EVEN if they do not live in the Maldives or Bangladesh, most people can appreciate the seriousness of rising sea levels. Much harder to grasp are most of the other consequences of global warming, and especially of the build-up of carbon dioxide in the atmosphere.
About a third of this CO2 ends up in the sea. Over geological time, virtually all the carbon released into the atmosphere has been taken out of it by living organisms and found its way into sediments, most of them in the sea (some has then gone into petroleum deposits). A vast amount of carbon is swilling about or sitting in the deep sea below 200 metres, where a biological pump pushes it round in such a way that any carbon atom entering the depths from the atmosphere will return to the surface every 500-1,200 years.
The pump is driven by phytoplankton, the tiny plants that constantly convert sunlight and CO2 into more plants, half of which die or are eaten by zooplankton. In the top 200 metres or so of the sea, some dead or faecal matter is turned back into CO2by microbes and may then re-enter the atmosphere. Any that sinks below the sunlit top layers will hang around in the depths for a long time, thanks to the colder temperature and greater density of the deep water. Eventually, currents and upwelling from the depths bring the dead stuff back to the top, where it stimulates more phytoplankton growth. For the duration of its cycle, though, this deep-sea carbon is locked away. In the short term it does not affect the concentration of CO2 in the atmosphere.
Short-term changes in that concentration are mostly affected by the carbon in the atmosphere itself, the carbon dissolved in the upper layers of the sea and the carbon in plants and animals on land. Until the Industrial Revolution, the exchanges among the three were more or less in equilibrium. But now the concentration of carbon dioxide in the atmosphere is higher than it has been for at least 365,000 years (some say 650,000), even though about half the CO2 produced in the past 200 years by burning fossil fuels and making cement has gone into the sea. As a result, the sea is 30% more acidic than it would have been without man’s new activities.
Even more alarmingly, the processes now set in train cannot easily be stopped, let alone reversed. Though CO2 in the surface layer is readily exchanged with the atmosphere, the mixing of that water with deeper layers takes several hundred years, meaning the acidification at the top is there for the duration. It is, said Britain’s Royal Society in 2005, “essentially irreversible” during the lifetime of anyone alive.
Exactly what this means for life in the sea no one quite knows. One fear is that the increasing acidity will kill off pteropods and similar creatures with calcium-carbonate shells or skeletons. A recent study found the seas acidifying ten times faster than previously believed, with disturbing effects on mussels, oysters and other animals living in coastal regions. But big fish in deeper waters might also be affected if entire species were lost at the bottom of the food web. Another worry is that the eggs and larvae of some fish may be unable to survive in more acidic water, and that creatures like squid which need a lot of oxygen will also die out. Any shortening of the food chain is likely to destabilise entire ecosystems, possibly leading to new ones in which just one or two species, such as jellyfish, predominate.
Heading for the exit (NaturePL)
A different set of concerns surrounds coral. Coral reefs, which have evolved over 400m years, are the biggest living structures on Earth and the richest in terms of marine biodiversity. A quarter of all sea species spend at least a part of their life in a reef—and many reefs are in cold or temperate waters. Their bounty already puts them at risk from fishermen. The Darwin Mounds to the north-west of Scotland, for instance, which support extensive colonies of cold-water coral, were smashed up soon after their discovery in 1998 by trawlermen eager to get at the fish. Similar damage has been done to reefs off Norway.
Yet coral is also at risk from acidification, and more so in colder waters even than in tropical ones, since colder seas tend to be more acidic. If the seas continue to become less alkaline at the current rate, the time will soon come when reefs will start to lose coral faster through erosion than they gain it through calcification. How soon? Some scientists think it could be in 60 or 70 years. Many fear that half the world’s coral will be gone by 2030.
The other aspect of man-made carbon-dioxide production that affects the sea is global warming itself. Though the oceans heat up more slowly than the land, higher temperatures are already causing them to expand. Indeed, expansion is now raising sea levels by at least 10-20cm per century (they rose about 30cm in the 20th century). The melting of glaciers, ice caps and polar ice sheets on land, however, has the potential to account for much more. The Arctic has lost over 40% of its year-round ice since 1985, 14% in 2004-05 alone. This will not do much directly to raise sea levels, because most Arctic ice is floating, but it suggests that the melting is speeding up, and that is confirmed by the flow of the Jakobshavn glacier in Greenland, which doubled in speed between 1997 and 2003.
