Sea Level Rise: Important Article in New Scientist10 July
or
and following the article is a copy of the NewScientist Editorialon the same subject:
These reports are both a good reference (the NewScientistis a great UK science policy magazine — for science what theEconomist is for political economics), and AN IMPORTANT REMINDERthat we need to feature future SEA LEVEL RISE issues in all of ourreports. In fact, I regret not including a climate change/sea levelimpacts point in the “Lessons Learned” paper for our recentNevis Peak Park Inventory and Management Plan.
I wouldadd only one thing from the perspective of small islands and our manyyears of natural resource conservation work: It’s even MORE WORSERthan that! The NewScientist does not discuss the need torecover, restore and rebuild critical coastal ecosystems whosedestruction has ALREADY badly compromised the ability of coastal andmarine ecosystems to provide critical ecosystem services. For example,there is new evidence that coastal ecosystem losses are directlyimplicated in the catastrophic failure of reef and other marineecosystems in the Caribbean. Rebuilding these systems would be hard atany time; rebuilding them in the face of multi-meter seal level riseAND increasing storm frequency is going to be virtually impossibleunless we build public acceptance and the legal and institutionalreforms to enable public authorities to take the steps necessary torebuild mangroves, wetlands and fringing reefs.
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Sea level rise:It’s worse than we thought
01July 2009 byAnil Ananthaswamy
Magazine issue 2715.
Forsimilar stories, visit the Climate Change Topic Guide
photocaption
Greenland is alreadylosing enough ice to raise sea level by 0.8 millimetres per year(Image: Nick Cobbing)
See our relatededitorial
FOR a few minutes David Holland forgets about his work and screamslike a kid on a roller coaster. The small helicopter he’s riding in isslaloming between towering cliffs of ice – the sheer sides of giganticicebergs that had calved off Greenland’s Jakobshavn glacier. “Itwas like in a James Bond movie,” Holland says afterwards.”It’s the most exciting thing I have everdone.”
Jakobshavn has doubled its speed in the past 15 years, drainingincreasing amounts of ice from the Greenland ice sheet into the ocean,and Holland, an oceanographer at New York University, has been tryingto find out why. Scientists like him are more than a little astonishedat the rate at which our planet’s frozen frontiers seem to beresponding to global warming. The crucial question, though, is whatwill happen over the next few decades and centuries.
That’s because the fate of the planet’s ice, from relatively small icecaps in places like the Canadian Arctic, the Andes and the Himalayas,to the immense ice sheets of Greenland and Antarctica, will largelydetermine the speed and extent of sea level rise. At stake are thelives and livelihoods of hundreds of millions of people, not tomention millions of square kilometres of cities and coastal land, andtrillions 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 by2100, but this excluded “future rapid dynamical changes in iceflow”. Crudely speaking, these estimates assume ice sheets are abit like vast ice cubes sitting on a flat surface, which will stay inplace as they slowly melt. But what if some ice sheets are more likeice cubes sitting on an upside-down bowl, which could suddenly slideoff into the sea as conditions get slippery? “Larger rises cannotbe excluded but understanding of these effects is too limited toassess their likelihood,” the IPCC report stated.
Even before it was released, the report was outdated. Researchers now know far more. And whilewe still don’t understand the dynamics of ice sheets and glaciers wellenough to make precise predictions, we are narrowing down thepossibilities. The good news is that some of the scarier scenarios,such as a sudden collapse of the Greenland ice sheet, now appear lesslikely. The bad news is that there is a growing consensus that theIPCC estimates are wildly optimistic.
The oceans are already rising. Global average sea level rose about 17centimetres in the 20th century, and the rate of rise is increasing.The biggest uncertainty for those trying to predict future changes ishow humanity will behave. Will we start to curb our emissions ofgreenhouse gases sometime soon, or will we continue to pump ever moreinto the atmosphere?
