Sea level rise: It's worse than we thought

by Anil Ananthaswamy, New Scientist, July 1, 2009

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Even before it was released, the report was outdated.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 cm 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 cm 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 m in my view."

This might sound dramatic, but we know sea level has swung from 120 m lower than today during ice ages to more than 70 m 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 10-20 cm 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 cm.

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 cm by 2100, and by another 10 cm by 3000. At the other extreme, if emissions peak at 1200 ppm, thermal expansion alone would lead to a 0.5-m rise by 2100, and another 1.4 m by 3000 (see "How high, how soon?").

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 cm 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 mm. Antarctica is losing about 200 gigatonnes per year, almost all of it from West Antarctica and the Antarctic Peninsula, raising levels by 0.55 mm. "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 m 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 km/year, three years ago, to over 4 km/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 m -- less than the 5 m 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 km/year to more than 9 km/year by 2000 and 13 km/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 m -- 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 m by 2100 from Greenland, and 1.5 m 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 km/year, and Pine Island and Thwaites glaciers at 50 km/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 cm 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 m 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 (pictured above) in the Yucatan Peninsula, formed during the last interglacial period when sea level peaked at around 6 ms higher than today. His findings suggest that at one point the sea rose 3 m within 50-100 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."

Anil Ananthaswamy is a contributing editor for New Scientist

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

Sea levels rising twice as fast as predicted

Melting ice sheets in Greenland and Antarctica force UN scientists to issue dramatic warning

by Michael McCarthy, Environment Editor, The Independent, March 11, 2009

Sea levels are predicted to rise twice as fast as was forecast by the United Nations only two years ago, threatening hundreds of millions of people with catastrophe, scientists said yesterday in a dramatic new warning about climate change. Rapidly melting ice sheets in Greenland and Antarctica are likely to push up sea levels by a metre or more by 2100, swamping coastal cities and obliterating the living space of 600 million people who live in deltas, low-lying areas and small island states.

Low-lying countries with increasing populations, such as Bangladesh, Burma and Egypt, could see large parts of their surface areas vanish. Experts in Bangladesh estimate that a one-metre rise in sea levels would swamp 17 per cent of the country's land mass. Pacific islands such as Tuvalu, where 12,000 people live just a few feet above sea level, and the Maldives, would face complete obliteration.

Even Britain could face real challenges in lower-lying areas along the east coast, from Lincolnshire to the Thames estuary, with a much greater risk of catastrophic "storm surges" such as the great flood of 1953 that killed 307 people.

Yesterday's urgent wake-up call to governments about global warming – telling them the data on which they are basing their official advice is flawed – came from four scientists from the US, Australia, France and Germany, who gave a press conference at a scientific meeting on climate change in Copenhagen, Denmark.

Professor Konrad Steffen, from the University of Colorado, Dr John Church, of the Centre for Australian Weather and Climate Research in Tasmania, Dr Eric Rignot, of NASA's Jet Propulsion Laboratory in Pasadena, and Professor Stefan Rahmstorf, from the Potsdam Institute for Climate Impact Research, are all experts in sea-level rise. Their views represent the mainstream opinion of researchers in the field, taking account of the most recent data.

Only two years ago, the UN's Intergovermental Panel on Climate Change (IPCC) said in its Fourth Assessment Report, or AR4, that the worst-case prediction for global sea-level rise was 59cm by 2100. But the scientists in Copenhagen suggested that the 2007 report was a drastic underestimation of the problem, and that oceans were likely to rise twice as fast.

Yesterday's meeting was a scientific overture to the global conference on climate change, which takes place in the Danish capital in December. The four researchers underlined how critical it is that world leaders act to slash the emission of carbon dioxide and other greenhouse gases from industry and transport which are causing the atmosphere to overheat.

Advance negotiations begin in three weeks in Bonn. On pages 20 and 21, we illustrate in detail just how great the task is, profiling the main emitters of CO2 and what they are doing – or not doing – to cut back. Yesterday's alarm call was clearly intended to inject more urgency into the process.

Rising sea levels are caused by the thermal expansion of the ocean – where water increases in volume as it warms. But although the melting of ice already floating in the sea does not add to the level, because it is already displacing its own mass, melting into the sea of land-based ice most definitely does.

It is the accelerated melting of the vast, land-based ice sheets in Greenland and Antarctica, caused by rapidly rising temperatures at high latitudes, which is now speeding up the increase beyond anything previously forecast. The Greenland ice sheet, in particular, is not simply melting but melting "dynamically" – that is, it is collapsing in parts as meltwater seeps down through crevices and speeds up its disintegration. Critically, the four scientists said, this process was not taken into account in the AR4 report, leading to estimates of sea-level increase which were far too low.

They revealed remarkable figures showing just how fast it is now happening. Professor Steffen said Greenland was losing 200–300 cubic kilometres of ice into the sea each year – about the same amount as all the ice in Arctic Europe. This on its own is causing the global sea level to rise by more than a millimetre a year, he added, whereas a decade ago Greenland's contribution to sea level rise was non-existent.

Dr Church said that the most recent satellite and in situ data showed seas were now rising by more than 3 mm a year – more than 50% faster than the average for the 20th century.

