February 4, 2008

'TIPPING POINTS' COULD COME THIS CENTURY

A number of key components of the earth's climate system could pass their 'tipping point' this century, according to new research led by a scientist at the University of East Anglia.

Published today by the prestigious international journal Proceedings of the National Academy of Science (PNAS), the researchers have coined a new term, 'tipping elements', to describe those components of the climate system that are at risk of passing a tipping point.

The term 'tipping point' is used to describe a critical threshold at which a small change in human activity can have large, long-term consequences for the Earth's climate system.

In this new research, lead author Prof Tim Lenton of the University of East Anglia (UEA) and colleagues at the Postdam Institute of Climate Impact Research (PIK), Carnegie Mellon University, Newcastle University and Oxford University have drawn up a shortlist of nine tipping elements relevant to current policy-making and calculated where their tipping points could lie. All of them could be tipped within the next 100 years.

The nine tipping elements and the time it will take them to undergo a major transition are:

• Melting of Arctic sea-ice (approx 10 years)
• Decay of the Greenland ice sheet (more than 300 years)
• Collapse of the West Antarctic ice sheet (more than 300 years)
• Collapse of the Atlantic thermohaline circulation (approx 100 years)
• Increase in the El Nino Southern Oscillation (approx 100 years)
• Collapse of the Indian summer monsoon (approx 1 year)
• Greening of the Sahara/Sahel and disruption of the West African monsoon (approx 10 years)
• Dieback of the Amazon rainforest (approx 50 years)
• Dieback of the Boreal Forest (approx 50 years)

The paper also demonstrates how, in principle, early warning systems could be established using real-time monitoring and modelling to detect the proximity of certain tipping points.

"Society must not be lulled into a false sense of security by smooth projections of global change," said Prof Lenton.

"Our findings suggest that a variety of tipping elements could reach their critical point within this century under human-induced climate change. The greatest threats are tipping of the Arctic sea-ice and the Greenland ice sheet, and at least five other elements could surprise us by exhibiting a nearby tipping point."

##

Contact:

Simon Dunford
University of East Anglia
44-016-035-92203
press@uea.ac.uk

This text derived from:
http://www1.uea.ac.uk/cm/home/services/units/mac/comm/media/press/2008/feb/homepagenews/%25E2%2580%2598Tipping%2Bpoints%25E2%2580%2599%2Bcould%2Bcome%2Bthis%2Bcentury

Greenland warming speeds rise in sea level
By Nic Fleming, Science Correspondent in St Louis, Missouri
(Filed: 21/02/2006)

Sea levels are rising quicker than previously thought because the amount of water Greenland's glaciers are dumping into the Atlantic Ocean has almost doubled in five years, according to research to be published today.

Scientists who carried out the first comprehensive analysis of changing speeds of the glaciers on the world's largest island were shocked to discover that many have doubled in speed within the past decade.

Warmer temperatures are "lubricating" the glaciers and have driven a 150 per cent increase in the amount of ice they are delivering to the ocean between 1996 and last year. The latitude at which this is happening is moving north.

The researchers fear that as a result current estimates that sea levels will rise by up to 90cms during the 21st century could underestimate the problem.

While there have been previous isolated reports of particular glaciers speeding up, the research presented yesterday at the American Association for the Advancement of Science conference in St Louis is the first detailed study showing the effect is widespread across Greenland.

Eric Rignot, of Nasa's Jet Propulsion Laboratory at the California Institute of Technology in Pasadena, the leading author of the study, said: "Climate change can work in different ways, but generally speaking, if you warm up the ice sheet, the glacier will flow faster.

"The southern half of Greenland is reacting to what we think is climate warming. The northern half is waiting but I don't think it is going to take long. If more glaciers accelerate farther north, especially along the west coast, the mass loss from Greenland will continue to increase well above predictions."

The Intergovernmental Panel on Climate Change has estimated that global sea levels will increase by 10-90cms over the next century. Last century they rose by 10-20cm.

Previously models of the melting of the Greenland ice sheet have been based mainly on airborne laser altimetry and have suggested that while the interior is reasonably stable, the periphery was thinning, especially in areas where glaciers meet the sea.