Even if all emissions stopped today, sea level would continue to rise.”The current rate of rise would continue for centuries iftemperatures are constant, and that would add about 30 centimetres percentury to global sea level,” says Stefan Rahmstorf of thePotsdam Institute for Climate Impact Research in Germany. “If weburn all fossil fuels, we are likely to end up with many metres of sealevel rise in the long run, very likely more than 10 metres in myview.”
This might sound dramatic, but we know sea level has swung from 120metres lower than today during ice ages to more than 70 metreshigher during hotperiods. There isno doubt at all that if the planet warms, the sea will rise. The keyquestions are, by how much and how soon?
To pin down the possibilities, researchers have to look at what willhappen to all the different contributors to sea level under variousemissions scenarios. The single biggest contributor to sea level riseover the past century has been the melting of glaciers and ice caps outside of Greenlandand Antarctica,from Alaska to the Himalayas. According to one recent estimate, the continued loss of thisice will add another 10 to 20 centimetres to sea level by 2100. Itcannot get much worse than this: even if all this ice melted, sealevel would only rise by about 33 centimetres.
Expandingwaters
The second biggestcontributor has been thermal expansion of the oceans. Its futurecontribution is relatively simple to predict, as we know exactly howmuch water expands for a given increase in temperature. A study published earlier thisyear found thateven if all emissions stopped once carbon dioxide levels hit 450 partsper million (ppm) – an unrealistically optimistic scenario – thermalexpansion alone would cause sea level to rise by 20 centimetres by2100, and by another 10 centimetres by 3000. At the other extreme, ifemissions peak at 1200 ppm, thermal expansion alone would lead to a0.5-metre rise by 2100, and another 1.4 metres by 3000 (see “Howhigh, how soon?”).
Then there are the great ice sheets of Greenland and Antarctica, whichhold enough water to raise sea level by about 70 metres. Untilrecently, their contribution to sea level rise was negligible, and theIPCC predicted that Greenland would contribute 12 centimetres at mostto sea level rise by 2100, while Antarctica would actually gain iceoverall due to increased snowfall. “A lot of new results havebeen published since then to show that this very conservativeconclusion does not hold,” says Eric Rignot of the University ofCalifornia, Irvine.
To study the ice sheets, Rignot and colleagues have combinedsatellite-based radar surveys, aircraft altimetry and gravitymeasurements using NASA’s GRACE satellite. They found that ice loss isincreasing fast. Greenland is now losing about 300 gigatonnes of iceper year, enough to raise sea level by 0.83 millimetres. Antarctica islosing about 200gigatonnes per year, almost all of it from West Antarctica and theAntarctic Peninsula, raising levels by 0.55 millimetres. “Themass 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 1metre by 2100. Sounderstanding why Greenland and Antarctica are already losing icefaster than predicted is crucial to improving ourpredictions.
The main reason for the increase is the speeding up of glaciers thatdrain the ice sheets into the sea. One cause is the knock-on effect ofwarmer air melting the surface of the ice: when the surface ice melts,the water pours down through crevasses and moulins to the base ofglaciers, lubricating their descent into the sea. Fears about theimpact of this phenomenon have receded somewhat, though: Antarctica isthought to be too cold for it to be a big factor, and even inGreenland it is only a summertime effect. “It’s significant, butI don’t think it’s the primary mechanism that would be responsible fordramatic increases in sea level,” says glaciologist RobertBindschadler 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 untilthey reach all the way down to the base of the ice. This can have adramatic effect on floating ice shelves. “Essentially, you arechopping up an ice shelf into a bunch of tall thin icebergs, likedominoes standing on their ends,” says Bindschadler. “Andthey are not very stable standing that way.” They fall over, andpush their neighbours out to sea.
