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SEA LEVEL RISE: IT'S WORSE THAN WE THOUGHT
By Anil Ananthaswamy
New Scientist
July 1, 2009

http://www.newscientist.com/article/mg20327151.300-sea-level-rise-its-worse-
than-we-thought.html

For a few minutes David Holland forgets about his work and screams like a
kid on a roller coaster. The small helicopter he's riding in is slaloming
between towering cliffs of ice -- the sheer sides of gigantic icebergs that
had calved off Greenland's Jakobshavn glacier. "It was like in a James Bond
movie," Holland says afterwards. "It's the most exciting thing I have ever
done."

Jakobshavn has doubled its speed in the past 15 years, draining increasing
amounts of ice from the Greenland ice sheet into the ocean, and Holland, an
oceanographer at New York University, has been trying to find out why.
Scientists like him are more than a little astonished at the rate at which
our planet's frozen frontiers seem to be responding to global warming. The
crucial question, though, is what will happen over the next few decades and
centuries.

That's because the fate of the planet's ice, from relatively small ice caps
in places like the Canadian Arctic, the Andes and the Himalayas, to the
immense ice sheets of Greenland and Antarctica, will largely determine the
speed and extent of sea level rise. At stake are the lives and livelihoods
of hundreds of millions of people, not to mention millions of square
kilometres of cities and coastal land, and trillions of dollars in economic
terms.

In its 2007 report, the Intergovernmental Panel on Climate Change (IPCC)
forecast a sea level rise of between 19 and 59 centimetres by 2100, but this
excluded "future rapid dynamical changes in ice flow". Crudely speaking,
these estimates assume ice sheets are a bit like vast ice cubes sitting on a
flat surface, which will stay in place as they slowly melt. But what if some
ice sheets are more like ice cubes sitting on an upside-down bowl, which
could suddenly slide off into the sea as conditions get slippery? "Larger
rises cannot be excluded but understanding of these effects is too limited
to assess their likelihood," the IPCC report stated.

Even before it was released, the report was outdated. Researchers now know
far more. And while we still don't understand the dynamics of ice sheets and
glaciers well enough to make precise predictions, we are narrowing down the
possibilities. The good news is that some of the scarier scenarios, such as
a sudden collapse of the Greenland ice sheet, now appear less likely. The
bad news is that there is a growing consensus that the IPCC estimates are
wildly optimistic.

The oceans are already rising. Global average sea level rose about 17
centimetres in the 20th century, and the rate of rise is increasing. The
biggest uncertainty for those trying to predict future changes is how
humanity will behave. Will we start to curb our emissions of greenhouse
gases sometime soon, or will we continue to pump ever more into the
atmosphere?

Even if all emissions stopped today, sea level would continue to rise. "The
current rate of rise would continue for centuries if temperatures are
constant, and that would add about 30 centimetres per century to global sea
level," says Stefan Rahmstorf of the Potsdam Institute for Climate Impact
Research in Germany. "If we burn all fossil fuels, we are likely to end up
with many metres of sea level rise in the long run, very likely more than 10
metres [32 feet] in my view."

This might sound dramatic, but we know sea level has swung from 120 metres
lower than today during ice ages to more than 70 metres higher during hot
periods. There is no doubt at all that if the planet warms, the sea will
rise. The key questions are, by how much and how soon?

To pin down the possibilities, researchers have to look at what will happen
to all the different contributors to sea level under various emissions
scenarios. The single biggest contributor to sea level rise over the past
century has been the melting of glaciers and ice caps outside of Greenland
and Antarctica, from Alaska to the Himalayas. According to one recent
estimate, the continued loss of this ice will add another 10 to 20
centimetres to sea level by 2100. It cannot get much worse than this: even
if all this ice melted, sea level would only rise by about 33 centimetres.

Expanding waters

The second biggest contributor has been thermal expansion of the oceans. Its
future contribution is relatively simple to predict, as we know exactly how
much water expands for a given increase in temperature. A study published
earlier this year found that even if all emissions stopped once carbon
dioxide levels hit 450 parts per million (ppm) -- an unrealistically
optimistic scenario -- thermal expansion alone would cause sea level to rise
by 20 centimetres by 2100, and by another 10 centimetres by 3000. At the
other extreme, if emissions peak at 1200 ppm, thermal expansion alone would
lead to a 0.5-metre rise by 2100, and another 1.4 metres by 3000.

