Why climate change could be catastrophic

2009-08-19

in Bombs and rockets, Economics, Politics, Science, Security, The environment

Wrenches and sockets

The basic equation of climate change is simple enough: add greenhouse gases to the atmosphere and you warm the planet. Of course, there are endless complications in areas like changes in precipitation, sea level, etc.

There are, however, a relatively small set of reasons for which climate change could potentially enormously problematic, or even a civilizational threat. That is to say, one that has the capacity to eliminate or virtually eliminate civilizations with the major characteristics of being industrial, such as level of technological advancement and share of the population whose primary vocation is farming.

The probability of all of these is unknown, and may be very low. Still, they bear consideration when we are deciding how precautionary an approach we should take when it comes to reducing emissions. Also, more than one of these could happen simultaneously.

1) It could happen very quickly

While the normal order of business for climatic changes seems to be gradual change, there is some evidence that it is possible to cross some threshold and experience massive sudden changes. An example would be the mass melting of icecaps in Greenland and Antarctica, producing several metres of sea level rise in a matter of decades. Rapid sea level rise (albeit perhaps not to that degree) has happened before, as the result of other ‘forcings’ that affect the climate system.

More rapid change would be harder to adapt to than slower change. Rather than having to make gradual changes to how we grow food, use water, etc, we would be confronted with the immediate necessity of making big, expensive, and politically difficult changes.

2) It could take place to an extreme extent

The ultimate extreme – seeing our oceans boil away and the planet turn into a burning hell like Venus – is probably impossible. That being said, a much less dramatic change could still strain the ability of human beings to cope. There is a general consensus that warming of more than 2°C would be ‘dangerous’ and that each additional degree would heighten problems such as agricultural failure and lack of access to fresh water.

A business-as-usual course of greenhouse gas emissions that takes atmospheric concentrations to over 1000 ppm by 2100. If climate sensitivity is high (say, 8°C) then the warming that results could average 25°C above pre-industrial levels, worldwide (with more warming in high latitudes). That would surely cause massive agricultural problems and leave many areas uninhabitable. Even with sensitivity at the high end of the IPCCs probable range (4.5°C), 1000 ppm conditions could generate warming of over 15°C.

3) It could become self-sustaining

The climate system contains a number of positive feedback effects, where warming causes a change that produces more warming. Examples include sea ice melting to reveal more heat-absorbing ocean, melting permafrost releasing methane, and tropical forests drying out and burning.

If the natural world began to regularly emit more greenhouse gasses than it was removing from the atmosphere, even cutting human emissions to zero would not prevent further climate change. Our only options would be various forms of geoengineering: air capture to remove greenhouse gasses directly from the atmosphere, or techniques to alter how the planet absorbs and reflects solar radiation.

If there is a threshold beyond which runaway climate change begins, humanity might find itself trapped between facing an unknown level of warming (to stop only when the system finds a new equilibrium) or taking the desperate step of trying to actively engineer the climate.

4) It could foster conflict

Even without any of the scenarios above, it is plausible that climate change could kick off major conflicts. Bangladesh and Florida could be permanently submerged. Major river systems could see massively decreased flows. Major famines could result, etc.

If the climate changes experienced were abrupt, the danger of conflict would be further heightened, as states made desperate attempts to cope and populations relocated.

In the end, our best chance for dealing with climate change is for states to begin cooperating when they still have a good amount of time and lots of resources to direct at the issue. Also, when their will to cooperate isn’t being reduced by Hobbesian tensions. By investing reasonable amounts now in transforming our energy system and protecting carbon sinks – as well as by creating increasingly powerful incentives to reduce greenhouse gas emissions – states can not only prevent the scenarios above from occurring, they can also switch the energy basis of their society from dirty and unsustainable fossil fuels towards renewable forms of energy that can be relied upon indefinitely.

[Update: 4 February 2009] Here is a post on the danger of self-amplifying, runaway climate change: Is runaway climate change possible? Hansen’s take.