"As a result of improved estimates of the observed rise, the thermal expansion, the melting of the glaciers and of the ice sheets, we now have a much better quantitive understanding of why sea level is rising," he said. "Without significant, urgent and sustained emissions reductions, we will cross a threshold which will lead to continuing sea level rise of metres."

Professor Steffen added: "What we have learnt in the past three or four years is that the ice dynamic is much stronger than the models indicated, and the prediction has to be revised up to a metre or more – which is enormous if you look at the impact."

Britain's Environment Agency was apparently unique when it discarded the IPCC's 2007 advice as flawed. Based on its own estimates, it is planning flood defences for 2100 on the basis of a one-metre rise in sea levels – with a "worst-case scenario" of 2.7 metres.

"These startling new predictions spell disaster for millions of the world's poorest people," said Rob Bailey, Oxfam's policy adviser on climate change. "Poor coastal communities in countries such as Bangladesh are already struggling to cope with a changing climate, and it can only get worse. This must be a wake-up call for rich countries who are not doing anywhere near enough to prevent these cataclysmic predictions from becoming a reality."

Link to article: http://www.independent.co.uk/environment/climate-change/sea-levels-rising-twice-as-fast-as-predicted-1642087.html

In Greenland, ice and instability

by Andrew C. Revkin, The New York Time, January 8, 2008

The ancient frozen dome cloaking Greenland is so vast that pilots have crashed into what they thought was a cloud bank spanning the horizon. Flying over it, you can scarcely imagine that this ice could erode fast enough to dangerously raise sea levels any time soon.

Along the flanks in spring and summer, however, the picture is very different. For a lengthening string of warm years, a lacework of blue lakes and rivulets of meltwater have been spreading ever higher on the ice cap. The melting surface darkens, absorbing up to four times as much energy from the sun as unmelted snow, which reflects sunlight. Natural drainpipes called moulins carry water from the surface into the depths, in some places reaching bedrock. The process slightly, but measurably, lubricates and accelerates the grinding passage of ice toward the sea.

Most important, many glaciologists say, is the breakup of huge semisubmerged clots of ice where some large Greenland glaciers, particularly along the west coast, squeeze through fjords as they meet the warming ocean. As these passages have cleared, this has sharply accelerated the flow of many of these creeping, corrugated, frozen rivers.

All of these changes have many glaciologists “a little nervous these days — shell-shocked,” said Ted Scambos, the lead scientist at the National Snow and Ice Data Center in Boulder, Colo., and a veteran of both Greenland and Antarctic studies.

Some fear that the rise in seas in a warming world could be much greater than the upper estimate of about two feet in this century made by the Intergovernmental Panel on Climate Change last year. (Seas rose less than a foot in the 20th century.) The panel’s assessment did not include factors known to contribute to ice flows but not understood well enough to estimate with confidence. All the panel could say was, “Larger values cannot be excluded.”

A scientific scramble is under way to clarify whether the erosion of the world’s most vulnerable ice sheets, in Greenland and West Antarctica, can continue to accelerate. The effort involves field and satellite analyses and sifting for clues from past warm periods, including the last warm span between ice ages, which peaked about 125,000 years ago and had sea levels 12-16 feet higher than today’s.

The Arctic Council, representing countries with Arctic territory, has commissioned a report on Greenland’s environmental trends, to be completed before the 2009 climate-treaty talks in Copenhagen, at which the world’s nations have pledged to settle on a long-term plan for limiting human-caused global warming.

Konrad Steffen, a University of Colorado glaciologist who has camped on the shoulders of Greenland’s ice sheet each year since 1990, is the lead author of the chapter in the report on Greenland’s climate. Last August, he and a team focusing on the ways meltwater might affect ice movement dropped a camera 330 feet down a water-filled moulin to explore whether the plumbing system can be mapped.

Research on alpine glaciers shows that as more water flows through such apertures, ice can shift more quickly. But eventually large sewerlike conduits form, limiting the lubrication effect. The camera drop was only an initial test.

Alberto Behar, a NASA engineer who designed the camera, said some unconventional methods were being considered to chart the flow of such water. “We had ideas to send rubber ducks down and see if they pop out in the ocean,” he said. “They’d have a little note saying, ‘Please call this number if you find me.’”

The changes seen in Greenland may turn out to be self-limiting in the short run; surging glaciers can flatten out and slow, for instance. Or they may be a sign that the island’s ice — holding about the same volume of water as the Gulf of Mexico — is poised for a rapid discharge. Scientists are divided on that question, and on whether there is a near-term risk from a Texas-size portion of West Antarctica’s ice sheet that is also showing signs of instability. This split divides those foreseeing a rise in the sea level of a couple of feet this century from water added by Greenland, West Antarctica and mountain glaciers, and a few experts who speak of a couple of yards in that time.

Those holding a more conservative view of Greenland’s near-term fate include Richard Alley of Pennsylvania State University, who noted that ice cores and tests of organic material from beneath the ice implied that the main mass of the Greenland ice sheet clearly endured thousands of years of warming in the past without vanishing.

“It’s basically a big lump of ice sitting on this bedrock,” Dr. Alley said in describing Greenland’s behavior in warm conditions. “What it tries to do is snow more in the middle and melt more on the edges. If it pulls its edges back, then there’s less area to melt, and that helps it survive. That’s why you can have a stable ice sheet in a warmer climate.”