Then there are the great ice sheets of Greenland and Antarctica, which hold
enough water to raise sea level by about 70 metres. Until recently, their
contribution to sea level rise was negligible, and the IPCC predicted that
Greenland would contribute 12 centimetres at most to sea level rise by 2100,
while Antarctica would actually gain ice overall due to increased snowfall.
"A lot of new results have been published since then to show that this very
conservative conclusion does not hold," says Eric Rignot of the University
of California, Irvine.

To study the ice sheets, Rignot and colleagues have combined satellite-based
radar surveys, aircraft altimetry and gravity measurements using NASA's
GRACE satellite. They found that ice loss is increasing fast. Greenland is
now losing about 300 gigatonnes of ice per year, enough to raise sea level
by 0.83 millimetres. Antarctica is losing about 200 gigatonnes per year,
almost all of it from West Antarctica and the Antarctic Peninsula, raising
levels by 0.55 millimetres. "The mass loss is increasing faster than in
Greenland," Rignot says. "It'll overtake Greenland in years to come."

If this trend continues, Rignot thinks sea level rise will exceed 1 metre by
2100. So understanding why Greenland and Antarctica are already losing ice
faster than predicted is crucial to improving our predictions.

The main reason for the increase is the speeding up of glaciers that drain
the ice sheets into the sea. One cause is the knock-on effect of warmer air
melting the surface of the ice: when the surface ice melts, the water pours
down through crevasses and moulins to the base of glaciers, lubricating
their descent into the sea. Fears about the impact of this phenomenon have
receded somewhat, though: Antarctica is thought to be too cold for it to be
a big factor, and even in Greenland it is only a summertime effect. "It's
significant, but I don't think it's the primary mechanism that would be
responsible for dramatic increases in sea level," says glaciologist Robert
Bindschadler at the NASA Goddard Space Flight Center in Greenbelt, Maryland.

There is another way for surface melt to affect sea level, though. Meltwater
fills any crevasses, widening and deepening the cracks until they reach all
the way down to the base of the ice. This can have a dramatic effect on
floating ice shelves. "Essentially, you are chopping up an ice shelf into a
bunch of tall thin icebergs, like dominoes standing on their ends," says
Bindschadler. "And they are not very stable standing that way." They fall
over, and push their neighbours out to sea.

The most famous break-up in recent times -- that of the Larsen B ice shelf
on the Antarctic Peninsula in 2002 -- likely happened this way. While the
break-up of floating ice shelves does not raise sea level directly, the
disintegration of Larsen B had consequences that models at the time failed
to predict. With little to resist their advance, glaciers behind Larsen B
immediately began to move up to eight times faster. Five smaller ice shelves
in the rapidly warming Antarctic Peninsula have also broken up and many
others are disintegrating.

What lies beneath

Surface melt poses little threat in West Antarctica, as it is so much
colder. Here the danger comes from below. Take the ice shelf holding back
the massive Pine Island glacier, which is thinning in a strange pattern.
Radar scans have revealed giant "ripples" up to 100 metres deep on its
underside.

Bindschadler thinks that the currents created by winter winds raise
relatively warm water from a few hundred metres down in the Amundsen Sea off
West Antarctica. This melts the underside of the ice shelf and gets trapped
in the space it carves out, thus continuing to melt the ice from below over
a few seasons. As the ice shelf thins, the Pine Island glacier behind it is
speeding up, from 3 kilometres per year three years ago to over 4 kilometres
per year according to the latest unpublished measurements by Ian Joughin of
the University of Washington in Seattle.

What does this have to do with global warming? Climate change, aided and
abetted by the loss of ozone, has strengthened the winds that circle
Antarctica. This is speeding up the Antarctic circumpolar current and
pushing surface waters away from the coast, causing deeper, warmer water to
well up.

Along with the Thwaites glacier and some smaller ones, Pine Island glacier
drains a third of the West Antarctic ice sheet. This ice sheet is
particularly vulnerable to ocean heat because much of it rests on the
seabed, a kilometre or more below sea level. This submarine ice will not
raise sea level if it melts, but if it goes a lot of higher-level ice will
end up in the ocean. The vulnerable parts contain enough ice to raise sea
level 3.3 metres -- less than the 5 metres that was once estimated but more
than enough to have catastrophic effects.