Dr Rignot and colleagues set out to obtain accurate measurements to help to build a clearer picture of Greenland's current and future contribution to rising sea levels. They collected satellite data on the speeds of 27 glaciers and made estimates for two others.

The velocities of several large glaciers had doubled in recent years to 12km per year, making them among the fastest flowing in the world.

The scientists found that warmer air temperatures have increased the overall shrinkage of the Greenland ice sheet from 91 cubic km per year in 1996 to 138 cubic km per year in 2000 and to 224 cubic km per year in 2005.

About two-thirds of this was caused by the dumping of ice in the Atlantic by glaciers and so the ice loss attributable to glacier flow grew by 150 per cent from 60 cubic km a decade ago to 150 cubic km last year.

On this basis Dr Rignot, whose work is published in Science, concluded that Greenland's contribution to rising global sea levels increased from around 0.23mm per year in 1996 to around 0.57mm per year in 2005.

The Greenland ice sheet is 1.7 million sq km - a little smaller than Mexico - and three kilometres thick. If it melted completely sea levels would rise by seven metres.

9 February 2006: Krakatoa helps to keep Earth cool

31 January 2006: Change to our climate unstoppable, warns expert

Global warming 'past the point of no return'

http://news.independent.co.uk/world/science_technology/article312997.ece

By Steve Connor, Science Editor

Published: 16 September 2005

A record loss of sea ice in the Arctic this summer has convinced scientists that the northern hemisphere may have crossed a critical threshold beyond which the climate may never recover. Scientists fear that the Arctic has now entered an irreversible phase of warming which will accelerate the loss of the polar sea ice that has helped to keep the climate stable for thousands of years.

They believe global warming is melting Arctic ice so rapidly that the region is beginning to absorb more heat from the sun, causing the ice to melt still further and so reinforcing a vicious cycle of melting and heating.

The greatest fear is that the Arctic has reached a "tipping point" beyond which nothing can reverse the continual loss of sea ice and with it the massive land glaciers of Greenland, which will raise sea levels dramatically.

Satellites monitoring the Arctic have found that the extent of the sea ice this August has reached its lowest monthly point on record, dipping an unprecedented 18.2 per cent below the long-term average.

Experts believe that such a loss of Arctic sea ice in summer has not occurred in hundreds and possibly thousands of years. It is the fourth year in a row that the sea ice in August has fallen below the monthly downward trend - a clear sign that melting has accelerated.

Scientists are now preparing to report a record loss of Arctic sea ice for September, when the surface area covered by the ice traditionally reaches its minimum extent at the end of the summer melting period.

Sea ice naturally melts in summer and reforms in winter but for the first time on record this annual rebound did not occur last winter when the ice of the Arctic failed to recover significantly.

Arctic specialists at the US National Snow and Ice Data Centre at Colorado University, who have documented the gradual loss of polar sea ice since 1978, believe that a more dramatic melt began about four years ago.

In September 2002 the sea ice coverage of the Arctic reached its lowest level in recorded history. Such lows have normally been followed the next year by a rebound to more normal levels, but this did not occur in the summers of either 2003 or 2004. This summer has been even worse. The surface area covered by sea ice was at a record monthly minimum for each of the summer months - June, July and now August.

Scientists analysing the latest satellite data for September - the traditional minimum extent for each summer - are preparing to announce a significant shift in the stability of the Arctic sea ice, the northern hemisphere's major "heat sink" that moderates climatic extremes.

"The changes we've seen in the Arctic over the past few decades are nothing short of remarkable," said Mark Serreze, one of the scientists at the Snow and Ice Data Centre who monitor Arctic sea ice.

Scientists at the data centre are bracing themselves for the 2005 annual minimum, which is expected to be reached in mid-September, when another record loss is forecast. A major announcement is scheduled for 20 September. "It looks like we're going to exceed it or be real close one way or the other. It is probably going to be at least as comparable to September 2002," Dr Serreze said.