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{ 22 comments… read them below or add one }

. August 19, 2009 at 5:33 pm

Abrupt climate change
From Wikipedia, the free encyclopedia

An abrupt climate change occurs when the climate system is forced to transition to a new state at a rate that is determined by the climate system itself, and which is more rapid than the rate of change of the external forcing. Past events include the end of the Younger Dryas, Dansgaard-Oeschger events, and possibly also the Paleocene-Eocene thermal maximum, . The term is also used within the context of global warming to describe sudden climate change that is detectable over the time-scale of a human lifetime. One proposed reason for the observed abrupt climate change is that feedback loops within the climate system both enhance small perturbations and cause a variety of stable states.

Timescales of events described at ‘abrupt’ may vary dramatically. Changes recorded in the climate of Greenland at the end of the Younger Dryas, as measured by ice-cores, imply a sudden warming of +10°C within a timescale of a few years. Other abrupt changes are the +4 °C on Greenland 11 270 years ago or the abrupt +6 °C warming 22 000 years ago on Antarctica. By contrast, the Paleocene-Eocene Thermal Maximum may have initiated anywhere between a few decades and several thousand years.

Runaway climate change
From Wikipedia, the free encyclopedia

Runaway climate change is a situation in which the climate system passes a tipping point, after which internal positive feedback effects cause the climate to continue changing without further external forcings. The runaway climate change continues until it is overpowered by negative feedback effects which cause the climate system to restabilise at a new state.

Runaway terms are occasionally used in relation to climate change events in climatological literature. More generally, uses for these terms are found in the engineering journals, in books, and in the news media. Runaway terms are also used in the planetary sciences to describe the conditions that led to the current greenhouse state of Venus.

. September 9, 2009 at 1:01 pm

Prehistoric hothouse

THE Eocene ran from 56 million to 34 million years ago. Geological evidence from the early and middle part of this period offers troubling news: the average temperature in the tropics at this time could have been as high as 40°C while the poles were at temperatures of 15 or 20°C. None of our climate models accounts for how this “Eocene hothouse” might have arisen (New Scientist, 21 June 2008, p 34).

Whichever way you look at the Eocene enigma, it is bad news for life on Earth. For a start, any tweaks we make to our climate models to account for it will produce scarier predictions of warming. Secondly, it suggests that there is no feedback mechanism that will stabilise a warming world against runaway climate change. And third, there is geological evidence for plant extinctions in the Eocene.

If the modern Earth goes the same way and plants in the tropics start dying, that will provide yet another way for atmospheric carbon dioxide levels to rise faster. The Eocene hothouse anomaly suggests that our worst-case scenario is probably optimistic to say the least.

R.K. September 9, 2009 at 10:39 pm
. September 23, 2009 at 3:46 pm

“Veron’s gravest concern is a tipping point in the ocean’s chemistry that he calls “commitment,” which comes freighted with the darkest of Anthropocene ironies. “Commitment,” he writes, “embodies the concept of unstoppable inevitability, according to which the nature and health of future environments will be determined not by our actions at some future date but by what is happening today.” In the case of ocean acidification, “the lag time of the ocean will make acidification a fait accompli before it has barely started.” By the time we know for sure we’ve reached the tipping point, in other words, it will be far too late to alter the process.

As recently as 2005, scientists believed this commitment point (under a business-as-usual emissions scenario) was as far off as 2050, but many now date it to about 2030. Veron, ever the iconoclast, believes our current emissions trajectory could reach commitment “within a decade.””

. September 28, 2009 at 4:34 pm

Four degrees of warming ‘likely’
By David Shukman
Environment correspondent, BBC News

In a dramatic acceleration of forecasts for global warming, UK scientists say the global average temperature could rise by 4C (7.2F) as early as 2060.

The Met Office study used projections of fossil fuel use that reflect the trend seen over the last 20 years.

