SEA LEVEL RISE

Scientists are warning that the level of the sea may rise by slightly more than previously forecast - but they also say that the very worst predictions look much less likely.

Confused? If so, you're not alone.

The future of sea level rise is one of the most important questions in climate science because so many millions around the world live beside coasts - but it's also one of the most difficult to answer.

The last major report by the UN's Intergovernmental Panel on Climate Change (IPCC), its Fourth Assessment Report published in 2007, concluded that the global average level of the sea may rise by between 18cm-59cm. Much of that was expected to come from what's called "thermal expansion" - the physical process by which the volume of a body of water expands as it warms, as the oceans are.

But, crucially, the IPCC said that not enough was known about the other great potential contributor to sea level rise - the melting of the polar ice sheets and glaciers. Polar melting has been a kind of wild card in these calculations: a sudden acceleration in melting could cross some hidden threshold and trigger runaway melting that would cause an unexpected surge. Or not.

So what does the new study tell us? The headline is that forecasts of the changes to the ice-sheets and glaciers in the Arctic and Antarctic suggest their melting could contribute between 3cm-36cm to sea level rise.

This would add another 10cm to the upper range of the last IPCC forecast.

And satellite data only stretches back a few decades.

Interestingly, one conclusion is that the very scariest scenarios look less likely.

There is only a one in 20 chance, they reckon, that by 2100 coastal towns in the Thames Estuary, Holland and Ireland may get hit by extreme surges about one metre higher than now. The very gloomiest warnings look far less plausible.

The most certain piece of knowledge is that the global average sea level has been rising by about 3mm a year. But what really matters is whether this will accelerate - and by how much. And all that's still a work in progress.