But there is no significant debate on the long-term picture anymore. Should greenhouse-gas emissions follow anything close to a “business as usual” rise, the resulting warming and ice loss at both ends of the earth would cause coasts to retreat for centuries. While it was circumspect about near-term changes, the intergovernmental panel was confident about that long view.

The prospect of having no “normal” coastline for the foreseeable future has many scientists deeply concerned.

“What is at stake is the stability we have always taken for granted” both for coasts and climate itself, said Jason E. Box, an associate professor of geography at Ohio State University. Dr. Box presented fresh findings at the American Geophysical Union meeting last month showing that several Greenland glaciers accelerated sharply in direct response to warming, both in a warm spell starting in the 1920s and now.

Eric Rignot, a longtime student of ice sheets at both poles for NASA’s Jet Propulsion Laboratory, said he hoped the public and policymakers did not interpret uncertainty in the 21st-century forecast as reason for complacency on the need to limit risks by cutting emissions.

Dr. Rignot recently proposed that unabated warming could result in three feet of global sea rise just from water flowing off Greenland, three feet from Antarctica and 18 inches as the remaining alpine glaciers shrivel away.

This is similar to projections by the most prominent NASA climate scientist, James E. Hansen, but more than twice the three-foot rise that many glaciologists seem to agree on as an outer bound for what is possible by the end of the century.

“It is too early to reassure that all will stabilize, and similarly there is no way to predict a catastrophic collapse,” Dr. Rignot said. “But things are definitely far more serious than anyone would have thought five years ago.”

[BLOGGER'S NOTE: And yet another year has passed and we know that things are much worse than these scientists were willing to speculate on last year.]

Link to article: http://www.physsci.uci.edu/psnews/?id=358

Geophysical Research Letters, Vol. 35, L20502, doi:10.1029/2008GL035417, 2008

Mass balance of the Greenland ice sheet from 1958 to 2007

E. Rignot (Department of Earth System Science, University of California, Irvine, California, USA; Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA), J. E. Box (Byrd Polar Research Center, Ohio State University, Columbus, Ohio, USA), E. Burgess (Department of Geography, University of Utah, Salt Lake City, Utah, USA), and E. Hanna (Department of Geography, University of Sheffield, Sheffield, UK)

Abstract

We combine estimates of the surface mass balance, SMB, of the Greenland ice sheet for years 1958 to 2007 with measurements of the temporal variability in ice discharge, D, to deduce the total ice sheet mass balance. During that time period, we find a robust correlation (R² = 0.83) between anomalies in SMB and in D, which we use to reconstruct a continuous series of total ice sheet mass balance. We find that the ice sheet was losing 110 ± 70 Gt/yr in the 1960s, 30 ± 50 Gt/yr or near balance in the 1970s–1980s, and 97 ± 47 Gt/yr in 1996 increasing rapidly to 267 ± 38 Gt/yr in 2007. Multi-year variations in ice discharge, themselves related to variations in SMB, cause 60 ± 20% more variation in total mass balance than SMB, and therefore dominate the ice sheet mass budget.

Received 21 July 2008; accepted 22 September 2008; published 22 October 2008.

Key words: glaciology, mass balance, sea level

Index Terms: 0720 Cryosphere: Glaciers; 0726 Cryosphere: Ice sheets; 0762 Cryosphere: Mass balance (1218, 1223); 1621 Global Change: Cryospheric change (0776); 1620 Global Change: Climate dynamics (0429, 3309).

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Citation: Rignot, E., J. E. Box, E. Burgess, and E. Hanna (2008), Mass balance of the Greenland ice sheet from 1958 to 2007, Geophys. Res. Lett., 35, L20502, doi:10.1029/2008GL035417.

Copyright 2008 by the American Geophysical Union.

Geophysical Research Letters, Vol. 35, L20502; doi:10.1029/2008GL035417, 2008

Mass balance of the Greenland ice sheet from 1958 to 2007

E. Rignot (Department of Earth System Science, University of California, Irvine, CA, U.S.A.; Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, U.S.A.), J. E. Box (Byrd Polar Research Center, Ohio State University, Columbus, OH, U.S.A.), E. Burgess (Department of Geography, University of Utah, Salt Lake City, UT, U.S.A.), and E. Hanna (Department of Geography, University of Sheffield, Sheffield, U.K.)

Abstract

We combine estimates of the surface mass balance (SMB) of the Greenland ice sheet for the years 1958 to 2007 with measurements of the temporal variability in ice discharge (D) to deduce the total ice sheet mass balance. During that time period, we find a robust correlation (R² = 0.83) between anomalies in SMB and in D, which we use to reconstruct a continuous series of total ice sheet mass balance. We find that the ice sheet was losing 110 ± 70 Gt/yr in the 1960s, 30 ± 50 Gt/yr or near balance in the 1970s–1980s, and 97 ± 47 Gt/yr in 1996 increasing rapidly to 267 ± 38 Gt/yr in 2007. Multi-year variations in ice discharge, themselves related to variations in SMB, cause 60 ± 20% more variation in total mass balance than SMB and therefore dominate the ice sheet mass budget.

Received 21 July 2008; accepted 22 September 2008; published 22 October 2008.

