Scientists fear that global warming will bring climatic turbulence, with changes coming in big jumps rather than gradually.
RICHARD ALLEY's eyes glint as we sit in his office in the University of Pennsylvania discussing how fast global warming could cause sea levels to rise. The scientist sums up the state of knowledge: "We used to think that it would take 10,000 years for melting at the surface of an ice sheet to penetrate down to the bottom. Now we know it doesn't take 10,000 years; it takes 10 seconds."
That quote highlights most vividly why scientists are getting panicky about the sheer speed and violence with which climate change could take hold. They are realising that their old ideas about gradual change the smooth lines on graphs showing warming and sea level rise and gradually shifting weather patterns simply do not represent how the world's climate system works.
The story of research into sea level rise is typical of how perceptions have changed in the past five years. The conventional view holds that sea levels will start to rise as a pulse of warming works its way gradually from the surface through the 2 km- and 3 km-thick ice sheets on Greenland and Antarctica, melting them. The ice is thick and the heat will penetrate only slowly. So we have hundreds, probably thousands, of years to make our retreat to higher ground.
Recent research, however, shows that this idea is wholly wrong. Glaciologists forgot about crevasses. What is actually happening is that ice is melting at the surface and forming lakes that drain down into the crevasses. In 10 seconds, the water is at the base of the ice sheet, where it lubricates the joint between ice and rock. Then the whole ice sheet starts to float downhill towards the ocean. "These flows completely change our understanding of the dynamics of ice sheet destruction," says Professor Alley. "Even five years ago, we didn't know about this."
This summer, lakes several kilometres across formed on the Greenland ice sheet, and drained away to the depths. Scientists measured how, within hours of the lakes forming, the vast ice sheets physically rose up, as if floating on water, and slid towards the ocean. That is why Greenland glaciers are flowing faster, and there are more icebergs breaking off into the Atlantic Ocean. Jim Hansen of NASA, George W. Bush's top climate modeller, predicts that sea level rise will be 10 times faster within a few years, as Greenland destabilises. "Building an ice sheet takes a long time," he says. "But destroying it can be explosively rapid." Alarmist? No. It has happened before, he says. During the final few centuries of the last ice age, the sea level rose 20 metres in 400 years, an average of 20 times faster than now.
There is more evidence of abrupt and violent change, most of it culled from ice cores, lake sediments, tree rings and other natural archives of climate. We now know that the last ice age was not a stable cold era but near-permanent climate change. Towards the end, around 11,000 years ago, average temperatures in parts of the Arctic rose by 16 degrees C or more within a decade.
All this comes as a surprise to us because, in the 10,000 or so years since the end of the last ice age, the climate has been, relatively speaking, stable. We have had warm periods and mini ice ages; but they were little compared with events before.
It is arguable that this rather benign world has been the main reason why our species was able to leave the caves and create the urban, industrial civilisation we enjoy today. But there is a growing fear among scientists that, thanks to man-made climate change, we are about to return to a world of climatic turbulence, where tipping points are constantly crossed. Their research into the workings of the planet's ecosystems suggests why such sudden changes have happened in the past, and are likely again in the future.
One driver of fast change in the past has been abrupt movements of carbon between the atmosphere and natural reservoirs such as rainforests and oceans. Hundreds of billions of tonnes of carbon dioxide can burp into the atmosphere, apparently at the flick of a switch.
That is why the U.K. Met Office's warning that the Amazon rainforest could die by mid-century, releasing its stored carbon from trees and soils into the air, is so worrying.
Other threats lurk on the horizon. We know that there are trillions of tonnes of methane, a virulent greenhouse gas, trapped in permafrost and in sediments beneath the ocean bed. There are fears this methane may start leaking out as temperatures warm. It seems this happened 55 million years ago, when gradual warming of the atmosphere penetrated to the ocean depths and unlocked the methane, which caused a much greater warming that resulted in the extinction of millions of species.
All this suggests that, in one sense, the climate sceptics are right. They say the future is much less certain than the climate models predict. They have a point. We know less than we think. But the sceptics are wrong in concluding that the models have been exaggerating the threat.
Far from it. Evidence emerging in the past five years or so suggests the presence of many previously unknown tipping points that could trigger dangerous climate change.
Can we call a halt? Hansen says we have 10 years to turn things round and escape disaster. James Lovelock, author of the Gaia theory, which considers the Earth a self-regulated living being, reckons we are already past the point of no return. I don't buy that. For one thing, there is no single point of no return. We have probably passed some, but not others. The water may be lapping at our ankles, but I am not ready to head for the hills yet. I am an optimist.
[Fred Pearce is author of The Last Generation How Nature Will Take Her Revenge for Climate Change (Eden Project Books).]