Bindschadler has calculated that a change in ocean currents could
potentially deliver up to 1019 joules of heat per year to the continental
shelf off West Antarctica -- and only about 109 joules per year would be
required to melt the ice shelves that hold back the Pine Island and Thwaites
glaciers. "The ocean has an enormous amount of heat compared to the
atmosphere," he says.

Even in Greenland, where the ice sheet rests on land above sea level, ocean
heat still matters. When not dodging giant icebergs, Holland has been trying
to find out why Greenland's Jakobshavn glacier started moving faster in
1997, speeding up from around 6 kilometres per year to more than 9
kilometres per year by 2000 and 13 kilometres per year by 2003. The glacier
continues to drain ice from the Greenland ice sheet at a higher rate than
before.

The increase had been attributed to lubrication by meltwater, but Holland's
team recently stumbled across data from local fishing boats, which deploy
thermometers in bottom-trawling nets. One fact stood out: the temperature of
the subsurface waters around West Greenland jumped in 1997, prior to the
massive calving of Jakobshavn.

As the team reported last year, though, the real trigger lay in what
happened in 1996. That year, the winds across the North Atlantic weakened,
slowing down the warm Gulf Stream. The weakened current meandered aimlessly
and hit west Greenland. "A modest change in wind gives you a big bang in
terms of ice sheet dynamic response," says Holland.

Findings like these suggest that predicting sea level rise is even trickier
than previously thought. If relatively small changes in winds and currents
could have a big impact on ice sheets, we need extremely good models of
regional climate as well as of ice sheets. At the moment we have neither --
and while regional climate models are improving, ice sheet models are still
too crude to make accurate predictions.

"They are coarse models that don't include mechanisms that allow glaciers to
speed up," says Rignot. "And what we are seeing today is that this is not
only a big missing piece, this could be the dominant piece. We can't really
afford to wait 10 to 20 years to have good ice sheet models to tell people,
'Well, sea level is actually going to rise 2 metres and not 50 centimetres',
because the consequences are very significant, and things will be pretty
much locked in at that point."

So climate scientists are looking for other ways to predict sea level rise.
Rahmstorf, for instance, is treating the Earth as one big black box. His
starting point is the simple idea that the rate of sea level rise is
proportional to the increase in temperature: the warmer Earth gets, the
faster ice melts and the oceans expand. This held true for the last 120
years at least. "There is a very close and statistically highly significant
correlation between the rate of sea level rise and the temperature increase
above the pre-industrial background level," says Rahmstorf.

Extrapolating this to the future, based on IPCC emissions scenarios,
suggests sea level will rise by between 0.5 and 1.4 metres -- and the higher
estimate is more likely because emissions have been rising faster than the
IPCC's worst-case scenario. Rahmstorf's study got a mixed reception when it
first appeared, but he can feel the winds of change. "I sense that now a
majority of sea level experts would agree with me that the IPCC projections
are much too low," he says.

Could even Rahmstorf's estimate be too low? It assumes the relation between
temperature and sea level is linear, but some experts, most prominently
James Hansen of NASA's Goddard Institute for Space Studies in New York,
argue that because there are multiple positive feedbacks, such as the
lubrication of glaciers by meltwater, higher temperatures will lead to
accelerating ice loss. "Why do I think a sea level rise of metres would be a
near certainty if greenhouse gas emissions keep increasing?" Hansen wrote in
New Scientist (28 July 2007, p 30). "Because while the growth of great ice
sheets takes millennia, the disintegration of ice sheets is a wet process
that can proceed rapidly."

Hansen has made no specific prediction, however. So just how bad could it
get? Tad Pfeffer of the University of Colorado in Boulder decided to work
backwards from some of the worst-case scenarios: 2 metres by 2100 from
Greenland, and 1.5 metres from West Antarctica, via the Pine Island and
Thwaites glaciers. Just how fast would the glaciers have to be moving for
the sea level to rise by these amounts? Pfeffer found that glaciers in
Greenland would need to move at 70 kilometres per year, and Pine Island and
Thwaites glaciers at 50 kilometres per year, from now until 2100. Since most
glaciers are moving at just a few kilometres per year, to Pfeffer and many
others, these numbers seem highly unrealistic.