Key words: glaciology, mass balance, sea level

Index Terms: 0720 Cryosphere: Glaciers; 0726 Cryosphere: Ice sheets; 0762 Cryosphere: Mass balance (1218, 1223); 1621 Global Change: Cryospheric change (0776); 1620 Global Change: Climate dynamics (0429, 3309).

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Citation: Rignot, E., J. E. Box, E. Burgess, and E. Hanna (2008), Mass balance of the Greenland ice sheet from 1958 to 2007, Geophys. Res. Lett., 35, L20502, doi:10.1029/2008GL035417.

Copyright 2008 by the American Geophysical Union.

Link to abstract: http://www.agu.org/pubs/crossref/2008/2008GL035417.shtml

Science, 9 May 1997, Vol. 276, No. 5314, pp. 934–937 DOI: 10.1126/science.276.5314.934

Reports

North and Northeast Greenland Ice Discharge from Satellite Radar Interferometry

E. J. Rignot, ^* S. P. Gogineni, W. B. Krabill, S. Ekholm

Ice discharge from north and northeast Greenland calculated from satellite radar interferometry data of 14 outlet glaciers is 3.5 times that estimated from iceberg production. The satellite estimates, obtained at the grounding line of the outlet glaciers, differ from those obtained at the glacier front, because basal melting is extensive at the underside of the floating glacier sections. The results suggest that the north and northeast parts of the Greenland ice sheet may be thinning and contributing positively to sea-level rise.

E. J. Rignot, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109-8099, U.S.A.
S. P. Gogineni, Radar Systems and Remote Sensing Laboratory, The University of Kansas, 2291 Irving Hill Road, Lawrence, KS 66045-2969, U.S.A.
W. B. Krabill, NASA Goddard Space Flight Center, Wallops Flight Facility, Laboratory for Hydrospheric Processes, Wallops Island, VA 23337, U.S.A.
S. Ekholm, Kort and Matrikelstyrelsen, Geodetic Division, Rentemestervej 8, DK-2400 Copenhagen NV, Denmark
^* To whom correspondence should be addressed. e-mail: eric@adelie.jpl.nasa.gov

Link to abstract: http://www.sciencemag.org/cgi/content/abstract/276/5314/934
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Science, 17 February 2006, Vol. 311, No. 5763, pp. 986–990 DOI: 10.1126/science.1121381

Reports

Changes in the Velocity Structure of the Greenland Ice Sheet

Eric Rignot^1* and Pannir Kanagaratnam^2*

Using satellite radar interferometry observations of Greenland, we detected widespread glacier acceleration below 66° north between 1996 and 2000, which rapidly expanded to 70° north in 2005. Accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year. As more glaciers accelerate farther north, the contribution of Greenland to sea-level rise will continue to increase.

¹ Jet Propulsion Laboratory, California Institute of Technology, Mail Stop 300-319, Pasadena, CA 91109–8099, U.S.A.
² Center for Remote Sensing of Ice Sheets (CReSIS), University of Kansas, Lawrence, KS 66045, U.S.A.

^* To whom correspondence should be addressed. e-mail: eric.rignot@jpl.nasa.gov (E.R), pannir@ku.edu (P.K.)

Link to abstract: http://www.scienceonline.org/cgi/content/abstract/311/5763/986
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Science, 20 June 2008, Vol. 320, No. 5883, pp. 1622–1625 DOI: 10.1126/science.1153929

Reports

Natural Variability of Greenland Climate, Vegetation, and Ice Volume During the Past Million Years

Anne de Vernal^* and Claude Hillaire-Marcel

The response of the Greenland ice sheet to global warming is a source of concern notably because of its potential contribution to changes in the sea level. We demonstrated the natural vulnerability of the ice sheet by using pollen records from marine sediment off southwest Greenland that indicate important changes of the vegetation in Greenland over the past million years. The vegetation that developed over southern Greenland during the last interglacial period is consistent with model experiments, suggesting a reduced volume of the Greenland ice sheet. Abundant spruce pollen indicates that boreal coniferous forest developed some 400,000 years ago during the "warm" interval of marine isotope stage 11, providing a time frame for the development and decline of boreal ecosystems over a nearly ice-free Greenland.

¹GEOTOP Geochemistry and Geodynamics Research Center–Université du Québec à Montréal, Case Postale 8888, succursale Centre-Ville, Montréal, Québec H3C 3P8, Canada.

^* To whom correspondence should be addressed. e-mail: devernal.anne@uqam.ca

Link to abstract: http://www.sciencemag.org/cgi/content/abstract/320/5883/1622

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Marc Kaufman and Eric Rignot
Washington Post Staff Writer and NASA Scientist, respectively
Monday, January 14, 2008; 12:00 PM

Washington Post staff writer Marc Kaufman will be online at Noon ET on Monday, Jan. 14 to discuss the shrinking of the Antarctic ice sheet.
He will be joined by Eric Rignot, who has written a report in the scientific journal, Nature, that shows evidence that climactic changes may be destabilizing vast ice sheets of western Antarctica. Previously, this area of the continent was thought to be relatively protected from the impact of global warming.

As Kaufman, writes of Rignot's report, this raises the prospect of faster sea level rise than current estimates. Read more in Escalating Ice Loss Found in Antarctica (The Post, Jan. 14).

Rignot, a professor of earth system science at University of California, Irvine, is also a senior scientist with NASA's Jet Propulsion Laboratory.

A transcript follows.

