It takes the equivalent of a massive chunk of ice of 390 cubic kms (150 cubic miles) to raise world sea levels by one millimetre, according to David Carlson, director of the International Programme Office of the International Polar Year.

As an example, he says that works out as a lump 39 kms long, 10 wide and 1 km thick. Or I reckon it could be a blockbuster ice cube with sides 7.3 kms long — that would smother most of  a large city such as Paris (top left — you can see the Eiffel Tower in the middle).

David’s numbers give an idea of the scale of the thaw under way — seas have been rising at about 3 millimetres a year in recent years in a trend that almost all climate scientists blame on global warming caused by human activities. That’s equivalent to a rate of 30 cms a century.

And it’s also a lot faster than a rise of 1.8 mm a year from the 1960s, according to the U.N. Climate Panel. The thaw is one of the spurs to action under plans for a new U.N. treaty to fight global warming due to be agreed in Copenhagen in December.

Some scientists reckon seas could rise by one metre this century. Most of the rise projected by 2100, however, is likely because water expands as it gets warmer, rather than because of a thaw of glaciers or of ice sheets smothering Greenland or Antarctica.

One bit of good news on the ice front is that it looks as if sea ice in the Arctic will not shrink to a new record low this summer, after 2007 marked the smallest since satellite records began in the 1970s (and probably a lot longer than that).

Ice shrinks to its annual low in September before freezing out again: so far the ice is still far bigger than in 2007 at the same time although it is also far smaller than the 1979-2000 average, according to the U.S. National Snow and Ice Data Center. And ice floating on the sea doesn’t really contribute to raising sea levels — it’s effectively part of the water already.

(Picture: undated satellite image of the Eiffel Tower and the surrounding area in Paris, France. REUTERS/DigitalGlobe TZ)

 Welcome to the front lines of global warming in the United States - the Harding Ice Field in Alaska, the biggest icefield in the United States.
   At the Exit Glacier north of Seward - the only glacier in the Kenai Fjords National Park reachable by foot - the giant cerulean blue ice sheet gives every sign of staying put.
   But one only has to glance at the many signs along the roadway and footpath to the glacier’s edge to mark its retreat  - it hit its peak size in 1815 and has been receding ever since. Signs along a footpath leading to the base of the glacier show just how far it has retreated.
   The glacier lost about 10 feet from its front face over the summer of 2008.
   Since the 1980s, land-based glaciers and ice caps like this one in Alaska have contributed the most to sea level rise than any other source within their category, which includes other land-based glaciers like Tanzania’s Mount Kilimanjaro and the Chacaltaya Glacier near La Paz, Bolivia, said Brenda Ekwurzel, a climate expert with the Union of Concerned Scientists.
   Unlike the ice cover around the North Pole or giant floating ice sheets, land-based ice contributes directly to sea-level rises.
   According to a 2007 report by the U.N.’s Intergovernmental Panel on Climate Change, glaciers and ice caps have the potential to raise global sea levels by between .15 meters and .37 meters.
   That pales in comparison to the giant ice sheets in Greenland and Antarctica, which could raise sea levels by 63.9 meters if they fully melted.
   At the Aialik Glacier in the Harding Icefield - reachable by boat or plane, the living nature of the ice was more evident.
   On a visit to the glacier via tourboat on Aug. 15 on a trip hosted by the Knight Center for Environmental Journalism, several chunks of ice broke apart and crashed into Aialik Bay.
   Throughout the visit, the ice cracked and groaned, with a sound like thunder claps that punctuated the still air.