Another Life: Sea-level debate heats up as Greenland’s ice melts

Whenever I feel the need to know where the wind is coming from, it helps that our hillside house straddles the compass exactly, so that one corner of my workroom is definitely pointing west. This doesn’t, for some reason, help persuade me that looking out to sea past my computer takes me straight to Newfoundland, home to grandsons of emigrant Irish cod fishermen and the weather we would rather not share with there and Labrador.

I must attend, nonetheless, to the news of a big patch of the North Atlantic, on the direct line between Ireland and Newfoundland, that is perversely cooling down while the rest of the world is warming up. The cold of its water has been even more abnormal than the recent icy winter of the US northeast.

It seems, moreover, to offer disturbing confirmation that the flow of freshwater from melting Greenland ice is indeed slowing down what scientists call the Atlantic meridional overturning circulation, or Amoc. This is the vital pump that carries oxygen through the whole world’s ocean system and happens, in the process, to draw the Gulf Stream north to warm the winters of western Europe.

The water of the Gulf Stream, issuing from the tropics, is salty and dense. So is water pouring out from the Mediterranean at Gibraltar. As the heavy, highly saline water flows north in the Atlantic it is chilled by winds and storms and plunges to the bottom of the ocean. As much as 1,300cu km a day descends from the Norwegian and Labrador seas to become North Atlantic deep water.

This creeps along southwards, brushes the deep Antarctic currents, rolls on around the Cape of Good Hope and India and surfaces in the north Pacific after about 1,000 years. This mixed, upwelling water travels back the way it came, but at the surface, warming again at the equator before turning north, there to complete the oceanic conveyor belt.

Fears that masses of cold freshwater from melting Greenland ice could block the saline plunge have been around since the 1980s. New research, led by the climate scientists Stefan Rahmstorf, Michael Mann and Jason Box, rests on the study of sea-surface temperatures from 1900 to 2013. The weakening of the Gulf Stream system, says Dr Rahmstorf, "is apparently unique in the last thousand years".

The effects of a large slowdown in ocean overturning would, writes Rahmstorf, "look nothing like the Hollywood film The Day After Tomorrow. But they would not be harmless either – eg for sea level, particularly along the US east coast, marine ecosystems and possibly even storminess in Europe."

The northern Atlantic is perhaps 50cm lower than the northern Pacific, which is much less salty and dense, so that water flows through the Bering Strait and around into the Arctic Ocean. Thus, it is argued, any major check to the deep-water process can cause big sea-level changes around the northern Atlantic.

Even without this possibility, a consensus of expert opinion – 90 sea-level scientists in 18 countries – published in 2013, showed the range of sea level rises expected from global warming: 40cm to 60cm this century “with successful, strong mitigation measures” and 70cm-120cm without them. The city of Boston, on a slowly tilting American coast, is already considering a network of canals to absorb a higher inflow from the sea.

Computer models must, of course, juggle with a hundred uncertainties, not least the output of Greenland’s melting ice cap. Even devising a basic, or mean, sea level for the world has to reckon with the highs and lows of the ocean surface, shaped by earthly mass and gravity plus the wilful pull of the moon.

Before satellites, mean world sea level was determined by a scattered multitude of gravitational measurements. Now it can be measured directly by a radar altimeter mounted on a satellite. Adjusted for the effects of waves, storms and currents, the mean sea level for the planet, so it is said, can be precise to a few centimetres.

There are denialists. The extent of global sea-level rise is contested by sceptics such as Nils-Axel Morner, a retired professor of the University of Stockholm, who has been tracking tides at the Kattegat for 60 years.

But no one knows more about the implications of sea-level rise for Ireland than Dr Robert Devoy, professor of geography at University College Cork's coastal and marine resources centre. A lead author in reports of the intergovernmental panel on climate change, he has long been urging action on Ireland's own shoreline defences.

He is the final speaker at a big conference on Dublin Bay's history and environment (dublinbayconference.org), marking the 300th anniversary of Dublin Port, to be held at the Gibson Hotel in Point Village on April 29th. His address on climate change and the future of Dublin Bay seems certain to draw a full audience.