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Marc Kaufman: Good morning, and thanks for joining us. We also are fortunate to have Eric Rignot, author of the Nature study on Antarctica, on the chat, and he and I will do our best to answer your questions. Eric has studied Antarctica and Greenland for 15 years and is well known for his research. Before we start, a brief correction from me: Mt. Kilimanjaro is not, as I wrote, in Kenya, but in Tanzania.

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Clarksville, Md.: The article today referenced Western Antarctica as on land, but also below sea level. Could you please explain this?

Eric Rignot: West Antarctica ice is grounded below sea level. But the top of the ice sheet is of course above sea level. Think about a vast expanse of ice sitting in the middle of the sea and reaching all the way down to the sea floor.
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Clarksville, Md.: Using the best scientific common sense with data to date, what is the acceleration trend for the melting that you understand to be happening in Antarctica, and how does that translate into sea level rise by year or decade?

Eric Rignot: The actual trend in Antarctica is a contribution to sea level rise that increased from about 0.3 mm/yr in 1996 to 0.5 mm/yr in 2006. It was certainly lower prior to 1996, and will continue to increase in years to come.

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Jefferson, N.C.: Sirs:

I have not seen Dr. Rignot's paper in NATURE, so my question may have already been answered.

For several years, there has been an indication that the temperatures over the Antarctic have cooled, as seen in the satellite data provided by MSU instruments as analyzed by UAH. This same data set was also analyzed by RSS and they chose to exclude any results poleward of 70S because of contamination due to the high elevations of interior Antarctica. Other researchers have reported that there is a warming trend seen in data collected near the coasts of Antarctica. In a paper published in the GRL, I found an apparent flaw in a comparison between sonde data and that produced by the UAH team (2003GL017938), perhaps another indication of surface contamination.

My question is, does the data in the NATURE paper support the claim that the Antarctic is warming and if so, how might that finding be reconciled with the cooling trend found by John Christy et al. at UAH?

R. E. Swanson

Eric Rignot: Good point. Antarctica's interior is cooling. The only air warming is in the Peninsula. But we are seeing at present in the glaciers is not related to atmospheric forcing. We think it is related to thermal forcing from the ocean. Changes in tropospheric circulation and wind patterns have enabled warm sources of water to reach glacier grounding lines and melt them from the bottom. This was the trigger for glacier acceleration in a large section of Antarctica.Melting of the frontal sections reduces the backforce on the glaciers and allows them to flow faster, much like what would happen if you slowly uncork a wine bottle .. or water bottle ..

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Bowdoinham, Maine: If the western shelf is largely below sea level, please explain how melting could contribute to sea level rise.

Eric Rignot: The portion of West Antarctica which is below will contribute little to sea level change if it melts in the ocean; only the portion that is sitting above sea level will contribute in full. There is enough ice sitting above sea level however to raise total sea level by 5-6 m if West Antarctica were to melt to sea

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Camden, Maine: It seems to me that the experts have consistently (almost) been too conservative in their estimates of coming changes in ice melt and other climatic changes.

Here's my question: Can't they somehow get the bigger picture and therefore predict with more accuracy? There are tons of inputs from all over the planet and taken collectively it would seem that even I, a simple lay person, can project that the future is going to be very, very exciting and challenging. I sometimes get the sense that the scientists are trying to downplay the obvious so as not to be too overbearing. Many did that in pre-Katrina times and look where it got us. I'd prefer to hear the unvarnished truth and have time to adjust and deal with it.

I'm working under the current assumption that climate change is going to be H-U-G-E and dealing with it is going where no man has gone before. Your thoughts?

Thanks.

Eric Rignot: Hello.

Prediction of what is going to happen in the polar regions is difficult. The existing models are not good enough. The first step in the last few years has been to get this accepted by the community, but this was tough. WE are now developing better models.

Scientists do not try to downplay. They have to go by the facts,by what they have, whether it is data or model results. We cannot make things up based on opinion. This is why we may at times sound too conservative.

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College Park, Md.: I understand the data and the possible implications. I want to know how we also track our psychological and emotional responses to unprecedented change? What happens to researchers (and all of us) when the facts "enter your system", as Patchauri says?

Eric Rignot: As a scientist, I am trying to do the best research I can do and get objective views on changes in the polar regions. When talking to medias and the press or students about our results, however, I can present a more personal view, but it has to remain within the realms of what I know as a scientist.

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New York City: How does the Antarctic ice sheets melt compare to the Greenland ice sheets...are they melting at the same rate?

Eric Rignot: Antarctic ice sheet loss is nearly comparable, though a bit smaller, than Greenland ice sheet loss. The rate of increase in the last ten years is also comparable.

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Cary, N.C.: What is the source of the claim from global warming skeptics that Antarctic ice is growing, not shrinking, despite the collapse of the Larsen ice shelves?

Eric Rignot: Climate models have been predicting climate warming would increase precipitation in polar regions (because of enhanced evaporation on the oceans), which has indeed been the case in a few places (e.g. Antarctic Peninsula), but the effect is very modest. Since there is no melt in Antarctica and these models ignored the influence of glaciers, Antarctica could only grow. Reality shows otherwise. Reality shows that glaciers speed up and drive the ice sheet mass budget. This is a major shortcoming of models which we will now try to improve.

