Which offers a perfect opportunity to refute them with facts and figures. This is something that has to be done again and again. If for no other reason, it's because Google and even Dogpile have gotten so they won't dig up links to previously posted but older data. This isn't just the entropic link decay of a constantly changing internet, because when crosschecked against my own files to directly assess the site, the original links will usually still be there.
Despite the drain on the expense accounts of the oil barons, despite their best attempts at green posturing, despite monkeying with search algorithms, reality and the perception of reality continues to belie them.
Nature 441, 802-805 (15 June 2006) | doi:10.1038/441802a
Climate change: The tipping point of the iceberg
Gabrielle Walker
...The idea that passing some hidden threshold will drastically worsen man-made climate change has been around for decades, normally couched in technical terms such as 'nonlinearity', 'positive feedback' and 'hysteresis'. Now it has gained new prominence under a new name. In 2004, 45 newspaper articles mentioned a 'tipping point' in connection with climate change; in the first five months of this year, 234 such articles were published. "Warming hits tipping point," one UK newspaper recently warned on its front page; "Climate nears point of no return," asserted another. The idea is spreading like a contagion...
A tipping point usually means the moment at which internal dynamics start to propel a change previously driven by external forces. The idea raises two questions. First, when will that moment be reached? Second, after it has been passed, is the system now destined to run its course regardless of what goes on elsewhere — is a tipping point a point 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.
On thin ice
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. "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 years (Stroeve, J. C. et al. Geophys. Res. Lett. 32, L04501 (2005)). 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 period (Rothrock, D. A. et al. J. Geophys. Res. 108, 3083 (2003))...
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 west (Sewall, J. O. & Sloan, L. C. Geophys. Res. Lett. 31, L06209 (2004)).
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.
Although satellite measurements of Greenland's interior suggest that snow has recently been accumulating there, the margins are receding (Johannessen, O. M. et al. Science 310, 1013–1016 (2005)). Laser measurements taken from planes suggest that this coastal melting is probably enough to outweigh the build-up of snow inland (Krabill, W. et al. Geophys. Res. Lett. 31, L24402 (2004)). 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 suit (Rignot, E. & Kanagaratnam, P. Science 311, 986–990 (2006)). 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."
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 Greenland (Gregory, J. M. & Huybrechts, P. Phil Trans. R. Soc. Lond. A (in the press)). 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 group (Gregory, J. M. et al. Nature 428, 616 (2004)). Their revision is a measure of how quickly the field is changing. "It's not just Greenland that is going fast," says Alley. "The rate of publications, the rate of new papers, and the rate of disagreement have multiplied amazingly."
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 quickly (Zwally, H. J. et al. Science 297, 218–222 (2002)). 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...
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