Monday, June 19, 2006

The Science of Global Warming: Are we reaching the "tipping points?"

Gabrielle Walker reported in Nature last week on whether we are reaching the tipping point in climate change.  The phrase "global warming" suggests to the uninitiated a gentle, linear increase in temperature with predictable linear effects on the earth.  But both the complex system that is climate, and the more subtle and difficult to identify biological systems affected by climate, cannot be captured by neat linear equations.  They have non-linearities: cliffs that are points of no return and tipping points when internal dynamics start to propel changes and small changes produce exponential impacts.  See Real Science post on tipping points. (tipping point post)  Are there tipping points or cliffs in climate change?  When will they be reached?  When and if they are reached, are they not just tipping points, but cliffs -- points of no return? 

Although there's no strong evidence that the climate as a whole has a point beyond which it switches neatly into a new pattern, individual parts of the system could be in danger of changing state quickly, and perhaps irretrievably. And perhaps the most striking of these vulnerable components are in the Arctic. Farthest north is the carapace of sea ice over the Arctic Ocean. South of that is the vast ice sheet that covers Greenland. And then there is the ocean conveyor belt, which originates in a small region of the Nordic seas and carries heat and salt around the world.  All three seem to have inbuilt danger zones that may deserve to be called tipping points. And the outside forces pushing them towards those points are gathering.

Even as it published the piece on tipping points, Nature noted in its editorial that there are dangers in focusing on those concepts:

there are three dangers attendant on focusing humanity's response to the climate crisis too much on tipping points. The first is the uncertainty of the science; the second is the tendency of such an emphasis to distort our responses; the third is the danger of fatalism.

The models through which our understanding of the climate system are channelled into assessments of how it might behave in the future are impressive by the standards of human investigation, but crude with respect to the details of the Earth system. All sorts of phenomena, from the formation of clouds to the respiration of soils, are hard to capture accurately, and it is on such details that an understanding of possible tipping points depends. Anyone claiming to know for sure when a particular tipping point will be reached should be treated with suspicion — and so must anyone who suggests that no tipping point will ever be reached.

The second problem is that an emphasis on tipping points not yet reached increases the focus on the future. Such an increase tips the balance away from adapting to climate change and in favour of trying to avoid it. A rational response to the challenge of the twenty-first century's climate is to do both: to reduce the rate at which greenhouse gases force climate change, but at the same time build up the ability to cope with adverse climates.

The third issue is that tipping points can induce fatalism. The concept may encourage the belief that a complete solution is the only worthwhile one, as any other course may allow the climate system to tumble past the crucial threshold. This sort of all-or-nothing approach is already over-stressed in climate policy by the Framework Convention on Climate Change, which calls for the complete avoidance of dangerous anthropogenic climate change, rather than the more reasonable and more feasible goal of minimizing and controlling it.

The first tipping point is Artic ice, which shrank 20% in the last 20 years of the 20th century:

"There is near-universal agreement that we are now seeing a greenhouse effect in the Arctic," says Mark Serreze from the US National Snow and Ice Data Center in Boulder, Colorado.  Serreze studies sea ice, the member of the arctic triumvirate that has had most recent attention. In the winter, sea ice more or less covers the Arctic Ocean basin. Summer sun nibbles at the pack ice, shrinking it at the edges and creating patches of open water within. Open water reflects much less sunlight than ice — it has what is known as a lower albedo — so the greater the area of dark open water, the more summer warmth the ocean stores. More stored heat means thinner ice in the next winter, which is more vulnerable to melting the next summer — meaning yet more warmth being stored in the open water in the following year, a cycle known as the 'ice–albedo feedback'. "Once you start melting and receding, you can't go back," says Serreze.  It seems that some of this process is under way. Serreze and his colleagues have found that the summer sea ice has shrunk by an average of 8% a decade over the past thirty years2. The past four years have seen record lows in the extent of September sea ice, and in 2005 there was 20% less ice cover than the 1979–2000 average, a loss of about 1.3 million square kilometres, which is more than the area of France, Germany and the United Kingdom combined. It was this finding that triggered a raft of alarming headlines.  The ice's volume, rather than its extent, would be a more useful figure, but this is hard to estimate. Radar measurements showing how proud the ice sits with respect to nearby water would help, but the European Cryosat mission intended to provide these data was lost on launch in October 2005. A reflight is planned, but at present the only way to determine the pack thickness is from below. In 2003 Andrew Rothrock and Jinlun Zhang of the University of Washington in Seattle analysed results from a series of submarine cruises from 1987–97 and concluded that the ice thinned by about one metre during that period3.
 

 
A natural swing in wind and weather known as the Arctic Oscillation may have played a key role in the decline.In 1989, this index began to approach its positive mode,in which a ring of strong winds circles the pole. Zhangand his colleague Roger Lindsay, also at the University of Washington, believe these winds flushed large amountsof thick ice out of the Arctic through the Fram Strait, eastof Greenland. Last year, they published a model suggesting that because the replacement ice was thinner and morevulnerable to the ice–albedo feedback, this extra loss pushed the Arctic over the edge. Their paper's title: "The thinning of Arctic Sea Ice, 1988–2003: Have We Passed a Tipping Point?"4.