Their computer models also factored in new findings on how carbon dioxide is absorbed by the oceans and forests.

The finding was presented at an Oxford University conference exploring the implications of a 4C rise.

The results show a “best estimate” of 4C being reached by 2070, with a possibility that it will come as early as 2060.

Richard Betts of the Met Office Hadley Centre described himself as “shocked” that so much warming could occur within the lifetimes of people alive today.

“If greenhouse gas emissions are not cut soon then we could see major climate changes within our own lifetimes,” he said.

“Four degrees of warming averaged over the globe translates into even greater warming in many regions, along with major changes in rainfall.”

The model finds wide variations, with the Arctic possibly seeing a rise of up to 15C (27F) by the end of the century.

Western and southern parts of Africa could warm by up to 10C, with other land areas seeing a rise of 7C or more.

. October 15, 2009 at 12:47 pm

Participants in a Clean Energy Economy Forum at the White House included J. Wayne Leonard, the Chairman and CEO of Entergy Corporation, the utility giant based in New Orleans, La. Speaking at the White House event, Leonard called for action on climate change and clean energy not just for economic reasons but starkly moral ones:

“We are virtually certain that climate change is occurring, and occurring because of man’s activities. We’re virtually certain the probability distribution curve is all bad. There’s no good things that’s going to come of this. But what’s uncertain is exactly which one of those things are going to occur and in what time frame. In the probability distribution curve is about a 50 percent probability that about half of all species will become extinct or be subject to extinction over this period of time. What we will never know on an ex ante basis is whether or not man be one of those casualties or not.

We condemn Wall Street for taking risks with our economy—risks that all of you are trying very hard to reverse—but at the same time we’re taking exactly the same kind of risks, with no upside whatsoever, with regard to our climate, failing to practice even the basic risk management techniques in terms of climate change reduction.”

. October 19, 2009 at 10:58 am

PM warns of climate ‘catastrophe’

The UK faces a “catastrophe” of floods, droughts and killer heatwaves if world leaders fail to agree a deal on climate change, the prime minister has warned.

Gordon Brown said negotiators had 50 days to save the world from global warming and break the “impasse”.

He told the Major Economies Forum in London, which brings together 17 of the world’s biggest greenhouse gas-emitting countries, there was “no plan B”.

World delegations meet in Copenhagen in December for talks on a new treaty.

R.K. October 19, 2009 at 4:28 pm

Brown makes a good point about shifting baselines:

“The extraordinary summer heatwave of 2003 in Europe resulted in over 35,000 extra deaths.

On current trends, such an event could become quite routine in Britain in just a few decades’ time. And within the lifetime of our children and grandchildren the intense temperatures of 2003 could become the average temperature experienced throughout much of Europe.”

. December 3, 2009 at 2:25 pm

“Uncertainty about the consequences of climate change makes it hard to persuade people to spend money on it, for where the damage is uncertain, so are the benefits of averting it. Yet uncertainty is also why mankind needs to take the problem seriously. If we were sure that the temperature would rise by 2-3ºC, then we could choose to live with that. But we do not know how far the rise might go. The Intergovernmental Panel on Climate Change (IPCC), the body set up by the UN to establish a scientific consensus on the subject, puts the range of possible increases by the end of this century at 1.1-6.4ºC. At the bottom end of the range, the difference would be barely noticeable. At the top end of the range—well, guesses about what the world would look like then read rather like science fiction.

Although the benefits of averting that sort of catastrophe are incalculably large, the costs of doing so should not be enormous—as little as 1% of global output, if policy is well designed. This newspaper reckons that the world should fork out, rather as householders spend similar proportions of their income on insuring their homes against disaster.

. January 14, 2010 at 4:20 pm

“We can live without a new trade agreement; we can’t live without a new climate agreement. One of the failings of the people who have tried to mobilise support for a climate treaty is that we have made the issue too complicated. So here is the simplest summary I can produce of why this matters.