The most famous break-up in recent times – that of the Larsen B iceshelf on the Antarctic Peninsula in 2002 – likely happened this way.While the break-up of floating ice shelves does not raise sea leveldirectly, the disintegration of Larsen B had consequences that modelsat the time failed to predict. With little to resist their advance,glaciers behind Larsen B immediately began to move up to eight timesfaster. Five smaller ice shelves in the rapidly warming AntarcticPeninsula have also broken up and many others aredisintegrating.
What liesbeneath
Surface melt poseslittle threat in West Antarctica, as it is so much colder. Here thedanger comes from below. Take the ice shelf holding back the massive Pine Islandglacier, which isthinning in a strange pattern. Radar scans have revealed giant”ripples” up to 100 metres deep on itsunderside.
Bindschadler thinks that the currents created by winter winds raiserelatively warm water from a few hundred metres down in the AmundsenSea off West Antarctica. This melts the underside of the ice shelf andgets trapped in the space it carves out, thus continuing to melt theice from below over a few seasons. As the ice shelf thins, the PineIsland glacier behind it is speeding up, from 3 kilometres per yearthree years ago to over 4 kilometres per year according to the latestunpublished measurements by Ian Joughin of the University ofWashington in Seattle.
What does this have to do with global warming? Climatechange, aided andabetted by the loss of ozone, has strengthened the winds that circle Antarctica.This is speeding up the Antarctic circumpolar current and pushingsurface waters away from the coast, causing deeper, warmer water towell up.
Along with theThwaites glacier and some smaller ones, Pine Island glacier drains athird of the West Antarctic ice sheet. This ice sheet is particularlyvulnerable to ocean heat because much of it rests on the seabed, akilometre or more below sea level. This submarine ice will not raisesea level if it melts, but if it goes a lot of higher-level ice willend up in the ocean. The vulnerable parts contain enough ice to raise sea level3.3 metres – lessthan the 5 metres that was once estimated but more than enough to havecatastrophic effects.
Any increase in thetemperature of seawater in contact with ice can lead to relativelyrapid melting, as with the cavities discovered by Bindschadler.”The ocean has an enormous amount of heat compared to theatmosphere,” 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 hasbeen trying to find out why Greenland’s Jakobshavn glacier startedmoving faster in 1997, speeding up from around 6 kilometres per yearto more than 9 kilometres per year by 2000 and 13 kilometres per yearby 2003. The glacier continues to drain ice from the Greenland icesheet at a higher rate than before.
The increase had been attributed to lubrication by meltwater, butHolland’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 jumpedin 1997, prior to the massive calving of Jakobshavn.
As the teamreported last year, though, the real trigger lay in what happened in1996. That year, the winds across the North Atlantic weakened, slowingdown the warm Gulf Stream. The weakened current meandered aimlesslyand hit west Greenland. “A modest change in wind gives you a bigbang in terms of ice sheet dynamic response,” saysHolland.
Findings like these suggest that predicting sea level rise is eventrickier than previously thought. If relatively small changes in windsand currents could have a big impact on ice sheets, we need extremelygoodmodels of regionalclimate as well as of ice sheets. At the moment we have neither – andwhile regionalclimate models areimproving, ice sheet models are still too crude to make accuratepredictions.
“They are coarse models that don’t include mechanisms that allowglaciers to speed up,” says Rignot. “And what we are seeingtoday is that this is not only a big missing piece, this could be thedominant piece. We can’t really afford to wait 10 to 20 years to havegood ice sheet models to tell people, ‘Well, sea level is actuallygoing to rise 2 metres and not 50 centimetres’, because theconsequences are very significant, and things will be pretty muchlocked in at that point.”
So climate scientists are looking for other ways to predict sea levelrise. Rahmstorf, for instance, is treating the Earth as one big blackbox. His starting point is the simple idea that the rate of sea levelrise is proportional to the increase in temperature: the warmer Earthgets, the faster ice melts and the oceans expand. This held true forthe last 120 years at least. “There is a very close andstatistically highly significant correlation between the rate of sealevel rise and the temperature increase above the pre-industrialbackground 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 thehigher estimate is more likely because emissions have been risingfaster than the IPCC’s worst-case scenario. Rahmstorf’s study got amixed receptionwhen it first appeared, but he can feel the winds of change. “I sensethat now a majority of sea level experts would agree with me that theIPCC projections are much too low,” he says.