But given that sea ice was disappearing even before the Arctic Oscillation lurched into its positive state, it is unlikely to have been the sole trigger. "The Arctic Oscillation was a strong kick in the pants," says Serreze, "but if we hadn't had it we would still have seen the ice loss."

Whatever the precise mechanisms, the decrease in ice seems to be warming the atmosphere, as heat pours from the open water into the air above it. Springtime temperatures began rising throughout the Arctic basin in the 1990s5. This year, the Arctic archipelago of Svalbard experienced a remarkable heatwave. January was warmer than any previously recorded April, and April was more than 12°C warmer than the long-term average.

Lindsay and Zhang suggest that the ice–albedo effect has indeed passed a tipping point, with the internal dynamics more important than external factors. But neither observations nor models suggest that the effect will now run away without outside help. According to climate modeller Jason Lowe of the UK Met Office in Exeter, the relationship between sea ice and temperature is reassuringly linear. "When you plot sea ice against temperature rise, whether from observations or models, it forms a remarkably straight line," he says. "It's not a runaway effect over the sorts of temperature ranges that we're predicting here." Lowe says that although the planet will almost certainly lose more ice, it does not have to lose it all. But if current trends in greenhouse-gas emissions and global warming continue, a planet that used to have two permanent polar caps will have only one.

Losing the sea ice would be bad news not only for polar bears and other charismatic megafauna, but also for some of the Arctic's smaller inhabitants. Photosynthetic plankton that live in pores and channels within the ice are the foundation of the area's food supply, and are not well adapted to ice-free life. Open-ocean plankton might benefit, but the Arctic is so poor in nutrients that this would probably not be much compensation6.

Compared with the overall scale of human-induced climate change, the additional warming expected if the ice–albedo feedback goes all the way would not be immense. The 4.5% of the Earth's surface above the Arctic Circle is simply too small to make a radical difference to the planet's energy balance. There are, however, some hints that the loss of sea ice may have more far-reaching effects beyond the simple number of watts absorbed per square metre. Tim Lenton, an Earth-systems scientist at the University of East Anglia in Norwich, UK, points out that our current, relatively stable pattern of winds, which is caused by three circulatory air systems in each hemisphere, depends in part on a white and cold North Pole.

Sinking air in the Arctic is an integral part of an air system called a Hadley cell; there is another Hadley cell over the tropics. Between these two cells are the fierce westerlies and the high-altitude jet streams that drive storms around the middle latitudes. "If any part of the current structure broke down, that would be profound," says Lenton. "If the system starts to switch seasonally between three cells and a less stable structure, you change the position of the jet streams, you change everything." Models of this possibility are scarce, but Jacob Sewall and Lisa Sloan of the University of California, Santa Cruz, have shown that an ice-free Arctic could shift winter storm tracks over North America, drying the American west7.

The second tipping point, with much more potential to dramatically change life on Earth, is the melting of Greenland ice.

The local warming caused by less sea ice could also affect the second tipping point, the size of the Greenland ice sheet. Here the effects could be dramatic, although delayed by centuries; there is enough ice on Greenland to raise sea levels by seven metres. "After hurricane Katrina, the deepest water in New Orleans was six metres," says glaciologist Richard Alley from Penn State University. "Greenland is more than that for all the coasts of the world. Do you move cities, do you build seven-metre walls and hope they stay, or what?" Until recently, nobody had painted a convincing portrait of how Greenland is responding to Arctic warming. A glacier here may recede while one over there grows; ice may be accumulating inland and eroding near the coast. But in the past couple of years, almost all of the indicators have started to point in the same direction. Greenland is melting...

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Although satellite measurements of Greenland's interior suggest that snow has recently been accumulating there, the margins are receding8. Laser measurements taken from planes suggest that this coastal melting is probably enough to outweigh the build-up of snow inland9. Also, Greenland's glaciers seem to have been speeding up. A few months ago, Eric Rignot of NASA's Jet Propulsion Laboratory in Pasadena and Pannir Kanagaratnam of the University of Kansas, Lawrence, published satellite evidence that between 1996 and 2000, Greenland's more southerly glaciers had begun to accelerate, and that by 2005 the northerly ones had followed suit10. They estimate that over the past decade this lurching has more than doubled Greenland's annual loss of ice, from 90 to 220 cubic kilometres per year....
"In the past decade there has been a lot of warming," says Alley. "There's plenty of room to argue whether that's a natural fluctuation or not, but there's a clear relation between Greenland getting warmer and Greenland getting smaller."