"This will be four Septembers in a row that we've seen a downward trend. The feeling is we are reaching a tipping point or threshold beyond which sea ice will not recover."

The extent of the sea ice in September is the most valuable indicator of its health. This year's record melt means that more of the long-term ice formed over many winters - so called multi-year ice - has disappeared than at any time in recorded history.

Sea ice floats on the surface of the Arctic Ocean and its neighbouring seas and normally covers an area of some 7 million square kilometres (2.4 million square miles) during September - about the size of Australia. However, in September 2002, this dwindled to about 2 million square miles - 16 per cent below average.

Sea ice data for August closely mirrors that for September and last month's record low - 18.2 per cent below the monthly average - strongly suggests that this September will see the smallest coverage of Arctic sea ice ever recorded.

As more and more sea ice is lost during the summer, greater expanses of open ocean are exposed to the sun which increases the rate at which heat is absorbed in the Arctic region, Dr Serreze said.

Sea ice reflects up to 80 per cent of sunlight hitting it but this "albedo effect" is mostly lost when the sea is uncovered. "We've exposed all this dark ocean to the sun's heat so that the overall heat content increases," he explained.

Current computer models suggest that the Arctic will be entirely ice-free during summer by the year 2070 but some scientists now believe that even this dire prediction may be over-optimistic, said Professor Peter Wadhams, an Arctic ice specialist at Cambridge University.

"When the ice becomes so thin it breaks up mechanically rather than thermodynamically. So these predictions may well be on the over-optimistic side," he said.

As the sea ice melts, and more of the sun's energy is absorbed by the exposed ocean, a positive feedback is created leading to the loss of yet more ice, Professor Wadhams said.

"If anything we may be underestimating the dangers. The computer models may not take into account collaborative positive feedback," he said.

Sea ice keeps a cap on frigid water, keeping it cold and protecting it from heating up. Losing the sea ice of the Arctic is likely to have major repercussions for the climate, he said. "There could be dramatic changes to the climate of the northern region due to the creation of a vast expanse of open water where there was once effectively land," Professor Wadhams said. "You're essentially changing land into ocean and the creation of a huge area of open ocean where there was once land will have a very big impact on other climate parameters," he said.

A record loss of sea ice in the Arctic this summer has convinced scientists that the northern hemisphere may have crossed a critical threshold beyond which the climate may never recover. Scientists fear that the Arctic has now entered an irreversible phase of warming which will accelerate the loss of the polar sea ice that has helped to keep the climate stable for thousands of years.

They believe global warming is melting Arctic ice so rapidly that the region is beginning to absorb more heat from the sun, causing the ice to melt still further and so reinforcing a vicious cycle of melting and heating.

The greatest fear is that the Arctic has reached a "tipping point" beyond which nothing can reverse the continual loss of sea ice and with it the massive land glaciers of Greenland, which will raise sea levels dramatically.

Satellites monitoring the Arctic have found that the extent of the sea ice this August has reached its lowest monthly point on record, dipping an unprecedented 18.2 per cent below the long-term average.

Experts believe that such a loss of Arctic sea ice in summer has not occurred in hundreds and possibly thousands of years. It is the fourth year in a row that the sea ice in August has fallen below the monthly downward trend - a clear sign that melting has accelerated.

Scientists are now preparing to report a record loss of Arctic sea ice for September, when the surface area covered by the ice traditionally reaches its minimum extent at the end of the summer melting period.

Sea ice naturally melts in summer and reforms in winter but for the first time on record this annual rebound did not occur last winter when the ice of the Arctic failed to recover significantly.

Arctic specialists at the US National Snow and Ice Data Centre at Colorado University, who have documented the gradual loss of polar sea ice since 1978, believe that a more dramatic melt began about four years ago.

In September 2002 the sea ice coverage of the Arctic reached its lowest level in recorded history. Such lows have normally been followed the next year by a rebound to more normal levels, but this did not occur in the summers of either 2003 or 2004. This summer has been even worse. The surface area covered by sea ice was at a record monthly minimum for each of the summer months - June, July and now August.

