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What caused the ice ages that have punctuated our planet's history for the past 2 million years? We badly need to know, a recent workshop of the world's top climate scientists concluded, because it could help us predict how our planet will respond to climate change in the coming decades. The answer "may help us decide if we are pushing the system to the extent that we risk a catastrophe," the conference concluded.
Every hundred thousand years or so, a huge ice cap starts spreading across North America. Beginning in the Arctic islands of Canada, it pushes south, crossing the Great Lakes and moving on into the prairies. This huge accumulation of ice causes temperatures to crash worldwide. The planet freezes. And it stays frozen for tens of thousands of years till the ice suddenly breaks up and, within a few centuries, is gone.
The last great thaw happened about 10,000 years ago. It heralded the rise of human civilization. But it has happened before -- perhaps 20 times in the past 2 million years. But what controls this great climatic convulsion? Scientists say that if they can unlock the secrets of the ice ages, maybe they'll be closer to figuring out our climatic future. "The planet may have an Achilles' heel that can cause abrupt climatic changes; and, if it does, we badly need to know about it," said Will Steffen, the Stockholm-based director of the International Geosphere Biosphere program, a worldwide network of Earth system scientists.
Last May in Germany, some of the world's top glaciologists and climate scientists sat down to try to unravel the puzzle. What will be the effect of all the artificial greenhouse gases on this cycle? They concluded that there was a significant risk of a "catastrophic outcome," with temperatures rising far further and faster than under current predictions for global warming.
Researchers are none too sure why the ice ages started. But the first clue to their regular recurrence is derived from continuous and repeated wobbles in the Earth's orbit, named after an obscure Serbian mathematician called Milutin Milankovitch who discoverd them in the 1940s.
The wobbles slightly change the distribution of solar heating of the Earth's surface. And when less heat falls on the poles, it encourages the formation of ice caps. The effect is small, but it gets amplified. A growing ice cap reflects more heat back into space than the dark, heat-absorbing tundra that it smothers. This results in further cooling. The cooling causes more ice to form, and so on.
There are, in fact, a series of different wobbles that happen with different frequencies. But the critical one, coinciding with the ice ages, is caused by the slightly elliptical shape of the Earth's orbit around the sun and repeats every 100,000 years. It seems to trigger the growth of the North American ice sheet. And that turns out to be crucial, said Andy Watson, a professor at the University of East Anglia in England who helped organize the meeting.
"An extraordinary picture is emerging," Watson said, "of a complex but ordered planetary dance" in which the orbital wobbles, shifting ice and global temperatures move in step with one another. "It offers a unique opportunity to probe the dynamics of the Earth's climate system."
North America seems to be the key to the ice ages because it has more land in the very far north than Asia, on which large ice caps can grow. So, when the Arctic cools, ice caps grow first and fastest in North America. At its peak, the great North American ice cap could contain enough ice to cover the whole of Canada and the United States to a depth of more than a mile, said Tony Payne, a glaciologist at the University of Bristol in England. It lowers sea levels worldwide by more than 200 feet and cools the planet by 10 to 15 degrees Fahrenheit.
Without this North American amplifying effect, the Milankovitch wobbles would not be enough on their own to create an ice age, said Martin Claussen of the Potsdam Institute for Climate Impact Research in Germany.
But then, when at its greatest extent, the great North American ice cap starts to break up, dragging the world out of the ice age. Why? And why do the changes happen so fast?
The conference -- one of a series of elite scientific meetings held in Dahlem, Germany, at the Free University of Berlin -- concluded that the answers to both questions seems to involve a "hidden hand" on the planetary thermostat -- the greenhouse gas, carbon dioxide.
This is the gas that most scientists believe causes global warming when it accumulates in the atmosphere. But in the depths of the ice ages, it seems to have helped the planet beat the freeze and push back the North American ice cap.
We know about past levels of greenhouse gases in the atmosphere because researchers have been able to recover bubbles of ancient air trapped in ice. These bubbles reveal past temperatures and concentrations of gases in the air. They show that when the world was at its coldest, carbon dioxide levels in the air were low; and when it got warmer, they rose.
But they reveal something more startling. The changes between these two situations were not smooth and gradual. They were extremely quick. It is almost, Watson said, as if the planet has a rather crude thermostat, with just two settings -- ice age, and not ice age.
Put another way, there appear to have been two "stable states" for the planet's climate system. Once one of them broke down, the entire system switched within a few centuries to the other.
In the cool state, carbon dioxide levels in the air were low, at about 190 parts per million. In the warm state, they were at about 280 ppm. The current thinking is that the flips between these two levels probably involved some abrupt change in where the planet's carbon is stored, between the oceans, the atmosphere, and land.
One theory holds that as the ice cap advanced, cooling the planet, it shut down ecosystems and reduced their natural emissions of carbon dioxide to the atmosphere. This reduced atmospheric concentrations and reinforced the cooling. But later, as the ice advanced on the big carbon stores in the tropical rainforests, dying forests started pushing carbon dioxide from decomposing trees into the air. The warming effect began to exceed the cooling effect. The world started to warm again. The ice cap started to retreat, causing more warming. The process accelerated. Suddenly, the ice age was over.
None of this is proved. But whatever the precise mechanism, Watson said, we are left with the worrying fact that, in the past 2 million years or so, the world had two stable climatic states -- anchored at 190 ppm and 280 ppm of carbon dioxide. Why worrying? Because, Watson said, we have now slipped the anchors. By burning fossil fuels, we have forced up carbon dioxide levels to 370 ppm today. That is probably higher than for millions of years. And the level is still rising by almost 20 ppm a decade.
The question now is: How will the planet respond? Until now, climate scientists have mostly expected that a gradual rise in greenhouse gases will cause a gradual increase in temperatures. Now there are two other possibilities. The planet might find a way to keep temperatures down. Or it might make another jump -- to perhaps a third "stable state" about which we as yet know nothing.
How might that happen? Peter Cox of the British government's meteorological service, said that within 50 years, rainforests and their soils could begin to dry out and die as warming gathers pace. That would release carbon dioxide into the atmosphere and accelerate warming. Others predict changes in the ocean circulation systems that reduce the oceans' abilities to absorb carbon dioxide from the air.
Nothing is certain in this. Climate scientists are being forced to acknowledge how little they know about how the planet works. But that ignorance, they say, should make us more worried rather than less.
The take-home message, perhaps, is that the Earth isn't quite such a well-oiled machine as we thought. Its orbit wobbles; its climate lurches. And we could be on the threshold of giving another nudge to the planet's thermostat.