The release of America's spy satellite images of Arctic sea ice provides unexpected, dramatic new evidence about the dangers of global warming.

These visions of dwindling ice cover confirm that changes in climate in the planet's high latitudes are progressing much faster than originally expected. And what happens there is bound to have an impact elsewhere on our overheating world, in particular to its rising sea levels.

It is not the actual loss of Arctic sea ice that is the danger, of course. Its melting will add nothing, directly, to rises in sea levels. But its dwindling will almost certainly have a profound knock-on effect - mainly on the great ice sheets that cover Greenland and Antarctica. Without sea ice to prop them up at their edges, these sheets will break apart at faster and faster rates and tip more and more ice into the oceans. And once changes have been triggered at their edges, these will be transmitted into the hearts of these great glaciers at remarkably fast rates, scientists predict.

And here lies the threat to Earth. The destruction of the ice sheets of Antarctica and Greenland will feed vast amounts of meltwater into the oceans, far more than has been calculated until very recently. For example, the last report of the Intergovernmental Panel on Climate Change contained little input from melting ice sheets in its estimates and concluded, instead, that sea-level rises would be constrained to around 20 to 60 centimetres by the end of the century.

That figure now looks uncomfortably optimistic and current estimates put the likely rise at one metre or more by 2100 - a figure backed by the US Geological Survey, which this year warned that rises could reach as much as 1.5 metres. As a result, low-lying areas, including Bangladesh, Florida, the Maldives and the Netherlands, will undergo catastrophic flooding, while in Britain large areas of the Norfolk Broads and the Thames estuary could disappear. In addition, cities including London, Hull and Portsmouth will need new flood defences.

And that is just the beginning. No matter what we do about carbon dioxide emissions - the key cause of this heating and melting - the world will continue to warm and its sea levels to rise beyond 2100. Reversing global warming will be a very long process. However, we have, if nothing else, been warned.