Could even Rahmstorf’s estimate be too low? It assumes the relationbetween temperature and sea level is linear, but some experts, mostprominently James Hansen of NASA’s Goddard Institute for Space Studiesin New York, argue that because there are multiple positive feedbacks,such as the lubrication of glaciers by meltwater, higher temperatureswill lead to accelerating ice loss. “Why do I think a sea levelrise of metres would be a near certainty if greenhouse gas emissionskeep increasing?” Hansen wrote in New Scientist (28 July 2007, p 30). “Because while thegrowth of great ice sheets takes millennia, the disintegration of icesheets is a wet process that can proceed rapidly.”
Hansen has made nospecific prediction, however. So just how bad could it get? TadPfeffer of the University of Colorado in Boulder decided to workbackwards from some of the worst-case scenarios: 2 metres by 2100 fromGreenland, and 1.5 metres from West Antarctica, via the Pine Islandand Thwaites glaciers. Just how fast would the glaciers have to bemoving for the sea level to rise by these amounts? Pfeffer found that glaciers in Greenlandwould need to move at 70 kilometres per year, and Pine Island andThwaites glaciers at 50 kilometres per year, from now until 2100.Since most glaciers are moving at just a few kilometres per year, toPfeffer and many others, these numbers seem highlyunrealistic.
Worstcase
So what ispossible? For scenarios based on conservative assumptions, such as adoubling of glacier speeds, Pfeffer found sea level will rise byaround 80 centimetres by 2100, including thermal expansion. “Forthe high end, we took all of the values we could change and we pushedthem 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 foundthat sea level rise averaged up to 1.6 metres per century attimes. There is ahuge caveat in Pfeffer’s number crunching, though. “An importantassumption we made is that the rest of West Antarctica stays put. Andthis is the part of West Antarctica that is held behind the Ross iceshelf and the Ronne ice shelf,” says Pfeffer. “Those two iceshelves are very big, and very thick, and very cold. We don’t see away to get rid of those in the next century.”
Holland is not so sure. He has been studying computer models of oceancurrents around Antarctica, and he doesn’t like what he sees. Thesubsurface current of warm water near the frozen continent, known asthe circumpolar deep water, branches near the coast, and one branchhits Pine Island – which is probably why the ice there is thinning andspeeding up. “Another branch of it comes ever so close to theRoss ice shelf,” says Holland. “In some computer simulationsof the future, the warm branch actually goes and hitsRoss.”
While it is impossible to predict exactly what will cause this, thelessons from Jakobshavn show that a small change in the wind patternsover Antarctica might be enough to shift the warm current towards andeventually underneath the Ross ice shelf. Then even this gigantic massof ice – about the size of France – becomes vulnerable, regardless ofhow cold the air above it is. Pfeffer agrees that the Ross and Ronneice shelves are the wild cards. “If we pull the plug on thosetwo, then we create a very different world.”
Is there really a danger of a collapse, which would cause a suddenjump in sea levels? Paul Blanchon’s team at the National AutonomousUniversity of Mexico in Cancun has been studying 121,000-year-oldcoral reefs (pictured above) in the Yucatan Peninsula, formed duringthe last interglacial period when sea level peaked at around 6 metreshigher than today.His findingssuggest that at one point the sea rose 3 metres within 50 to 100years.
We just don’t know if this could happen again in the 21st century.What is clear, though, is that even the lowest, most conservativeestimates are now higher than the IPCC’s highest estimate. “Mostof my community is comfortable expecting at least a metre by the endof this century,” says Bindschadler.