Human beings can live in a wider range of conditions than almost any other species. But the climate of the past few thousand years has been amazingly kind to us. It has enabled us to spread into almost all regions of the world and to grow into the favourable ecological circumstances it has created. We currently enjoy the optimum conditions for supporting seven billion people.

A shift in global temperature reduces the range of places than can sustain human life. During the last ice age, humans were confined to low latitudes. The difference in the average global temperature between now and then was four degrees centigrade. Global warming will have the opposite effect, driving people into higher latitudes, principally as water supplies diminish.

Food production at high latitudes must rise as quickly as it falls elsewhere, but this is unlikely to happen. According to the body that summarises the findings of climate science, the Intergovernmental Panel on Climate Change, the potential for global food production “is very likely to decrease above about 3°C”. The panel uses the phrase “very likely” to mean a probability of above 90%. Unless a strong climate deal is struck very soon, the probable outcome is a rise of three or more degrees by the end of the century.

Even in higher latitudes the habitable land area will decrease as the sea level rises. The likely rise this century – probably less than a metre – is threatening only to some populations, but the process does not stop in 2100. During the previous interglacial period, about 125,000 years ago, the average global temperature was around 1.3 degrees higher than it is today, as a result of changes in the earth’s orbit around the sun. A new paper in the scientific journal Nature shows that sea levels during that period were between 6.6 and 9.4 metres higher than today’s. Once the temperature had risen, the expansion of sea water and the melting of ice caps in Greenland and Antarctica was unstoppable. I wonder whether the government of Denmark, whose atrocious management of the conference contributed to its failure, would have tried harder if its people knew that in a few hundred years they won’t have a country any more.

As people are displaced from their homes by drought and sea level rise, and as food production declines, the planet will be unable to support the current population. The collapse in human numbers is unlikely to be either smooth or painless: while the average global temperature will rise gradually, the events associated with it will come in fits and starts: sudden droughts and storm surges.”

. January 25, 2010 at 9:51 am

“Paleoclimatologists study how the Earth’s climate reacted in the past to natural warming forces, like a small change in the Earth’s tilt, or an increase in the sun’s heat. Hansen believes these studies provide stronger evidence than climate models, because they are looking at what happened the last times this experiment—of a rapidly warming world—was run. And the findings are seriously scary. Ice sheets can go fast, and when they do, sea levels rise remorselessly and do not settle for centuries. He reasons: “If ice sheets begin to disintegrate, there will not be a new stable sea level on any foreseeable time scale. We will have created a situation with continual change, with intermittent calamities at thousands of cities around the world. It will continue for as many generations as we care to think about. … Global chaos will be difficult to avoid.””

Milan March 4, 2010 at 11:10 am

In this ‘toy model’ discussion of climate change, Paul Krugman says that: “Marty Weitzman has managed to scare me, by pointing out that there’s a pretty plausible case that a rise of 5 degrees C – which is no longer an outlandish prediction – would be utterly catastrophic. You don’t have to be sure about this; just a significant probability is enough.”

. March 4, 2010 at 11:14 am

“Martin Weitzman” (2008-06-05). “On Modeling and Interpreting the Economics of Catastrophic Climate Change

With climate change as prototype example, this paper analyzes the implications of structural uncertainty for the economics of low-probability high-impact catastrophes. Even when updated by .nite Bayesian learning, uncertain structural parameters induce a critical .tail fattening. of posterior-predictive distributions, Such fattened tails have strong implications for situations, like climate change, where a catastrophe is theoretically possible because prior knowledge cannot place su¢ ciently narrow bounds on overall damages. The core problem is learning extreme-impact tail probabilities from .nite data. Fat-tailed structural uncertainty, along with great unsureness about high-temperature damages, can outweigh discounting in climate-change economics.