Models predicted a loss of Antarctic mass only after a warming of 4-5 degree Celsius. We are obviously there much sooner than expected.

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Chantilly, Va.: In your own opinions, what can an average earth citizen do to help to slowdown the glacier melting process?

Eric Rignot: We need to appreciate the seriousness of the situation and the seriousness of climate change. We need to anticipate future changes in the best possible manner. I think that if there is a collective realization that this problem is truly important, we can make a big difference in our every day lives, and elect the right people to make the right decisions.

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Toronto, Canada: Skeptics will be looking for holes in the climate change hypothesis. Please evaluate alternative explanations, such as ocean currents/temperature, or prevailing wind direction. Be a severe self-critic!

Eric Rignot: Antarctica has not been warming up. But climate change is complex and impacts many aspects of the climate system besides air temperatures. In the Antarctic, we believe that the main agent of change is the ocean and fluctuations in the sources of heat around the continent.

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Fairfax County, Va.: Many of us, including me, base our knowledge of this issue on the movie An Inconvenient Truth, which was helpful to me because it offered a broad survey of the whole subject in lay terms. Do you think that movie gives an accurate picture (for a lay audience) of the scientific consensus on this subject? If it does, how does this new finding change what I learned from that film? Thank you for your work on this subject.

Eric Rignot: Inconvenient truth was an excellent movie because it was entirely based on scientific facts. I do not thin our new finding change much of what this film is showing, except that loss of antarctic ice is larger than thought at the time the movie was made.

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New York, N.Y.: How similar or different on what is being observed in Antarctica being observed at the North Polar region?

Marc Kaufman: As I understand it, the sea ice of the Arctic is melting quickly in some areas, while until recently that was true only for the Antarctic peninsula. But a major difference is also that the sea ice in the Arctic typically does melt in the summer--with 5.8 million square miles winter sea ice that exist during winter shrinking on average to 2.7 sq miles in summer -- while that has not been the case typically in Antarctica. Also, much of the Arctic ice covers an ocean, while much of the Antarctic ice is on land in the form of glaciers, snow and ice sheets. Finally, most of the ice in the world is in the Antarctic.

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Ojai, Calif.: When I saw Richard Alley at the AGU this December, he was quite concerned about global warming, but nonetheless said that the ice sheets would melt over the course of centuries, not decades. Does this study suggest otherwise?

Eric Rignot: ICe sheets will NOT melt in the course of decades, it will take centuries, there is no doubt about that. Yet some models have predicted it would take 1,000 years. What we are seeing today says it would happen over time scales of centuries. If sea level rises 1-2m in the next 100 years because of ice sheet melting, it would have a major impact on coastal populations.

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Bowling Green, Ohio: A friend of mine says that "Global Warming" is occurring on other planets, specifically there is ice melt and that all the hoopla being caused by man is false. Any truth to this statement? Is global warming a cosmic occurrence that we have no control over?

-Dwight Taylor

Marc Kaufman: Climate changes are indeed common on other planets, and they have been on the Earth as well. But those natural warming processes occur over long time periods and seem to have different characteristics. But here are some factors that climate specialists--such as those in the Intergovernmental Panel on Climate Change--point to when they this is different. 1/ The build-up of carbon dioxide in the air has been dramatic and unprecedented. 2/ We now have glaciers in retreat not only in polar regions, but also in the high tropics -- the Andes, the Himalayas, and Mt. Kilimanjaro. 3/ Global average temperatures are quickly going up. Put it all together and you have a pretty dramatic picture. The scientists of the IPPC said that human activity was the primary cause, and they said it with a confidence level of 90 percent or so.

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Silver Spring, Md.: A question for Eric or Marc...In the Post article this morning, "Rignot said there has been evidence of ice loss going back as far as 40 years", I assume you mean net ice loss? If so, what is the specific evidence?

Eric Rignot: Ice has been lost in the Antarctica Peninsula for several decades as a result of sustained climate warming. Climate warming in the Peninsula has been proceeding at 5 times the global average. But this is an anomaly, most of the rest of Antarctica remains cool. In the main area of Antarctica that is losing mass at present, however, we believe that warm pools of water are able to reach the glaciers as a result of changes in wind patterns. Some of these changes are traced back to the 1970s, but the mass loss numbers get smaller as we go back to the 1970s, so I think the change in ocean forcing started around that time. It turns out to be a time when observations suggest a shift in antarctic oscillation took place. But then the story becomes a bit too technical ...

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Washington, D.C.: Could the wind pattern changes that are bringing more warm water to shore, and causing increased erosion of the glaciers and the ice sheets, be a result of the weather changes that are being caused by global dimming?

Eric Rignot: The change in wind patterns is related to global climate change. This is not my area of expertise, however, so I wont dwell into the details.

Climate models have (all) shown that climate change - while influenced by solar variability of course, as it always did in the past and will always do - was very strongly influence by greenhouse gases emissions. Climate models simply cannot match observations of climate change if they do not include human influence. This was a major conclusion of the last report from IPCC.

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Raleigh, N.C.: Almost every day brings another article/more evidence about the damage human beings continue to inflict on the environment and the earth, and my despair and sadness deepens. Is there anything an "average" person can do to help in any significant way?

