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 flesh 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, hut 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, "un 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 end 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 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 recant 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 arc 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 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 400; 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 us 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, lee 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 ice tongue growing out from the Greenland coast used to reject salt back into the water, making ______ heavier and helping it to sink.