Scientists analysing the latest satellite data for September - the traditional minimum extent for each summer - are preparing to announce a significant shift in the stability of the Arctic sea ice, the northern hemisphere's major "heat sink" that moderates climatic extremes.

"The changes we've seen in the Arctic over the past few decades are nothing short of remarkable," said Mark Serreze, one of the scientists at the Snow and Ice Data Centre who monitor Arctic sea ice.

Scientists at the data centre are bracing themselves for the 2005 annual minimum, which is expected to be reached in mid-September, when another record loss is forecast. A major announcement is scheduled for 20 September. "It looks like we're going to exceed it or be real close one way or the other. It is probably going to be at least as comparable to September 2002," Dr Serreze said.

"This will be four Septembers in a row that we've seen a downward trend. The feeling is we are reaching a tipping point or threshold beyond which sea ice will not recover."

The extent of the sea ice in September is the most valuable indicator of its health. This year's record melt means that more of the long-term ice formed over many winters - so called multi-year ice - has disappeared than at any time in recorded history.

Sea ice floats on the surface of the Arctic Ocean and its neighbouring seas and normally covers an area of some 7 million square kilometres (2.4 million square miles) during September - about the size of Australia. However, in September 2002, this dwindled to about 2 million square miles - 16 per cent below average.

Sea ice data for August closely mirrors that for September and last month's record low - 18.2 per cent below the monthly average - strongly suggests that this September will see the smallest coverage of Arctic sea ice ever recorded.

As more and more sea ice is lost during the summer, greater expanses of open ocean are exposed to the sun which increases the rate at which heat is absorbed in the Arctic region, Dr Serreze said.

Sea ice reflects up to 80 per cent of sunlight hitting it but this "albedo effect" is mostly lost when the sea is uncovered. "We've exposed all this dark ocean to the sun's heat so that the overall heat content increases," he explained.

Current computer models suggest that the Arctic will be entirely ice-free during summer by the year 2070 but some scientists now believe that even this dire prediction may be over-optimistic, said Professor Peter Wadhams, an Arctic ice specialist at Cambridge University.

"When the ice becomes so thin it breaks up mechanically rather than thermodynamically. So these predictions may well be on the over-optimistic side," he said.

As the sea ice melts, and more of the sun's energy is absorbed by the exposed ocean, a positive feedback is created leading to the loss of yet more ice, Professor Wadhams said.

"If anything we may be underestimating the dangers. The computer models may not take into account collaborative positive feedback," he said.

Sea ice keeps a cap on frigid water, keeping it cold and protecting it from heating up. Losing the sea ice of the Arctic is likely to have major repercussions for the climate, he said. "There could be dramatic changes to the climate of the northern region due to the creation of a vast expanse of open water where there was once effectively land," Professor Wadhams said. "You're essentially changing land into ocean and the creation of a huge area of open ocean where there was once land will have a very big impact on other climate parameters," he said.

Oceanic plankton have largely disappeared from the waters off Northern California, Oregon and Washington, mystifying scientists, stressing fisheries and causing widespread seabird mortality.

The phenomenon could have long-term implications if it continues: a general decline in near-shore oceanic life, with far fewer fish, birds and marine mammals. No one is certain how long the condition will last. But even a short duration could severely affect seabird populations because of drastically reduced nesting success, scientists say.

The plankton disappearance is caused by a slackening of what is known as "upwelling:" the seasonal movement of cold, nutrient-rich offshore water into areas near shore.

This cold water sustains vast quantities of phytoplankton and zooplankton, which are the basis of the marine food web. During periods of vigorous upwelling and consequent plankton "blooms," everything from salmon to blue whales fattens and thrives on the continental shelf of the West Coast.

The larger fish and baleen whales eat mostly krill: free-floating, shrimp- like crustaceans ranging from one to two inches, the upper size limit of the zooplankton realm.

When the water is cold, krill swarm off the Northern California coast by the tens of thousands of tons. Now that they are largely absent, fisheries and wildlife are feeling the effects.