Marc Kaufman: Experts say that we can address the issue on both individual and societal levels. Individually we can try to reduce our use of gasoline, home heating oil and electricity, and try to learn more about how much energy goes into producing the products (including food) that we purchase. Based on that information, we can make more or less energy-efficient choices.

On a larger societal level, it is clear that many states and quite a few businesses have embraced reducing greenhouse gas emissions as a priority. Others have not, and perhaps they could be convinced over time. The big issue, of course, is the role of the federal government, with many saying the Bush administration has been missing in action. Bush officials say that is unfair, but whatever is the case, I think it's clear that his successor will have to do more. On this issue, I think that public pressure has shown itself to make a difference. The next step will be how to translate a desire to reduce global warming to an actual plan. For now, it certainly wouldn't hurt to plant some trees (which take up CO2.)

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Olney, Md.: I'm curious about the theory that the "ice melt" or "ice recession" in Antarctica is being caused by warmer underwater currents. Has there been testing of the currents and have actual temperatures been measured? You didn't mention specific data in your story. How much warmer is this current? Thanks!

Eric Rignot: Unfortunately, we do not have much data to quantify how warm these currents are. In places where we do, however, the waters are 3 degrees warmer than 'normal' water. This is a very high temperature differential and quite possibly one of the warmest around Antarctica. Unsurprisingly, this also corresponds to the sector that is changing most rapidly at present.

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Clarksville, Md.: Given what you know of Antarctic, Greenland, and global climate change, what can U.S. citizenry do to give you scientists better tools through government to develop the scientific evidence? Satellites, onsite monitoring stations, UAV's, etc.?

Eric Rignot: We need to make sure we have appropriate tools to observe ongoing changes. Unfortunately, the number of satellite missions flown to observe the Earth has been decreasing in the last decade, as a result of more efforts placed on the Mars program and the space station. I hope this will change. Studies have shown that if we were able to go back to the situation we were in the early 1990s, we would be ok.

Perhaps more important, we need better numerical models with the realization that existing ones have major shortcomings. Fortunately this can be solved by giving a job to a lot of smart young scientists interested in doing that.

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Milwaukee, Wis.: Am I correct that the melting of ice in Antarctica is a more serious threat than in the Arctic in terms of ocean level rise because this ice is on land, like Greenland?

If you were looking ahead 50 years, would we be seeing by then many of our coastal cities beginning to be submerged, as well as flooded during storms and surges? And if this is the case, when will scientists begin telling the world that we must start planning, and soon to retreat from the coasts? This is going to take decades to prepare, but it seems we will only start dealing with it when we are in crisis mode.

Do scientists have the responsibility to urge these kinds of preparations?

Margaret

Marc Kaufman: Yes, the loss of ice in Antarctica is the big issue in terms of ocean level rise. Some 90 percent of the world's ice is locked up in Antarctica, and that makes it the 900 pound gorilla. The large eastern section of the continent is on land and is not currently losing much ice -- indeed, some have found an increase in ice levels there. (Rignot's newest data shows a steady-state in terms of snowfall versus ice loss.) So for now, the drama is being played out in West Antarctica and the peninsula. The West Antarctic coast appears to be losing ice because of changed wind patterns and ocean movements that together bring more warm water from the circumpolar current to shore. The result is ice erosion and melting. But on the peninsula, the weather is also changing dramatically -- and is showing increased temperatures that are some of the most dramatic in the world. By the way, West Antarctica has as much ice as Greenland.

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Washington, D.C.: Is there a relation between ocean temperature and sea level rise. In other words, how does just temperature change relate to increases in sea levels?

Marc Kaufman: The connection is that an increase in ocean temperatures has the effect of eroding glaciers and ice sheets. When they melt, that increases the volume of water in the oceans and so they rise. This is not an issue so much of sea ice, but rather of ice on land, or glaciers that have displaced ocean (as in West Antarctica.)

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Clarksville, Md.: Sir, going back to your Camden, Maine, answer. I have to concur 100% with the questioner, but I also understand that scientifically-established truth has not caught up with the unraveling massive climate events. However, why can't the scientific community in this seemingly dire emergency for civilization, take what data they have, along with te 25-yr. history of scientific underestimates, and provide an un-scientific but extremely valuable "forecast" of where civilization is truly headed...without politicians having any say?

Eric Rignot: Good point. A lot of the changes we are observing today however have virtually no equivalent in the past. In the past, we did not have an outburst of greenhouse gases generated by an inhabitant of the planet. Also, despite all our accumulated knowledge of past climate, we do not know well how fast climate can change. Cores taken in Greenland in the early 1990s said: very fast, perhaps less than decades. New cores taken a few years ago said: even faster, perhaps years. This was unthinkable before these results were obtained.

In terms of ice sheets, we have been relatively data poor. It is hard to get there, get data over large areas, and measurements are difficult. Satellite remote sensing has truly changed our capability to understand what is happening the polar regions. But this only started a few decades ago, and as far as ice sheets are concerned, it really only started in the early 1990s. That's quite a short time to allow us to fully understand what is going and what the implications will be. But we will catch up. In March 2007, the International Polar Year started. There could not have been a better time for it.

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Iowa City, Iowa: How many years have the glaciers been melting on Greenland? It would appear the comparison is not valid to the Antarctic regions. Please watch the film 90 Degree's South by Henry Ponting 1910 words added in 1933 to see some of the calving of icebergs 100 years ago. Can you explain Geologic Time against what you call real time today to help us all gain a little more understanding?

