Climatic Changes That Make The World Flip - global warming

Global warming's impact on the environment is not necessarily a drawn-out affair. Recent evidence shows that dramatic changes or 'climatic flips' could happen virtually overnight.

The once-green land of Ireland turned into a frozen wilderness. Harp seals swimming among ice-floes off the coast of France. Polar bears prowling the streets of Amsterdam. These are the images conjured up by the latest research into global warming.

Yes, you read that correctly: global warming - the rise in the world's average temperature caused by the trapping of the sun's heat by pollution in the atmosphere.

If you are baffled by that, then prepare to be shocked. For the same research is now suggesting that such dramatic changes in the climate of northern Europe could take place in as few as 10 years.

Again, this figure is not a misprint: no zero has gone missing. Scientists have recently uncovered compelling evidence that global warming can have a devastating impact on timescales far shorter than anyone believed possible. Not centuries, not even decades, but years, in what are being called "climatic flips". One leading expert has recently gone on record to warn that some north Atlantic countries could find themselves plunged into Arctic conditions over the space of just 10 years.

Risk of sudden upheaval

In geological terms, that is as fast as the blink of an eye. But even in human terms, such a rate of climatic change is incredibly - and quite probably intolerably - rapid. It is far from clear whether any economy or agricultural system could cope with such sudden upheaval.

Yet evidence is now mounting that such "climatic flips" not only can happen, but have happened in the past. It is evidence that adds new urgency to the global warming debate, which has lost much of its momentum in recent years. It also highlights the frightening complexity of the task facing scientists trying to predict the earth's response to human activity.

Arguments about climatic change typically focus on how increasing levels of so-called greenhouse gases - principally carbon dioxide from burning fossil fuels - in the earth's atmosphere trap ever more of the sun's heat.

Huge efforts have been put into predicting the likely global temperature rise caused by the extra greenhouse gases, and current best estimates point to a rise of 1.5 degrees Celsius or so over the next century.

But while scientists warn that even so apparently small a rise in temperature could cause upheaval in everything from agricultural practices to the spread of disease, the rate of change hardly seems terrifying. Surely we can cope and have coped with events that change over several generations?

Such arguments are buttressed by another, apparently compelling, argument against rapid climate change. The earth's oceans have colossal thermal inertia, and would surely iron out any sudden upheaval: weight for weight, it takes ten times more energy to heat water than it does solid iron.

Small wonder, then, that scientists were unsurprised when they failed to find any signs of rapid climatic changes when they first studied ancient ocean sediments, the isotope levels of which retain a record of past temperatures.

The end of the lee Age: a puzzling discovery

But this apparently comforting confluence of theory and data is now known to contain two huge loopholes. The first reared its head in the early 1980s, when a joint U.S.-European team of scientists working in Greenland made a puzzling discovery. They had extracted an ice-core from a site in the southern part of the country, and had measured isotope levels in the gas trapped at different depths in an attempt to gauge the temperature in the region over thousands of years.

Because the ice builds up relatively rapidly, the ice-core was expected to give the researchers the most fine-detailed picture yet of temperature changes in the region. Plotting out the corresponding temperatures, the researchers discovered something puzzling - and disturbing.

As expected, the core showed the rise in temperature corresponding to the end of the last Ice Age around 11,000 years ago. But it also showed that the bulk of that warm-up had taken place in the space of just 40 years.

At the time, no one knew what to make of the result, which flew in the face of everything scientists then knew about climate change - or thought they knew. In the years that followed, however, further ice-cores were extracted, and they revealed an even more dramatic story: a 5- to 10-degree increase in temperature and doubling of precipitation over Greenland in the space of just 20 years.

Nothing in the earlier ocean sediment core data had prepared scientists for such a finding - nor could it. For this was the first loophole in the argument against sudden climatic flips: the absence of evidence from the original ocean sediment cores simply reflected the very broad-brush picture they gave of temperature change. They lacked the detail offered by ice-cores.

Prompted by the Greenland findings, scientists have since tracked down locations where ocean sediment builds up fast enough to give a record of temperature comparable in detail to that from the ice-cores. And, sure enough, they reveal the same story of rapid climatic change in locations as far apart as California and India.