The paper has five broad conclusions: (1) from deep structural uncertainty about the prospects for disastrously large temperature changes, there is a strong prima facie case that the relevant probability density function (PDF) of climate change catastrophes has an extreme tail that is heavy with probability; (2) when this heavy tail is combined with very unsure high-temperature damages, this aspect can dominate the discounting aspect in calculations of expected present discounted utility, even at empirically plausible real-world interest rates; (3) all of this translates into placing severe limitations on the reliability of policy advice coming from standard cost-bene.t analysis (CBA) of climate change; (4) the conventional economic advice of spending modestly on abatement now but gradually ramping up expenditures over time is an extreme lower bound on what is reasonable rather than a best estimate of what is reasonable; (5) removing the arti.cial limitation on conventional CBA that comes from excluding high-impact disasters can shift a more inclusive economic-welfare analysis strongly away from the gradualism of a climate-change policy ramp.

Perhaps in the end the climate-change economist can help most by not presenting a cost-bene.t estimate for what is inherently a fat-tailed situation with potentially unlimited downside exposure as if it is accurate and objective .and perhaps not even presenting the analysis as if it is an approximation to something that is accurate and objective .but instead by stressing somewhat more openly the fact that such an estimate might conceivably be arbi- trarily inaccurate depending upon what is subjectively assumed about the high-temperature damages function along with assumptions about the fatness of the tails and/or where they have been cut o¤. Even just acknowledging more openly the incredible magnitude of the deep structural uncertainties that are involved in climate-change analysis . and explain- ing better to policy makers that the arti.cial crispness conveyed by conventional IAM-based CBAs here is especially and unusually misleading compared with more-ordinary non-climate- change CBA situations .might go a long way towards elevating the level of public discourse concerning what to do about global warming. All of this is naturally unsatisfying and not what economists are used to doing, but in rare situations like climate change where DT applies we may be deluding ourselves and others with misplaced concreteness if we think that we are able to deliver anything much more precise than this with even the biggest and most-detailed climate-change IAMs as currently constructed and deployed.

. March 5, 2010 at 2:02 pm

“So far, governments have proven themselves unable to impose prices on carbon high enough to meet the recommendations of the ramp-up supporters, to say nothing of those urging drastic action. The idea that those same governments might somehow be able to adopt far more painful measures in order to forestall catastrophe is absurd. The public won’t stand for it. And that’s part of the political challenge of climate change—by the time the public is able to observe the catastrophic effects of warming in ways that might steel them to accept painful emissions reduction measures, it’s too late to do anything about the problem.

What this suggests, then, is that while pursuit of a carbon price is a worthwhile goal, it can’t be the end of the policy response. A carbon price, even one insufficiently small, would create some incentive to change behaviour and innovate. But what’s also needed is a set of technological leaps sufficient to quickly and drastically increase the elasticity of demand for carbon. And that militates in favour of large-scale investment in green infrastructure and a broad push to encourage research into alternatives to carbon-intense activities. These activities wouldn’t be cheap, but they’d be easier to bear than an immediate price of $200 per tonne of carbon. And politically, such measures should go down easier, given that they entail the doling out of government resources rather than the imposition of a tax on a negative externality.

Of course, there will be costs to moving away from a purely market-driven approach to climate policy. Non-Pigouvian taxes to fund the above measures will generate bigger economic distortions, and targeted government spending is likely to breed some inefficiencies and opportunities for rent-seeking. Politics being politics, it’s also possible that some counter-productive steps are taken—the example of ethanol subsidies looms large in these discussions. But for some expected cost and likelihood of tail climate impacts, these downsides become acceptable.”

. May 5, 2010 at 10:25 am

Global Warming: Future Temperatures Could Exceed Livable Limits, Researchers Find

ScienceDaily (May 4, 2010) — Reasonable worst-case scenarios for global warming could lead to deadly temperatures for humans in coming centuries, according to research findings from Purdue University and the University of New South Wales, Australia.