Eric Rignot: Greenland glaciers have been melting ever since the end of the last glacial maximum. It is an ongoing story. Except that the rate of melting at present is far larger than what it has been over the last century and there is no sign of it slowing down.

Similarly, Antarctica has been deglaciating since the last glacial maximum. In some parts of it, it is still ongoing, very slowly. In others, the deglaciation process is pretty much over.

Now this is a process over 100,000 years. What we are concerned about right now is the evolution of ice in the next 100 years.

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Rockville, Md.: Given the rapid decrease of ice in the polar regions, how much longer are they saying it's going to be before Boston, NYC, other coastal cities are under water? I would think that would force us to do whatever it takes to stop global warming. Is it fair to say also, that if we plant millions of trees, that might help in stopping the warming?

Marc Kaufman: I think was has become clear over the past few years is that the global warming, and the related consequences, are taking place faster than expected. But that said, there are still so many imponderables involved that I don't think anyone has firm predictions about what will happen and when.

As for planting trees, it certainly couldn't hurt. I've seen studies suggesting that even massive tree planting in temperate climates would not make much difference in terms of global warming, but that it could have some effect on localized climates.

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Clarksville, Md.: Sir, would you agree that the scientific community, through no fault of their own, have been disadvantaged over the past 25 years in trying to understand global climate change, but in attempting to address it, the scientific community has consistently underestimated the change rate and global dimensions? My understanding is that science cannot grasp the enormously complex and interdependent feedback loops in the warming trend...with new ones being discovered every few months. Your thoughts?

Eric Rignot: Yes, you are correct, but I am a bit more optimistic. I think the scientific community is coming to grasp with the Earth climate system. Slowly but surely. We are not doing very well in terms of predicting the future of ice sheets, but we are now gathering important information which will help improve the next generation of models. Global climate models were not doing very well 30 years ago. They came a long way and are now becoming more reliable. I remain optimistic that in years to come, not decades, we will produce more realistic predictions of the evolution of Greenland and Antarctica. We are learning tremendously right now about the dynamics of these systems. An unfortunate byproduct of changing the climate of the Earth so rapidly ...

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Silver Spring, Md.: If Antarctic ice loss is accelerating - particularly in West Antarctica - as your work shows ... what do you think odds are it will collapse this century?

Eric Rignot: That is a very difficult question. I think a collapse of West Antarctica in the next 100-200 yr is now a concept that is back on the table; it was not on the table anymore 10 years ago; it was first put on the table in the early 1970s. But even if the ice sheet does not collapse, a loss of a significant portion of Antarctica and Greenland could raise sea level 1-2 m in the next century, and I think this is already something to worry about.

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Milwaukee: Just want to reiterate the question: given how long a timeline would be needed, shouldn't scientists begin recommending development of a long-term plan for retreating from the coasts? Shouldn't there be some policy recommendations around the whole issue of coastal development that would be informed by this science -- not just here in the US, but also internationally? When does science interact with politics and economics -- neither or which is anywhere near catching up to the new alarms about melting ice sheets.

Marc Kaufman: I'm not sure it's up to scientists to come up with a long-term plan for pulling back from coastal areas -- that's far more a political, social and economic issue. The grim truth is that a huge proportion of human activity occurs within 100 miles of a coastline, and a large percentage of humans live in that region. So my guess is that nobody will do much until we are forced to by a number of tragedies like Katrina. There is just too much human and economic capital invested along coasts, and not enough certitude about the nature of the risk. But this said, it certainly seems to me that it's time for government to look seriously at limiting new development along coastal areas, and to do more about restoring coastal wetlands.

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Normal trends versus altered trends: Just for the sake of clarity, can you explain briefly the difference between the normal global warming/cooling that we know has taken place from time to time and the influence of external factors including man-made issues that may increase or decrease these trends? I'm thinking in particular of things like magnetic polar shift as a normal trend compared to greenhouse gases that are clearly influenced by humanity.

Eric Rignot: This is a climate question. I am a glaciologist. From looking at ice melting, I cannot trace the origin of the change. I can only say: air temperatures have been getting warmer, or the ocean has been getting warmer, and the glacier changes we are observing are quite large compared to the background noise.

Ice sheets have growing and shrinking over time, over geologic times, over 100,000 year cycle. My view is that what we are witnessing right now is of a different nature. Ice sheets are responding to a spike in climate change created by us, and we are not well prepared to predict what may happen next. The only thing I can add to that is that this is serious problem. The way ice sheets are changing today, the rate at which they change worries me.

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Clarksville, Md.: To Marc,

You & the Washington Post have done an excellent job of reporting global warming news over the past several years. I commend you! I hope that even as the Post has played a key role in national politics in the past in the name of truth, on the topic of awakening people everywhere to the coming global warming crises, the Post will take this subject where scientists cannot by their required discipline. As Dr. Rignot cited, there are not past parallel events to reference where we are. His dedication and that of his colleagues will be cited years hence as having saved many from disaster. Thank you!

Marc Kaufman: Many thanks for your comment. My colleagues Juliet Eilperin and Doug Struck have also written many stories on the subject, which clearly the paper sees as very important.

We thank you all for your questions, which have been most interesting and stimulating.

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