Most glaciologistswho study Greenland and Antarctica are expecting at least a metre riseby the end of the century
And it will notstop at a metre. “When we talk of sea level rising by 1 or 2metres by 2100, remember that it is still going to be rising after2100,” Rignot warns.
All of which suggests we might want to start preparing. “Peoplewho are trying to downplay the significance say, ‘Oh, the Earth hasgone through changes much greater than this, you know, in thegeological past’,” says Pfeffer. “That’s true, but it’scompletely irrelevant. We weren’t there then.”
What it allmeans
If a 1 metre risein sea level doesn’t sound like much, consider this: about 60 millionpeople live within 1 metre of mean sea level, a number expected togrow to about 130 million by 2100.
Much of this population lives in the nine major river deltas in southand southeast Asia. Parts of countries such as Bangladesh, along withsome island nations like the Maldives, will simply be submerged.
According to a2005 report, a1-metre rise in sea level will affect 13 million people in fiveEuropean countries and destroy property worth $600 billion, with theNetherlands the worst affected. In the UK, existing defences areinsufficient to protect parts of the east and south coast, includingthe cities of Hull and Portsmouth.
Besides inundation,higher seas raise the risk of severe storm surges and dangerousflooding. The entire Atlantic seaboard of North America, including NewYork, Boston and Washington DC, and the Gulf coast will become morevulnerable to hurricanes. Today’s 100-year storm floods might occur asoften as everyfour years – inwhich case it will make more sense to abandon devastated regions andtowns than to keep rebuilding them.
Now is the timeto prepare for the great floods
01July 2009
Magazine issue 2715.
Forsimilar stories, visit the Editorials Topic Guide
THREE key facts about rising sea levels need to be hammered home tothe world’s politicians and planners: sea-level rise is now inevitable, it will happenfaster than most of us thought, and it will go on for a very longtime.
Even if greenhouse gas emissions stopped tomorrow, the oceans willcontinue to swell as they warm, and as glaciers and ice sheets melt orslide into the sea(see “Going, going…”). The growing consensus among climate scientists isthat the “official” estimate of sea-level rise in the lastreport of the Intergovernmental Panel on Climate Change – 0.2 to 0.6metres by 2100 – is misleading. It could well be in the region of 1 to2 metres, with a small risk of an even greater rise. And barring amegaproject to cool the planet, it could take several thousand yearsfor the system to reach equilibrium – by which time sea level will besomewhere between 10 and 25 metres higher than it istoday.
For many islands and low-lying regions, including much of theNetherlands, Florida and Bangladesh, even small rises will spellcatastrophe. Most countries, however, will only lose a tiny percentageof their land, even with a very big rise. The problem is what has beenbuilt on that land: large parts of London, New York, Sydney and Tokyo,to mention just a few cities. Unless something can be done, greatswathes of urban sprawl will vanish beneath the waves. It will take amassive engineering effort to protect these cities – an effort thatmay be beyond economies that have been brought to their kneesby climatechange.
In a few hundredyears, large parts of London, New York and Sydney will vanish beneaththe waves
None of this meanswe should despair, and stop trying to curb emissions; the more we pumpinto the atmosphere, the higher and faster the seas will rise. Butalongside these efforts, we need to start acting now to minimise theimpact of future sea-level rise. That means we must stop building inthe danger zone.
Countless billions are being spent on constructing homes, offices,factories and roads in vulnerable coastal areas. For instance, theglittering skyscrapers of Shanghai, China’s economic powerhouse, arebeing built on land that is a mere 4 metres above sea level onaverage, and which is sinking under the weight of its buildings and aswater is extracted from the rocks beneath them.
In cities that have been around for hundreds of years, this sort ofdevelopment may be understandable. But planning for new coastaldevelopments is to fly in the face of reality. If we want to build alasting legacy for our descendants, we should do so on the plentifulland that is in no danger from the sea. It is one of the easiestways to mitigate climate change, and we should be acting on itnow.
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