While the Intergovernmental Panel on Climate Change central estimates of business-as-usual warming by 2100 are seven degrees Fahrenheit, eventual warming of 25 degrees is feasible, he said.

“We found that a warming of 12 degrees Fahrenheit would cause some areas of the world to surpass the wet-bulb temperature limit, and a 21-degree warming would put half of the world’s population in an uninhabitable environment,” Huber said. “When it comes to evaluating the risk of carbon emissions, such worst-case scenarios need to be taken into account. It’s the difference between a game of roulette and playing Russian roulette with a pistol. Sometimes the stakes are too high, even if there is only a small chance of losing.”

Steven Sherwood, the professor at the Climate Change Research Centre at the University of New South Wales, Australia, who is the paper’s lead author, said prolonged wet-bulb temperatures above 95 degrees would be intolerable after a matter of hours.

“The wet-bulb limit is basically the point at which one would overheat even if they were naked in the shade, soaking wet and standing in front of a large fan,” Sherwood said. “Although we are very unlikely to reach such temperatures this century, they could happen in the next.”

. June 21, 2011 at 6:11 pm

Atmospheric carbon dioxide buildup unlikely to spark abrupt climate change
Published: Monday, June 20, 2011 – 09:03 in Earth & Climate

There have been instances in Earth history when average temperatures have changed rapidly, as much as 10 degrees Celsius (18 degrees Fahrenheit) over a few decades, and some have speculated the same could happen again as the atmosphere becomes overloaded with carbon dioxide. New research lends support to evidence from numerous recent studies that suggest abrupt climate change appears to be the result of alterations in ocean circulation uniquely associated with ice ages.

“There might be other mechanisms by which greenhouse gases may cause an abrupt climate change, but we know of no such mechanism from the geological record,” said David Battisti, a University of Washington atmospheric sciences professor.

Battisti was part of a team that used a numerical climate model coupled with an oxygen-isotope model to determine what caused climate shifts in a computer-generated episode that mimicked Heinrich events during the last ice age, from 110,000 to 10,000 years ago. Heinrich events produced huge numbers of North Atlantic Ocean icebergs that had broken off from glaciers.

. March 12, 2012 at 7:33 am

Greenland ice sheet may melt completely

The Greenland ice sheet appears more vulnerable to global warming than previously thought, and could disappear altogether if global temperatures rise more than 1.6 degrees Celcius above pre-industrial levels.

Indeed, say scientists from the Potsdam Institute for Climate Impact Research (PIK) and the Universidad Complutense de Madrid, just 0.8 degrees could do it – and we’ve reached that point already.

. April 23, 2012 at 7:49 am

Arctic sea ice is leaking methane

NASA scientists have detected large releases of methane – a highly potent greenhouse gas – from the crumbling Arctic sea ice.

Scientists have long been concerned about methane release from melting Arctic tundra, hut have been alarmed to discover that the ocean is a culprit too.

The study was conducted as part of the HIAPER Pole-to-Pole Observations (HIPPO) mission. A Gulfstream V aircraft was flown over the Pacific Ocean, almost from pole to pole, collecting atmospheric measurements from the ground to an altitude of 8.7 miles.

And during five flights over the Arctic from 2009 to 2010, the team found increased methane levels at low altitudes over the remote Arctic Ocean, north of the Chukchi and Beaufort Seas – about one-half percent larger than normal background levels.

“While the methane levels we detected weren’t particularly large, the potential source region, the Arctic Ocean, is vast, so our finding could represent a noticeable new global source of methane,” says Eric Kort of the Keck Institute of Space Studies at Caltech.