Worst case

So what is possible? For scenarios based on conservative assumptions, such
as a doubling of glacier speeds, Pfeffer found sea level will rise by around
80 centimetres by 2100, including thermal expansion. "For the high end, we
took all of the values we could change and we pushed them forward to the
largest numbers we imagined would be reasonable," says Pfeffer. The answer:
2 metres.

These estimates fit well with recent studies of comparable periods in the
past, which have found that sea level rise averaged up to 1.6 metres per
century at times. There is a huge caveat in Pfeffer's number crunching,
though. "An important assumption we made is that the rest of West Antarctica
stays put. And this is the part of West Antarctica that is held behind the
Ross ice shelf and the Ronne ice shelf," says Pfeffer. "Those two ice
shelves are very big, and very thick, and very cold. We don't see a way to
get rid of those in the next century."

Holland is not so sure. He has been studying computer models of ocean
currents around Antarctica, and he doesn't like what he sees. The subsurface
current of warm water near the frozen continent, known as the circumpolar
deep water, branches near the coast, and one branch hits Pine Island --
which is probably why the ice there is thinning and speeding up. "Another
branch of it comes ever so close to the Ross ice shelf," says Holland. "In
some computer simulations of the future, the warm branch actually goes and
hits Ross."

While it is impossible to predict exactly what will cause this, the lessons
from Jakobshavn show that a small change in the wind patterns over
Antarctica might be enough to shift the warm current towards and eventually
underneath the Ross ice shelf. Then even this gigantic mass of ice -- about
the size of France -- becomes vulnerable, regardless of how cold the air
above it is. Pfeffer agrees that the Ross and Ronne ice shelves are the wild
cards. "If we pull the plug on those two, then we create a very different
world."

Is there really a danger of a collapse, which would cause a sudden jump in
sea levels? Paul Blanchon's team at the National Autonomous University of
Mexico in Cancun has been studying 121,000-year-old coral reefs in the
Yucatan Peninsula, formed during the last interglacial period when sea level
peaked at around 6 metres higher than today. His findings suggest that at
one point the sea rose 3 metres within 50 to 100 years.

We just don't know if this could happen again in the 21st century. What is
clear, though, is that even the lowest, most conservative estimates are now
higher than the IPCC's highest estimate. "Most of my community is
comfortable expecting at least a metre by the end of this century," says
Bindschadler.

And it will not stop at a metre. "When we talk of sea level rising by 1 or 2
metres by 2100, remember that it is still going to be rising after 2100,"
Rignot warns.

All of which suggests we might want to start preparing. "People who are
trying to downplay the significance say, 'Oh, the Earth has gone through
changes much greater than this, you know, in the geological past'," says
Pfeffer. "That's true, but it's completely irrelevant. We weren't there
then."

............

WHAT IT ALL MEANS

If a 1 metre rise in sea level doesn't sound like much, consider this: about
60 million people live within 1 metre of mean sea level, a number expected
to grow to about 130 million by 2100.

Much of this population lives in the nine major river deltas in south and
southeast Asia. Parts of countries such as Bangladesh, along with some
island nations like the MaldivesMovie Camera, will simply be submerged.

According to a 2005 report, a 1-metre rise in sea level will affect 13
million people in five European countries and destroy property worth $600
billion, with the Netherlands the worst affected. In the UK, existing
defences are insufficient to protect parts of the east and south coast,
including the cities of Hull and Portsmouth.

Besides inundation, higher seas raise the risk of severe storm surges and
dangerous flooding. The entire Atlantic seaboard of North America, including
New York, Boston and Washington DC, and the Gulf coast will become more
vulnerable to hurricanes. Today's 100-year storm floods might occur as often
as every four years -- in which case it will make more sense to abandon
devastated regions and towns than to keep rebuilding them.

............

NHNE's Climate Change Resource Page:
http://www.nhne.org/tabid/490/Default.aspx

NHNE's 1000 Most Recent Climate Change Articles:
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Published by David Sunfellow
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