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WEATHER

ON THIS DAY

EDITORS' BLOG

Last Updated: Friday, 18 May 2007,
08:59 GMT 09:59 UK

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Earth - melting in the heat?

By Richard Black

Environment Correspondent, BBC News
website

Glaciers: Coming to a sticky
end?

Predictions
vary from the catastrophic to the cataclysmic.

Glaciers are melting, the ice caps disappearing into the oceans.
Sea levels may rise by many metres as a consequence.

Indigenous 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
unliveable.

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

The
Arctic

The
temperate zones

THE ANTARCTIC

2002: The Larsen B ice shelf shatters and
drifts out to sea

Huge,
pristine, 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, Rio... 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,
UK.

"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 2C 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.

Glaciers in retreat

Enlarge Map

The
ice dumped into the ocean as the glaciers retreat should not make
much difference to global sea levels - perhaps a few cm.

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 2,000km across - there are
few research stations, and temperatures vary naturally by 2-3C 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 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."

Antarctic ups and downs

Enlarge Image

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 20cm 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 1.8cm/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 5-10C 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
Antarctic

The
Arctic

The
temperate zones

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 US 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,
since the 1970s when satellites were able to map it," comments Peter
Wadhams, Professor of Ocean Physics at Cambridge University, UK, and
currently at the Laboratoire Océanographique 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 de-classify
their data.

Annual Arctic air temperatures relative to the
1961-1990 average

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 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 millimetre 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.

Great expanse

Temperatures, meanwhile, show a similar pattern to that seen
along the Antarctic Peninsula; an average warming of about 2C in the
last 50 years, about twice the global average, albeit with
significant variations between different parts of the Arctic.

This is reflected in changes to ice cover on land as well as on
sea.

Greenland heats up

Enlarge Image

The
Greenland ice sheet is, after Antarctica, the second biggest expanse
of ice in the world.

Its fringes expand and contract with the seasons; but images show
it is melting more each summer now than a decade ago.

In February 2006 researchers discovered glaciers in Greenland
were moving much faster than before, meaning that more of its ice
was entering the sea.

In 1996, Greenland was losing about 100 cubic km per year in mass
from its ice sheet; by 2005, this had increased to about 220 cubic
km.

A complete melt of the ice sheet would cause a global sea level
rise of about 7m; but the current picture indicates that while some
regions are thinning, others are apparently getting thicker.

Other land masses surrounding the Arctic, such as Siberia and
Alaska, are also beginning to feel the impact of rising
temperatures.

Much ground here is permafrost - ground that is, or at least
should be, permanently frozen.

Click here to see how much Arctic permafrost is
projected to disappear in the coming decades

More details

Construction
methods have been adapted to the rock-hard ground. Buildings can be
erected with minimal foundations until the ground begins to melt.

That it is now melting deeper than before in some areas is beyond
dispute. Projections indicate that the permafrost line - its
southern boundary - will migrate northwards.

"As permafrost melts more rapidly," says Peter Wadhams, "a lot of
methane and methane hydrates will be released to the atmosphere; and
these are greenhouse gases."

So melting here would create a positive feedback, stirring the
atmospheric mix to generate further warming.

Wider impact

To people living in the region, the melting brings mixed news.

Current lifestyles and staples 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 - 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.

Local melt, global change?

Enlarge Image

The Arctic
is intimately tied to the global climate system, and disruptions
here have the potential to create worldwide changes - albeit over
long timescales.

Possibly the most powerful link is via the thermohaline
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 20C) 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
which alarms many researchers as much as the change itself.

The
Antarctic

The
Arctic

The
temperate zones

THE TEMPERATE ZONES

Glacier loses its snout

Enlarge Image

Glaciers
snake over many of the world's high regions - the Himalayas, the
Andes, the Alps, Alaska.

The recent signs are that these, like the Arctic, are feeling the
impact of rising temperatures.

Over the last five years, various teams have reported glaciers
shrinking in Peru, Kazakhstan, Nepal and Alaska.

"There is a global pattern of melting in most of the world's
mountain glaciers," says Michael Hambrey, director of the Centre for
Glaciology at Britain's University of Aberystwyth.

"There are exceptions - some glaciers are advancing - but overall
the state of mountain glaciers is a dramatic shrinking since the
1970s.

"Some have disappeared completely, and most could be gone by the
end of this century."

Snow and snouts

The data on this comes principally from a number of glaciers -
perhaps less than 100 - which scientists consider make up a
representative sample.

These are studied in some detail, though many more will be
surveyed periodically to measure the length of their snout.

Glaciers have lost an average 6m in height in
20
years

Satellite
measurements are also of some value.

Two years ago, a study in South America found that rates of
thinning in the Patagonian ice fields were double in the period
1995-2000 compared with what they had been in the previous 25 years.

The World Glacier Monitoring Service (WGMS), supported by the
United Nations Environment Programme (Unep), collates records from
across the globe and issues regular bulletins of area and volume
changes.

Two years ago, they concluded that 30 major glaciers - assessed
as being a representative global sample - had thinned by an average
6m between 1980 and 2001.

Dry season

None of this is without its impact on human society.

"It will have a major impact," says Professor Hambrey, "mainly
through reductions in the fresh water supply.

Will iconic mountains like the Matterhorn
become ice-free?

"Cities
like La Paz in Bolivia and Lima in Peru rely heavily on glacial
meltwater from the high Andes brought down into dry arid areas.

"Switzerland, by contrast, uses meltwater for hydroelectric power
generation. If the glaciers disappear, their generating capacity
will be very much reduced."

Melting glaciers can form lakes on mountainsides - lakes which
can suddenly burst, creating torrential streams which threaten life
and property.

Michael Hambrey says these incidents have already caused about
30,000 deaths in Peru. They are also a major issue in the Himalayas,
where a programme of work is in place to make the lakes safe.

If ice comes off the land and ends up as water in the sea that
should, logically, have some impact on sea level.

Its scale is minute compared with the potential melting of the
West Antarctic or Greenland ice sheets - perhaps half a metre
globally if all the mountain glaciers disappeared.

But that could happen before the end of the century - much
earlier than projected for Greenland and Antarctica.

There seems little doubt that the changes seen in mountain
glaciers, however, are going to have an impact on human societies in
coming decades, as populations expand and the supply of fresh water
shrinks with the ice.

A little under 70% of the world's fresh water is
locked up in ice

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to the top

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Printable version

SEE ALSO

Greenland
ice swells ocean rise
16 Feb 06 |
Science/Nature

Q&A:
Europe's ice mission
06 Oct 05 |
Science/Nature

Arctic
ice 'disappearing quickly'
28 Sep
05 |  Science/Nature

Antarctic
glaciers show retreat
21 Apr 05 |
Science/Nature

Hunting
climate change evidence
07 Dec
04 |  Science/Nature

Arctic
heads into warmer future
02 Nov
04 |  Science/Nature

Patagonian
ice in rapid retreat
27 Apr 04 |
Science/Nature

RELATED INTERNET LINKS

Arctic Climate Impact
Assessment

Cryosat

British
Antarctic Survey

Icrest,
University of Missouri

National Snow and Ice
Data Center

University
of Aberystwyth Centre for Glaciology

Glaciers
Online

World Glacier
Monitoring Service

United Nations
Environment Programme

The BBC is not responsible for the content of external
internet sites

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