Earth: Melting in the Heat
Glaciers are melting; the ice caps are disappearing into the oceans; sea levels may rise by many meters as a consequence. Indigenous (本土的) Arctic peoples will find their food stocks gone, while fresh water supplies in Asia and south America will disappear as the glaciers which provide them melt away; penguins, polar bears and seals will find their habitats gone, their traditional lives unlivable.
But how realistic is this picture Is the world’s ice really disappearing, or is it unscientific hot air
A European satellite named Cryosat was designed to provide definitive answers to some of these questions. A launcher fault destroyed the mission in October 2005, but the European Space Agency has approved a replacement. In the meantime, here is our global snapshot.
The Antarctic
Huge, pristine (质朴的), dramatic, unforgiving-the Antarctic is where the biggest of all global changes could begin.
There is so much ice here that if it all melted, sea levels globally would rise hugely—perhaps as much as 80m. Say goodbye to London, New York, Sydney, Bangkok … in fact, the majority of the world’s major cities.
But will it happen Scientists divide the Antarctic into three zones: the east and west Antarctic ice sheets; and the Peninsula, the tongue of land which points up towards the southern tip of South America.
"Everybody thinks that the Antarctic is shrinking due to climate change, but the reality is much more complex," says David Vaughan, a principal investigator at the British Antarctic Survey in Cambridge, U.K. "Parts of it appear to be thickening as a result of snowfall increases, but the Peninsula is thinning at an alarming rate due to warming. The West Antarctic sheet is also thinning, and we’re not sure of the mason why."
On the Up
Temperatures in the Peninsula appear to be increasing at around twice the global average—about 2℃ over the last 50 years. Those figures are based on measurements made by instruments at scientific stations.
Earlier this year, David Vaughan’s group published research showing that the vast majority of glaciers along the Peninsula—87% of the 244 studied—are in retreat. The ice dumped into the ocean as the glaciers retreat should not make much difference to global sea levels—perhaps a few centimeters.
More worrying, potentially, are the vast ice sheets covering the rest of Antarctica. Making temperature measurements for the continent as a whole is difficult; it is a vast place--more than 2,000 km across--them are few research stations, and temperatures vary naturally by 2～3℃ from year to year. But measurements indicate that in the west, reciting is underway.
"About one-third of the West Antarctic ice sheet is thinning," says Dr. Vaughan, "on average by about 10cm per year, but in the worst places by 3～4m per year."
The rock on which the West Antarctic ice rests is below sea level, and British Antarctic Survey researchers believe the thinning could be due to the ice sheet melting on its underside.
"It may be that the ocean is warming and that’s causing the ice to melt, but there may be other reasons as well; for example, there’s lots of volcanism in that area and so that could change how much heat is delivered to the underside of the ice sheet."
Cryosat should help to pin down what is happening at the West Antarctic fringe. The radar altimeters on board its predecessors ERSI and ERS2 have been unable to map the steep slopes at the coast, whereas Cryosat’s instrument should be able to cope.
If the entire West Antarctic ice sheet did melt, sea levels globally would rise, by around 5m. But at the moment, there is no sign of that happening.
One recent scientific paper attempted to calculate probabilities for how much West Antarctic melting would contribute to global sea-level rises during this century. The conclusions: a 30% probability of a 20 cm rise, and a 5% chance of a 1m rise.
Eastern Mass
And what of the big monster, the much larger East Antarctic sheet A recent study using altimeter data suggested it is getting thicker, by about 1.8 cm/yr; another, using the gravity satellite mission Grace indicates its mass remains stable. But could rising temperatures in time drain the ice away
"It is net going to happen on any realistic human timescale," says David Vaughan. "It’s so cold that you could raise temperatures by 5～10℃ without having much of an impact; it’s on rock above sea level, so warming in the ocean can’t affect it."
Largely insulated from global trends and so big as to generate its own climatic systems, most of Antarctica appears to be immune to the big melt for now, though answers to what is happening in the west are eagerly awaited.
The Arctic
At the top of the world, the Arctic is a region built on water. Around the Noah Pole is ocean, with ice floes crowding in each winter and thinning again in the summers.
In September, we learned from scientists at the U.S. National Snow and Ice Data Center that the extent of ocean covered by ice is getting smaller each year, the current rate of shrinkage they calculate at around 8% per decade. Their projection is that within about 60 years, there will be no summer ice at all on the Arctic Ocean.
"Overall, the extent has been declining, with some oscillations (摆动), since the 1970s when satellites were able to map it," comments Peter Wadhams, Professor of Ocean Physics at Cambridge University, U.K., and currently at the Laboratoire Oce anographique in Villefranche-sur-mer, France. "There’s been a slow decline, but now the thinning appears to be more rapid. In the last two decades, not only has the area shrunk but the ice has got thinner by about 40%; the prediction is that it will vanish altogether during summers in the second half of this century."
Military Records
Measurements of thickness come mainly from military submarines, which spent long periods under the Arctic ice during the Cold War.
Peter Wadhams was one of the scientists who afterwards persuaded the authorities in Britain and the United States to declassify their data.
But as a method of measurement, it is far from perfect; and satellites have given only limited help. The existing satellite fleet gives good measurements of ice extent, but is not so good at detecting thickness, partly because the orbits of satellites with radar altimeters do not cover every portion of the ocean.
This data deficit has led to a rival theory—that the ice is not melting at ail; it is simply piling up in another part of the ocean, perhaps along the north Canadian coast.
Peter Wadhams believes he has now disproved this idea. "We did an experiment where we installed a set of buoys (浮标) in that region which measure the thickness of the ice and transmit it back via satellite," he says. "The buoy sits on the ice, and as waves pass under it they make it rise and fall, just by a millimeter or two; measuring this allows you to calculate the thickness of the ice."
The preliminary results, announced at a scientific meeting in April 2005, show that the extra ice is not there; it really has melted away.
Wider Impact
To people living, in the region, the melting brings mixed news. Current lifestyles and staple foods will almost certainly change, but the open ocean may permit new opportunities for trade and agriculture.
A bigger question is what it means for the rest of the planet. Ice reflects the sun’s radiation; water absorbs it. More water and less ice—a lower albedo (反照率)—mean that the pace of warming could increase. In this scenario, the Earth would be losing one of its "natural checks and balances" against warming—another positive feedback mechanism.
The Arctic is intimately tied to the global climate system, and disruptions here have the potential to create worldwide changes—albeit (虽然) over long timescales. Possibly the most powerful link is via the thermohaline (热盐的) circulation, the global conveyor taking warm water along ocean surfaces and returning colder water at depth.
"One very sensitive place is the middle of the Greenland Sea," says Peter Wadhams. "That has been ice-free in the summer, but usually in winter it would be covered by a lobe of ice growing out from the Greenland coast. AS it formed, it rejected salt back into the water, making the water heavier and helping it to sink. Since 1997, the ice tongue has never termed. That will be having an impact on the thermohaline circulation."
Back in geological history, about 55 million years ago, the Arctic was a warm (possibly 20~C) shallow sea that would have been ice-free without the intervention of a human-enhanced greenhouse effect.
Natural variations may be playing a role in the picture seen now; hut, as with other parts of the planet, it is the speed of change that alarms many researchers as much as the change itself. Fresh water supplies in Asia and south America is disappearing as the glaciers melt away.