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 reason 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—there are few research
stations, and temperatures vary naturally by 2～3℃ from year to year. But
measurements indicate that in the west, melting is underway.
"About one-third of the West Antarctic ice sheet is thinning," says Dr.
Vaughan, "on average by about 10 cm 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 masons 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 ERS1 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 not 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 North 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 lee
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 all; 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 them; 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 formed. 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℃) 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; but, as with other parts of the planet, it is the speed of change that
alarms many researchers as much as the change itself. The existing satellites are not good at detecting the thickness of the ice partly because the orbits of satellites with ______ do not cover every portion of the ocean.