The history of science shows that finding evidence for some astonishing phenomenon is often only part of the story. To convince the scientific community at large, the evidence has to be backed up by a more comprehensive explanation. And for many years the standard explanation for why Ice Ages begin and end provided yet more reasons for thinking all climatic change must be slow and graceful.

That explanation rests on work by a Serbian scientist named Milutin Milankovitch, who in 1920 linked Ice Ages to changes in the shape of the earth's orbit. Caused by the push and pull of the other planets, these orbital changes altered the concentration of sunlight reaching the planet. Such changes would naturally take place very gradually, on timescales of many thousands of years - a recipe for climatic change that is anything but abrupt.

A global heat transporter

Yet, once again, there is a loophole in this comforting argument - as Wallace Broecker of Columbia University, New York State, realized around the time climate experts were puzzling over the ice-core data.

This loophole centres on a very specific feature of the earth's oceans: their circulation patterns. Ocean currents transport heat around the globe like a vast conveyor belt. In the Atlantic, for example, warm water travels northwards from the Gulf of Mexico, passing its heat to the air by evaporation as it goes. This makes the current progressively cooler, saltier and denser until eventually, near Iceland, the water is so heavy that it sinks, and begins a long journey southward, along the ocean floor.

Broecker realized that this complex, subtle process - which he called "The Conveyor" - could be the Achilles heel of the earth's climate, allowing subtle changes to be turned into dramatic upheaval. For instead of having to alter the whole body of the oceans, just a small change in temperature might be enough to alter the behaviour of the Conveyor - and trigger radical and rapid climatic change over a large area.

For example, gradually melting ice from the Arctic could dilute the saltiness of the Conveyor to a critical density where it no longer sinks and begins its journey southward to pick up more heat. The Conveyor would, in effect, be switched off, isolating the north Atlantic from the warming waters of the tropics. The result would then be distinctly paradoxical, with a slight warming of the Arctic causing temperatures of north Atlantic countries to plunge.

Broecker's explanation is now widely believed to lie at the heart of rapid climate change in the past. Worryingly, however, global warming is predicted to have precisely the type of warming effect on the Arctic ice that threatens the existence of the Conveyor. Computer projections of the effect of pollution on global temperatures predict an inflow of cold, fresh water into the northern Atlantic - water that could dilute the Conveyor enough to switch it off.

The Achilles heel of the earth's climate

If that happened, says Broecker, winter temperatures in the north Atlantic region would fall by 10 or more degrees Celsius within 10 years, giving places like Dublin the climate of Spitsbergen, 400 km north of the Arctic Circle. "The consequences could be devastating," he says.

It is a scenario that gains credibility from ice-core data, according to climate expert Kendrick Taylor of the Desert Research Institute in Reno, Nevada. He says that many cores suggest that around 8,000 years ago there was a sudden plunge back to a "mini Ice Age" which lasted around 400 years. The most likely cause, says Taylor, was the release of melted ice-water from lakes in Canada into the Atlantic, which switched off the heat-transporting Conveyor.

"The change in freshwater flux to the oceans was large, but not that much different from what greenhouse-induced changes may produce in the future," he said in a recent paper in American Scientist. "It is ironic that greenhouse warming may lead to rapid cooling in eastern Northern America, Europe and Scandinavia."

So just how close is the Conveyor to switching off once again?The short answer is: no one knows. Computer models have still to identify the critical density of seawater at which the Conveyor will switch off, or the greenhouse gas concentrations needed to release the requisite amount of melt-water.

Cutting pollution buys time

What computer models have shown, says Taylor, is that reducing pollution emissions buys time - both by slowing the rate of global warming, and also by driving the climate more gently, which seems to increase its stability against rapid change.

But while scientists struggle to capture the full complexity of the climate on their supercomputers, evidence of other causes of dramatic climate change is beginning to emerge.

Last July, Professor Martin Claussen and his colleagues at the Potsdam Institute for Climate Science, Germany, reported evidence that today's Sahara desert was created in a sudden climatic "flip" that took place just 5,500 years ago, turning vast areas of lush grassland into an arid wilderness and devastating ancient civilizations.

Using a sophisticated computer model of the land, sea and atmosphere, the team has discovered just how subtle are some of the effects that can turn Milankovitch-style changes in the earth's orbit into major climate upheaval.

