growth and no change in emissions patterns. A2 produces more or less the same sea level rise than other IPCC scenarios with much reduced carbon emission like B1, B2, A1B and A1T. Larger (good or bad) impacts on sea level appear only in A1F1, an unlikely scenario with more intensive use of fossil fuels than at present (per unit of output), which goes counter the trend of recent decades (when fossil fuel use per unit of output has rapidly decreased).

I think this paragraph is not warranted by current scientific knowledge.

1. IPCC PROJECTIONS.
The IPCC projected in fact sea level rises over the century from 28 to 34 cm in its main future scenarios. A1B predicts a SLR of 34.5cm ±13.5cm, along the century, whilst in the B1 scenario the expected rise would be 28 cm ±10cm (Meehl et al 2007, Table 10.7, p.820). The overall SLR over the century is estimated from 20 to 38cm in the most favourable B1 scenario and from 26 to 59cm in the worst (and unlikely) A1F1 scenario of intensified fossil-fuel use along the current century.
The enhanced melting of ice sheets due to melted surface ice percolating into deep ice was not included just because there is no sufficient scientific basis for it. Anyway, according to the same source, in case it would happen, it would add about 20cm to SLR during the 21st Century in the worst-case scenario, or just 9-13cm in the most likely scenarios.
Increase in storms and cyclones might be a consequence of climate change, but it is NOT linked to rising sea levels, two-thirds of which (17-29cm in the most likely scenarios) are simply a result of thermal expansion of sea water due to higher temperature, and the rest the sum of continental glacier melting (10.5-12.5cm) PLUS net ice loss in Greenland (3-4cm) MINUS net ice gain in Antarctica (6-8.5cm): see table in Meehl et al 2007,p.820. These figures are for the most likely scenarios (excluding fossil-fuel intensive A1F1, which is only slightly worse). It is also important to note that sea levels rose by about 20cm in 1870-2004, somewhat less than expected in this century but within the same order of magnitude (Church and White 2006).

2. THRESHOLD.
The idea that a threshold may be passed soon ensuring that the sea rises several meters in the next century is without foundation. The only theoretical possibility for this event is an “abrupt” collapse of the Northern Atlantic Meridional Overturning Circulation (NAMOC), causing an acceleration of Greenland melting. The odds of this happening are unknown, and the only recent model proposing it is also the only model not including ANY stabilizing feedback mechanism, while several other models predict that NAMOC would be resilient to warmer climate. The authors of the article (McInerney and Keller 2008) remark that averaging their model with ANY of the other models would reduce the odds by half. The existence of the threshold is unconfirmed, and even if it exists, its value is unknown (it could be +2°C, +5°C or +15°C). Even if the temperature threshold is reached, the melting of Greenland would take many centuries to complete. In the only model generating the total melting of Greenland, Ridley et al (2005) Greenland melts totally only after 3000 years of continued very high temperatures and carbon concentrations four times above pre-industrial levels.
McInerney & Keller 2008 thus explain the limitations of their model: However alarming their conclusions, the same authors introduce a number of important caveats about limitations of their results. “We analyze only the effects of a single threshold out of many possible climate responses, consider only uncertainty about a subset of the model parameters, and neglect structural uncertainty, stochastic variability, and the distinct possibility that we may learn in the future.” They mention as another limitation “the adopted choice of a single MOC model” because “Several models (e.g. Latif et al. 2000) suggest the existence of stabilizing feedbacks that result in a basically insensitive MOC”. The 2007 IPCC report does not incorporate any of these theories, and just mentions that “results suggest that there is a critical temperature threshold beyond which the Greenland Ice Sheet would be committed to disappearing completely, and that threshold could be cros¬sed in this century. However, the total melting of the Greenland Ice Sheet, which would raise global sea level by about seven metres, is a slow process that would take many hundreds of years to complete(Meehl 2007:819). In fact the only model predicting the speed of the process puts it at 3000 years under continued and very high carbon concentrations in the atmosphere (Ridley et al 2005).

Antarctica, on its part, will be detracting from sea level rise, since ice gain over the continent would be much larger than any loss of ice at its coasts. Some floating ice shelves around the Antarctic Peninsula (the warmest bit of Antarctica) have melted recently, but this does not add to sea levels because the ice was already floating. Some coastal points (esp. in Western Antarctica) may loss grounded ice along the century, but ice gain in the interior would be far greater. This is reflected in the effect of Antarctica reducing sea levels, envisaged in the 2007 IPCC report. Curiously, it detracts more water from the sea in the worst-case global warming scenarios, because a warmer climate causes more precipitation over Antarctica, i.e. more ice accumulation, since the entire continent is perpetually well below freezing (except the Northernmost points of the Peninsula in summer).

To sum up: rising sea levels will be a nuisance for places like Venice, New Orleans or low lying islands in Polinesia or elsewhere, but do not expect a rise of several meters anytime soon.

References:

Church, John & Neil White, 2006. A 20th century acceleration in global sea-level rise. Geophysical Research Letters 33: L01602.

Latif M, E.Roeckner, U.Mikolajewski & R.Voss, 2000. Tropical stabilization of the thermohaline circulation in a greenhouse warming simulation. Journal of Climate 13: 1809-1813.

McInerney, David & Klaus Keller. 2008. Economically optimal risk reduction strategies in the face of uncertain climate thresholds. Climatic Change 91(1-2):29-41.

Meehl, G.A., T.F. Stocker, W.D. Collins, P. Friedlingstein, A.T. Gaye, J.M. Gregory, A. Kitoh, R. Knutti, J.M. Murphy, A. Noda, S.C.B. Raper, I.G. Watterson, A.J. Weaver & Z.-C. Zhao, 2007. Chapter 10: Global Climate Projections. IN: Solomon, S., D. Qin, M. Manning, Z. Chen, M. Mar¬quis, K.B. Averyt, M. Tignor & H.L. Mi¬ller (eds.) Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK. Available at: http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter10.pdf.

Ridley, J.K., P. Huybrechts, J.M. Gregory, & J.A. Lowe, 2005. Elimination of the Green¬land ice sheet in a high CO2 climate. Journal of Climate, 17: 3409–3427.

Let’s keep the ball rolling!