You might compare it, in its implications, to Hitler marching his troops into the previously demilitarised Rhineland, in March 1936 the clearest possible sign that the world was in for serious trouble. Some people understood the potential consequences of Hitler's move at once, but the world as a whole carried on with business as usual, until three years later the storm burst upon it. And so it seems to be with the ice.
On Sunday 16 September 2007, the sea ice covering the Arctic ocean melted back to a record low point. It has always melted back in the summer, but in recent years it has retreated further and further, to new lows, strongly suggesting the influence of climate change. The 2007 retreat, however, shattered the previous record, set only two years earlier, by a quite colossal amount, an amount so enormous as to be scarcely credible. It exceeded the September 2005 low point by another 22 per cent an area of 1.2 million square kilometres, or more than 385,000 square miles. This represents an extra area of ice five times the size of the United Kingdom. Gone in a single summer. If you consider that and you don't think the world is rapidly warming up, what do you need to convince you?
This summer's Arctic melting astonished scientists around the world, and sent a chill down the backs of those who saw the implications. As the head of the Canadian Ice Service said, it wasn't predicted in any supercomputer-generated climate change scenario. The scale was entirely unexpected. It was not just the clearest signal yet that global warming is taking hold; it was an ominous indication that the warming process is proceeding far, far faster than anyone considered possible even five years ago, and that its catastrophic consequences may be upon us much sooner than we have hitherto imagined.
To take just one example: the rapid increase in the melt rate in the past two or three years has led to a revision of estimates of when the Arctic might be wholly ice-free in summer.
Early predictions by the UN's Intergovernmental Panel on Climate Change (IPCC), based on computer models of global warming, suggested that as climate change advances, this might happen by 2080. But now scientists are increasingly thinking the models have seriously underestimated the rate, and it may happen much earlier.
According to Mark Serreze, a researcher at the US National Snow and Ice Data Center (NSIDC) in Colorado, it might take only 25 years or less. "If we were talking even two or three years ago, I'd have said the transition to an ice-free Arctic summer might be between 2070 and 2100," he said. "But we're starting to see that that is rather optimistic, and an educated guess right now would be 2030. It's something that could be within our lifetime."
He added: "We're on strong spiral of decline; some would say a death spiral. I wouldn't go that far but we're certainly on a fast track. We know there is a natural variability, but the magnitude of change is too great to be caused by natural variability alone."
Yet other estimates in recent months have put an ice-free Arctic summer even closer, perhaps within 10 years. This spells doom for much of the wildlife of the region, led by polar bears, which need the ice to hunt seals. Polar bears were officially notified as threatened species last year by being included in the Red List of the World Conservation Union. Some conservationists think they could be gone by mid-century.
What the melting of the Arctic ice will not do is add to global sea-level rise because, following Archimedes' principle, it is already displacing its own volume of water when floating in the sea. (When the ice-cube melts in the gin and tonic it does not raise the level of liquid in the glass.) It is the melting of giant land-based ice sheets, such as those covering Greenland and Antarctica, which is likely to add substantially to sea levels. But breathe no sigh of relief this is also happening at an accelerating rate, and in 2007 there was an indication of it, just as graphic as the ice melt in the Arctic Ocean.
A new island appeared off the coast of Greenland. Several miles long, the island was once thought to be the tip of a peninsula halfway up Greenland's remote east coast, but a glacier joining it to the mainland melted away completely, leaving it surrounded by sea. Shaped like a three-fingered hand, some 400 miles north of the Arctic Circle, it was discovered by veteran American explorer Dennis Schmitt, who named it Warming Island. The US Geological Survey confirmed its existence with satellite photos that show it as an integral part of the Greenland coast in 1985, but linked by only a small ice bridge in 2002, and completely separate by the summer of 2005. It is now a striking island of high peaks and rugged, rocky slopes plunging steeply to a sea dotted with icebergs.
The Independent gave over the front page to a picture of Warming Island, and also published the satellite photos, which made it clear that the new isle had been created by a quite undeniable, rapid and enormous physical transformation. It was the most vivid image yet of the disintegration of the Greenland ice sheet, which, if it melted completely, would lead to a global sea level rise of 7.2 metres, or more than 23 feet.
The melting of the ice, at sea and on land, threw into sharp relief the other climate warning that 2007 brought us the official written one, in the shape of the fourth report of the Intergovernmental Panel on Climate Change (IPCC).
AR4, as it is known in the jargon, appeared in three parts, in February, April and May, dealing respectively with the science and the potential impacts of global warming, and the strategies for countering it. The report as a whole painted a dire picture of what was likely to happen to the world and to human society if the emission of greenhouse gases continued unabated, with a potential rise in average global temperature of 6.4C by the end of the century, which would make life on the planet as we know it unviable. Agriculture would be likely to fail over wide areas of Africa where people are poorest, while extreme weather events and sea-level rise would affect hundreds of millions.
The predictions were not entirely new they were a refinement of the three previous IPCC reports. What was new was the level of confidence with which they were made. The fact that the world was warming was now "unequivocal", the IPCC said an unusual strong word for cautious scientists to use and the likelihood that this warming was a direct result of the increased amount of greenhouse gases being put into the air by human society was greater than nine out of 10.
These conclusions, representing the considered and consensus view of the international community of climate scientists, in effect ended the major debate about climate change, for all but the most perverse sceptics. They were heard with all the more attention in a world that had been at last alerted to the real dangers of global warming by one man the former US Vice-President Al Gore, with his film An Inconvenient Truth. (He was awarded the Nobel Prize for his efforts, jointly with the IPCC, in September.)
However, some voices said the IPCC, even in its dire warnings, was underestimating the real danger, and that the rate of ice-melt instanced above was evidence of a much faster progression of global warming than the UN scientists were allowing for.
Chief among the critics was America's leading climate scientist, James Hansen of Nasa, who published an apocalyptic paper in May suggesting that by the end of the century the world would face a sea-level rise not of 59 centimetres, as the IPCC suggested, but of several metres.
This was because the land-based ice-sheets were melting in a "non-linear" way not just melting at a steady rate, but dynamically breaking up as well, and this process was not properly represented in the supercomputer climate models used to make global-warming predictions.
By the time the final synthesis of all three sections of AR4 was published in Spain in November, the authors had partially accepted this criticism, and indicated that sea-level rise might be more than they had first calculated.
Just as important was the fact that the report had been signed off by every government in the world, including that of the US, whose officials said that the basic scientific case for climate change had now been accepted. That did not mean that the world community was united in what to do about it. On the contrary, the meeting of the UN convention on climate change in Bali this month showed that there were still wide divisions between nations about how to go forward and tackle the most critical problem human society has ever faced. The world has two years at most to get its act together to build a new international climate treaty to replace the Kyoto protocol, which is coming to an end.
But there is no longer any excuse for inaction, as people can clearly see the future, and it is dire. They used to see it in the stars, in crystal balls, in the entrails of sacrificed birds, even in tea leaves.
Our generation is seeing it in the ice.
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