. May 17, 2012 at 11:19 am

In fact, according to the latest science, says Anderson, “a 4 degrees C future is incompatible with an organized global community, is likely to be beyond ‘adaptation’, is devastating to the majority of ecosystems, and has a high probability of not being stable.”

http://grist.org/climate-change/2011-12-05-the-brutal-logic-of-climate-change/

http://www.smh.com.au/environment/too-hot-to-handle-can-we-afford-a-4degree-rise-20110709-1h7hh.html

. May 17, 2012 at 11:35 am

Keynote speaker Professor Hans Joachim Schellnhuber, director of the Potsdam Institute and climate adviser to the German Chancellor and to the EU, has said that in a 4-degree warmer world, the population “carrying capacity estimates [are] below 1billion people”.

Professor Kevin Anderson, director of the Tyndall Centre for Climate Change in Britain, was quoted in The Scotsman ahead of the 2009 Copenhagen conference saying the consequences were ‘‘terrifying’’.
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‘‘For humanity it’s a matter of life or death … we will not make all human beings extinct, as a few people with the right sort of resources may put themselves in the right parts of the world and survive. But I think it’s extremely unlikely that we wouldn’t have mass death at 4 degrees.

‘‘If you have got a population of 9 billion by 2050 and you hit 4 degrees, 5 degrees or 6 degrees, you might have half a billion people surviving.’’

http://www.smh.com.au/environment/too-hot-to-handle-can-we-afford-a-4degree-rise-20110709-1h7hh.html

. August 24, 2012 at 3:43 pm

The worry that needs to be taken most seriously is climate change itself. The impact of the melting Arctic may have a calamitous effect on the planet. It is likely to disrupt oceanic circulation—the mixing of warm tropical and cold polar waters, of which the Gulf Stream is a part—and thawing permafrost will lead to the emission of masses of carbon dioxide and methane, and thus further warming. It is also raising sea levels. The Greenland ice sheet has recently shed around 200 gigatonnes of ice a year, a fourfold increase on a decade ago. If the warming continues, it could eventually disintegrate, raising the sea level by seven metres. Many of the world’s biggest cities would be inundated long before that happened.

. August 26, 2012 at 1:01 pm

Around 125,000 years ago—when the IPCC thinks the Arctic was last much warmer than today—polar meltwater raised the sea level by 4-6 metres. If that happened again it would displace a billion people and inundate most of the world’s biggest cities, including New York, London and Mumbai. The chances of that are uncomfortably high—though it might take a couple of centuries—because of another Arctic surprise.

The IPCC’s prediction in 2007 of a rise in sea levels of up to 59cm by the end of this century was attended by large uncertainties over the world’s big ice sheets. Greenland’s is up to 3km (1.9 miles) deep and contains enough water to raise the sea level by 7.5 metres; the Antarctic ice sheets are much bigger and could potentially cause a 57-metre rise. In recent years they have also been monitored by satellite, with the first data obtained in 1992. Until about 2000 the ice sheets seemed to be more or less stable, with increased snowfall on their tops compensating for increased melting at the margins.

But in Greenland something has changed. The ice sheet’s recent rate of loss—around 200 gigatonnes a year—represents a fourfold increase on a decade ago. Half this melting is thought to be due to the warming atmosphere. The other half is due to warmer seawater, caused by global warming or a shift in Atlantic currents, or both. As a result, the sea is eating away the edge of the ice sheet at a faster rate. Between 2002 and 2007 the Jakobshavn Isbrae, a big glacier in western Greenland, retreated by 3km a year, shedding a total of over 36 billion tonnes of ice.

If the climate stabilises soon, the ice cap might resettle at a slightly lower mass than it has now, raising sea levels by only a few centimetres. But if the warming continues, the ice cap will continue to melt. Sooner or later, it is thought, a tipping-point would be reached when the decline would become terminal. “How long will the ice cap last?” asks Dorthe Dahl-Jensen of the University of Copenhagen, who has studied the subject in depth. “We don’t know because we don’t fully understand what determines the velocity of its ice streams.” But such a collapse has happened before. Around 15,000 years ago the Barents Sea ice sheet, which stretched from northern England to Siberia, disintegrated in perhaps less than 1,000 years, probably because of warming seas.

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