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.