Blogger Alex Tingle has adapted Google Maps to put together a cool utility that gives an idea of the effects of varying degrees of sea level rise on coastlines around the world. You can simply navigate to your desired location, zoom in, then choose your... er... preferred level of sea water influx (currently only ranging from zero to fourteen metres, in one metre increments) to see what your world might look like.

Alex uses data from NASA to generate the results, and he is very candid about potential inaccuracies, stating that the results err on the conservative.

Click the picture below, showing Amsterdam and dozens of other cities in the Netherlands fully drowned after a sea level rise of fourteen metres, to get taken to the flood-maps utility.

How much sea level rise can we expect from global warming? The short answer is, I don't know. The long answer is, they don't know, exactly, either.

Understanding global sea-level changes is a difficult physical problem, because complex mechanisms with different time scales play a role ... , including thermal expansion of water due to the uptake and penetration of heat into the oceans, input of water into the ocean from glaciers and ice sheets, and changed water storage on land. Ice sheets have the largest potential effect, because their complete melting would result in a global sea-level rise of about 70 m. Yet their dynamics are poorly understood, and the key processes that control the response of ice flow to a warming climate are not included in current ice sheet models [for example, meltwater lubrication of the ice sheet bed ... or increased ice stream flow after the removal of buttressing ice shelves...]. Large uncertainties exist even in the projection of thermal expansion, and estimates of the total volume of ice in mountain glaciers and ice caps that are remote from the continental ice sheets are uncertain by a factor of two.... Finally, there are as yet no published physically based projections of ice loss from glaciers and ice caps fringing Greenland and Antarctica.

For this reason, our capability for calculating future sea-level changes in response to a given surface warming scenario with present physics based models is very limited, and models are not able to fully reproduce the sea-level rise of recent decades. -- Science (PDF)

The Fourth Assesment Report from the Intergovernmental Panel on Climate Change (IPCC) states that human influences have "very likely contributed to sea level rise during the latter half of the 20th century". Their 'best estimate' is a maximum rise of between 0.26 – 0.59 metres by the years 2090-2099, relative to 1980-1999. They state, however, that their model excludes "future rapid dynamical changes in ice flow".

IPCC projections on sea level rises have, historically, proved conservative:

... Rates of sea-level rise calculated with climate and ice sheet models are generally lower than observed rates. Since 1990, observed sea level has followed the uppermost uncertainty limit of the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR), which was constructed by assuming the highest emission scenario combined with the highest climate sensitivity and adding an ad hoc amount of sea-level rise for “ice sheet uncertainty” .... -- Science (PDF)

The IPCC, however, do recognise there are significant areas of unknown in attempts to make predictions, and warn against taking their upper values as the ceiling on sea level rises:

Because understanding of some important effects driving sea level rise is too limited, this report does not assess the likelihood, nor provide a best estimate or an upper bound for sea level rise. Table SPM.1 shows model-based projections of global average sea level rise for 2090-2099.... The projections do not include uncertainties in climate-carbon cycle feedbacks nor the full effects of changes in ice sheet flow, therefore the upper values of the ranges are not to be considered upper bounds for sea level rise. They include a contribution from increased Greenland and Antarctic ice flow at the rates observed for 1993-2003, but this could increase or decrease in the future. -- IPCC AR4 'Synthesis Report' (5.7mb PDF)

Sea levels rise from two direct contributors. One is thermal expansion of the water itself as it warms, the other is the simple addition of water volume -- from melting icesheets and glaciers. Regular readers will know that this year's ice melt, in the arctic in particular, blew all predictions from the IPCC out of the water (see report #3 on this page), confirming their conservative approach, and also justifying their own admission about uncertainties due to the complexities of 'climate feedbacks', like: melting permafrost releasing methane, oceans getting saturated and absorbing less CO2, less reflective ice surface, etc.- which all contribute to an escalating cycle of warming - as well as the actual mechanics of melting ice (for an excellent article on the 'mechanics of melting ice', head here).

Rising sea levels bring their own feedbacks as well. Imagine, for example, if the world's largest CO2 absorbing forest - the Amazon - was inundated with seawater?

Based on complete melting of polar ice sheets

Additionally, it should be noted that glaciers are the first and fastest to melt -- with significant consequences as well.

James Hansen, Head for NASA’s Goddard Space Flight Centre, criticised the IPCC over their underestimations back in June, and this before the effects on arctic ice from the summer of 2007 were fully realised.

They also implicitly criticise the UN’s Intergovernmental Panel on Climate Change (IPCC) for underestimating the scale of sea-level rises this century as a result of melting glaciers and polar ice sheets.

Instead of sea levels rising by about 40 centimetres, as the IPCC predicts in one of its computer forecasts, the true rise might be as great as several metres by 2100. That is why, they say, planet Earth today is in “imminent peril”. -- IPCC Criticised for Underestimations

New York City Under a 3-5 metre Sea Level Rise (Source)

A recent report from Nature Geoscience would appear to confirm Dr. Hansen's projections:

The last interglacial period, Marine Isotope Stage (MIS) 5e, was characterized by global mean surface temperatures that were at least 2 °C warmer than present.... Mean sea level stood 4–6 m higher than modern sea level.... -- Nature Geoscience [Note: Evidence seems to be mounting that we're already committed to a 2'C temperature rise - see here and here]

And Dr Rajendra Pachauri himself, Chairman of the IPCC and Co-Laureate of the Nobel Prize, indicated he's fully aware of studies that have drawn more drastic conclusions than the report he headed, stating the following in his Nobel Prize acceptance speech:

Another issue of extreme concern is the finding that anthropogenic factors could lead to some impacts that are abrupt or irreversible, depending on the rate and magnitude of climate change. For instance, partial loss of ice sheets on polar land could imply metres of sea level rise, major changes in coastlines, and inundation of low-lying areas, with greatest effects in river deltas and low-lying islands. -- Acceptance Speech for the Nobel Peace Prize Awarded to the Intergovernmental Panel on Climate Change (IPCC) (PDF)

Whatever the rise will be over the next century, we must also recognise that time doesn't stop at the year 2100:

Anthropogenic warming and sea level rise would continue for centuries due to the time scales associated with climate processes and feedbacks, even if GHG concentrations were to be stabilised. -- IPCC AR4 'Synthesis Report' (5.7mb PDF)

Based on Complete Melting of Greenland & Antarctic Icesheets, which is likely to occur over the next few centuries if we don't pull back from runaway climate change