From Greenland’s icy mountains
Most scientists think East Antarctica is stable, because it is high and dry, meaning a temperature rise of a few degrees will cause no melting. But if even half of the West Antarctic and Greenland ice sheets were to melt, sea levels would rise by six or seven metres, flooding many of the world’s big cities, and the outlook is discouraging. Over the past 50 years the fastest rise in temperatures on Earth has been on the Antarctic Peninsula, in the west of the continent; this has been matched only in Alaska. And last year the Wilkins shelf, a huge plate of floating ice attached to the peninsula, started to break up, losing some 2,000 sq km in six months. The eighth ice-shelf collapse on the peninsula in 30 years, it is further evidence of an acceleration in warming. West Antarctica as a whole lost ice about 75% faster in 2006 than in 1996.
The melting of an ice shelf, which is merely a floating projection into the sea, would not affect sea levels. It would take the melting of an ice sheet to do that. Yet that is just what is happening in Greenland, whose relatively warm and wet sheet is on course to melt completely, bringing the prospect of a sea-level rise of perhaps seven metres. The big question is when, and that is unanswered. The Intergovernmental Panel on Climate Change in 2007 forecast a rise of 18-59cm this century. Many think this much too conservative.
When sea ice melts, the newly exposed dark water absorbs radiation rather than reflecting it, as snow or ice would. That raises the temperature of the sea, making the ice melt even faster. Something similar happens when terrestrial ice melts. Either it exposes land, which then warms up; or it forms ponds (or lakes or streams) of meltwater on the surface of the ice, which absorb energy and melt more ice. Both mean a loss of heat-reflecting ice and a net addition to global warming.
One related concern seems to have abated, though. For some time scientists were puzzled by the speed with which meltwater could disappear through the ice as it drained down natural pipes known as moulins. A meltwater lake on the Greenland ice sheet that contained 44 billion litres and covered 5.7 sq km gurgled away within 24 hours in 2006. Most of it went in 90 minutes, at a rate that at its maximum was faster than the average flow of the Niagara Falls. The fear was that when this quantity of water hit the bottom, it would detach the ice from the bed on which it rested and lubricate its passage to the sea.
Not so, it seems, to judge by a study published last year by Sarah Das, of Woods Hole. The meltwater did indeed cut through nearly 1km of ice, as some had hypothesised and others had doubted, forcing its way down thanks to its greater weight and density. But it did not destabilise the ice sheet or provide a new reason to worry about rising sea levels.
Even if it meant nothing for sea levels, though, a melting Arctic (see video-graphic) still means a lot for the ecology of the region. Polar bears, for example, are becoming endangered, as the disappearance of ice obliges them to swim farther and farther to catch seals, their main prey. In contrast, some humans are delighted by the new warmth. Greenlanders can now grow potatoes, miners are eyeing newly accessible mineral reserves and trawlermen can more easily pursue fish into northern waters.
Most excited of all, though, are oil-drillers and shipping companies. Shell Oil, frustrated last year by lawsuits filed by environmentalists and Alaskan natives, is eager to start exploiting its leases in the Beaufort Sea. Russian companies such as Gazprom are already developing fields in the Barents Sea. And Russia, like America, thinks the Arctic holds lots more oil and gas. It believes that the territory it considers to be rightfully its own—it lodged a claim for 1.2m sq km of seabed in 2001—holds 586 billion barrels of oil, more than twice Saudi Arabia’s proven reserves. In this it may be wrong, but there is no doubt that fossil fuel, the very agent that is destroying the Arctic, will become far more available as a result of the destruction it wreaks. In all, the Arctic may hold 20-30% of the world’s undiscovered oil reserves.
Soon shipping will join the polluters of the north. A new seaway through an unfrozen Arctic Ocean would cut the journey from Rotterdam to Yokohama via the Suez canal by 4,700 miles, a saving of 42%. From Rotterdam to Seattle via the Panama canal, the saving would be 2,000 miles, over 20%. Ships too big to go through the Panama canal would save even more.
Would the financial savings be commensurate? If it cost no more to sail through the Arctic than anywhere else, yes: the savings per voyage would be huge. But even if the ice melts as fast as shippers hope (and scientists fear), navigation is likely to demand strengthened hulls, higher insurance and extra training for crews, all adding to the costs.
Everything depends on the speed at which the ice disappears. Computer models have been predicting that the Arctic will not be ice-free, even for a short time in late summer, until 2040, and at present only icebreakers and the occasional lone yachtsman are getting through. But some people believe change is coming so fast that the northern seas will open up much earlier than expected. They may be right.
If so, it will be seen as a harbinger of a another horror: the prospect of a shutdown of the North Atlantic conveyor. This is the current of water that takes enormous amounts of heat—about as much as would be generated by a million nuclear power plants—from the tropics and carries it to eastern North America and western Europe. The fear is that melting ice, along with increased snow and rain, could reduce the density and salinity of the top layers of the sea, making them more buoyant. At present, the conveyor depends on surface water sinking and travelling towards the equator, there to rise again and bring warmth back to the north (see map in the introduction). If this current stopped, the average temperature in Europe might fall by five to ten degrees Celsius.
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