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Modelling by Jonathan Gregory from the University of Reading and his colleagues suggests that it would require an average warming worldwide of 3.1 °C to drive this shrinking to its ultimate conclusion of an ice-free Greenland11. This climatic point of no return is around the middle of the range foreseen by the Intergovernmental Panel on Climate Change, but is higher than a previous estimate made by the same group12. ...

But these models do not take into account the dynamism of Greenland's glaciers. In 2002 Jay Zwally from NASA's Goddard Space Flight Center in Greenbelt, Maryland, found that as soon as summer meltwater appeared on the surface of west-central Greenland, the ice began to slip more quickly13. This is surprising, as slip rates should depend on processes at the base of the ice rather than at its surface. But Zwally points out that the great lakes of water produced by the melting could slip down conduits in the ice and be delivered directly to the bed.

This result doesn't necessarily make a big difference to the fate of Greenland, as the increase in the ice's speed was relatively small. But it points to a new way in which the ice sheet could react to climate change quicker than anyone had realized. "In places inland where the ice is frozen to its bedrock, if you warm the surface and wait for heat to get conducted to the bottom it takes 10,000 years," says Alley. "But if you send water down through a crack it takes maybe 10 minutes, maybe 10 seconds." If this process started to move inland, even the interior of Greenland's ice sheet could be vulnerable to warmer air. That could point to the sort of self-sustaining feedback that tipping points are made of.

The models don't incorporate this mechanism, because they can't. The cliff fronts of many Greenland glaciers are shot through with bright blue conduits, but nobody knows how widespread these veins are inside the ice. Still, the responsiveness of Greenland's glaciers makes that point-of-no-return figure of 3.1 °C even less comforting. What's more, a lot of damage can be done without losing all of the ice. The ice sheet did not vanish during the last interglacial, around 130,000 years ago, when temperatures in the north were a few degrees higher than they are today. And yet the latest analyses suggest that meltwater from Greenland increased the sea level by between two and three metres. The only good thing about such an increase is that it would take centuries....

The third tipping point is thermohaline circulation, the ocean conveyor belt that distributes heat and salt in the ocean.

Thanks to its cold temperatures and high salinity, water in the Nordic seas between Greenland and Scandinavia is unusually dense and sinks. Surface water is drawn northwards to replenish this. One result of this flow is that Britain is warmer than its latitude would seem to deserve.

The sinking process sets a global mass of water in motion, transporting vast amounts of heat around the oceans. In the 1980s, models began to suggest that melting ice in the north could weaken this system, by putting a plug of fresh water over the sinkhole. This led to fears of abrupt climate change and snap ice ages in Europe and eastern America. These days most scientists think that the power of this flow to affect European temperatures under current conditions, or in a globally warmed future, has been overestimated14. But changes in the system could still have far-reaching implications. And models suggest that the thermohaline circulation has its own tipping point.

Comparing the output from 11 different ocean and climate models, ocean modeller Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research (PIK), Germany, has concluded that it would take between 100,000 and 200,000 cubic metres of fresh water per second to shut down the thermohaline circulation — similar to the outflow from the Amazon River15. And once the circulation is stopped, restarting it would take a lot more cooling than just reversing the system into the conditions in which it was previously working.

The good news is that although the Arctic does seem to be getting fresher, it is nowhere near the danger point. Add together the increased output from disappearing sea ice (which moves fresh water from the point where sea water freezes to the point where the ice melts), the melting of Greenland and increased Arctic river flow and you still have barely a quarter of the lower bound of the model threshold.

However, measurements of flow in the deep ocean suggest that the circulation might be fluctuating in ways not considered by the models16. And if the melting of Greenland were to gather pace, the thermohaline circulation would be vulnerable. If the lower bounds of the models turn out to be right, a rate of melting that would get through the ice in 1,000 years would trouble the ocean overturning in centuries. "The fate of the thermohaline circulation will be decided by Greenland," says Rahmstorf. "If that goes quickly it will be bad news for the deep-water formation. But if Greenland is stable, the risk of shutting down the circulation completely is very small."


Any such shutdown would probably have only a small effect on European temperatures. But thanks to the Coriolis effect, says Rahmstorf, such a large shift in the ocean circulation would redistribute sea water so that the North Atlantic rose by up to a metre17. There are also suggestions that Atlantic fisheries could collapse.

But the biggest danger would come farther south. In the past, similar changes in ocean circulation seem to have led to significant shifts in tropical rainfall. "If you switch off the thermohaline circulation, the tropical rainfall belts shift. All the models show this. It's quite simple robust physics," says Rahmstorf. General circulation models, which try to simulate the workings of the climate system as a whole, often including the ocean, predict at least some weakening of the thermohaline circulation by the end of the century, with a knock-on effect on tropical rainfall — the system that provides much of Asia with food. And as with Greenland, the change doesn't have to be complete to have consequences. "Just weakening the system is by no means harmless," says Rahmstorf. "You'd get the same pattern of effects as for a total shutdown, but just a smaller amplitude."

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