The Sahara's quick-change act

They found that over the last 9,000 years the gravitational pull of the planets has altered the tilt of the earth's axis by about half a degree, and changed the timing of earth's closest approach to the sun by around five months.

By themselves, such subtle changes should not cause major climatic effects. But when Claussen and his colleagues included the effect of vegetation in their computer model, they found that it caused rainfall levels to plummet over the Sahara region.

They traced the cause to "feedback" effects, in which a slight drop in vegetation level makes the earth's surface slightly better at reflecting sunlight, which causes rainfall levels to drop - prompting more vegetation loss, and so on.

According to Claussen, these feedback effects turned the vast, once-green Sahara into a brown wasteland within just 300 years. "It was the largest change in land cover during the last 6,000 years,' he says. "It was very severe, ruining ancient civilizations."

The discovery is likely to force historians to rethink their explanations of events in the region. For according to Claussen, it contradicts the long-held belief that the collapse of agriculture in the region was caused by ancient farmers exhausting the soil: "Although humans lived in the Sahara and used the land to some extent, we think that ancient land use played only a negligibly small role."

The findings are also being seen as another warning of just how unstable even today's climate may be. "It is capable of changing very abruptly," says climate expert Andrew Goudie of Oxford University. "We've known that the extent of the Sahara has yo-yoed back and forwards for millions of years, and that about 8,000 years ago it was much wetter than today, with big rivers feeding into the Nile. But I hadn't realized just how rapid the changeover had been. It is salutary."

Temperature nose-dives

Also in July, a team of researchers from the universities of Illinois and Minnesota reported the discovery of another climatic "flip" in the northern hemisphere around 9,000 years ago, which temporarily plunged the region back into an Ice Age.

Using lake sediments from Minnesota, the team confirmed the existence of the cold snap around 8,200 years ago, as revealed by the ice core data. But they also found evidence for another dive in temperatures around 8,300 to 8,900 years ago. The team thinks this older cold snap was linked to the release of melted ice from lakes into the Atlantic - which may have switched off the Conveyor. But the researchers now think that the more recent flip most likely had another - and as yet unknown - origin.

What is clear is that until we know much more about the complexity of climate change, all bets about how much time we have to take action are definitely off. What evidence we do have increasingly points to the stark possibility that we may have far less time than we thought.

"I used to believe that change in climate happened slowly and would never affect me," admits Taylor. "Now I know that our climate could change significantly in my lifetime."

A MUDDY MYSTERY

The effects that scientists now believe can shape the earth's climate are astonishingly subtle, and one of the most bizarre centres on the connection between Ice Ages, earthquakes, and mud.

Last year a team from the Southampton Oceanography Centre in Britain announced in the journal Nature the discovery of a 450-billion-cubic-metre deposit on the sea-floor off the coast of Sardinia-the aftermath of a truly enormous slide of mud: enough to engulf the whole of France to a depth of a metre.

Carbon dating of the plankton above and below the mud bed suggest that the slide took place around 20,000 years ago - at the height of the last Ice Age. This was a time when so much water had been turned to ice that sea-levels were 120 metres lower than they are today-a fact that the team suspects is crucial to the cause of the colossal mud-slide.

Formed out of thousands of years of river deposits, the mud would have been rich in organic matter-material that rots to produce huge amounts of methane gas. Normally this gas would have been locked into the mud by the huge pressure of sea-water lying over the submerged sediments. But as the Ice Age deepened and sea-levels fell, the mud deposits would find themselves exposed, and thus able to release their pent-up methane gas.

This gas is a very potent source of global warming, and the sudden release of huge amounts of it might have helped push the earth back out of the Ice Age.

Certainly the timing of the Sardinian mudslide - at the height of the last Ice Age - is intriguing, says team member Professor Euan Nisbet of London's Royal Holloway College: "It is possible that a big slide could have released enough methane to act as a warming trigger."

It is an idea that has gained strength from a recent discovery by researchers at Duke University in North Carolina. In a paper also published in Nature last year, they argue that the sheer weight of ice pressing down on the earth's crust may have triggered huge earthquakes during the Ice Age.

Could these have triggered massive submarine mudslides, releasing methane that led to the end of Ice Age? As yet, no one knows. But it points to yet another astonishingly subtle link between the environment, climate - and us.

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