In perhaps the most ominous development, seabird nesting has dropped significantly on the Farallon Islands off San Francisco, the largest Pacific Coast seabird rookery south of Alaska.

Bill Sydeman, the director of marine ecology for the Point Reyes Bird Observatory, a science and conservation organization that maintains a research station on the Farallones, said the collapse of the nesting season is unprecedented in the three decades the group has monitored the islands.

Cassin's auklets -- a relatively rare seabird that feeds almost extensively on krill -- have been particularly hard hit, Sydeman said.

"Normally they breed in March," Sydeman said. "They got started late this year, and by May they had virtually disappeared. We expect zero nesting success for them this year, or close to it. We've never seen anything like it."

Sydeman said other seabirds are also showing the effects of the reduced marine productivity.

"We have little or no nesting of pelagic cormorants (at the Farallones), and Brandt's cormorants are nesting at reduced numbers," he said. "Double- crested cormorant nesting is down by 50 percent (in the Bay Area)."

Upwelling cessation is typically caused by El Niño events -- warm water intrusions from the equatorial Pacific. But what is happening off the coast right now is not a true El Niño, Sydeman said.

"We really don't have a clear idea of what it is," Sydeman said, noting that standard El Niños can be tracked as they progress from the equator to temperate waters, something that hasn't occurred in the current case.

"Some are calling it an El Niño Norte; others think it's some sort of anomalous intrusion of warm offshore blue water onto the continental shelf," he said.

A recent study indicated the phenomenon may be long term, and linked to global warming.

Last week, Fisheries and Oceans Canada -- the federal agency dealing with Canada's marine and inland waters -- released a report saying 2004's spring and summer ocean surface temperatures in the Gulf of Alaska and off British Columbia were the warmest in 50 years.

The study concluded the record high temperatures were caused by abnormally warm weather in Alaska and western Canada, as well as "general warming of global lands and oceans."

Some pulses of upwelling occurred off Northern California in June, Sydeman said, but they're unlikely to significantly increase marine productivity.

"Upwelling has slackened along all the West Coast, except for a little bit of recent activity off Northern California," Sydeman said. "At this point, it's too little and too late. Things aren't going to turn around. For krill predators in this system, it's a very serious situation."

Juvenile rockfish numbers are also way down.

"We annually survey (juvenile rockfish) from San Diego to Cape Mendocino, and this is the lowest catch we've recorded in the 23 years we've been doing it," said Stephen Ralston, a supervising research biologist at the Santa Cruz office for the National Marine Fisheries Service, the federal agency that oversees fisheries in federal waters.

Like krill, young rockfish are a significant food source for seabirds, large fish and marine mammals; they are also essential to maintaining healthy stocks of mature rockfish, esteemed by commercial fishermen and sport anglers.

Off the coast of Oregon, abnormally warm marine water is continuing unabated, affecting local birds and salmon.

"Things are pretty grim up here," said Bill Peterson, an oceanographer with the National Marine Fisheries Service office in Newport, Ore.

Peterson said a major die-off of double-crested cormorants recently occurred in Oregon, and juvenile salmon numbers have dropped precipitously. Both events, he said, are likely due to the warm water.

"We do salmon surveys every spring and summer," he said. "Normally, we catch several hundred salmon in the spring. This year we caught eight. And we usually get several thousand fish in the summer. This year, it was 80."

Peterson said the water temperature off Oregon in late June is normally 10 degrees Celsius (about 50 Fahrenheit), "and this year it's 16 degrees (about 61 F). Our (upper layer of warm water) is normally 15 meters thick, and this year it's 30 meters. Krill numbers are down, and the plankton we are seeing are as unusual as can be -- warm water species that you'd find off San Diego or Monterey."

Peterson said it is unlikely Oregon waters will cool significantly this summer.

"It takes an enormous amount of (offshore wind) energy to push that much warm water offshore, which is what we would need to see for significant upwelling," he said. "I don't see that happening anytime soon."

Near San Francisco, salmon have switched from krill to bait fish, and appear to be holding their own -- at least for now.

"The fishing is terrific," said Roger Thomas, the president of the Golden Gate Fishermen's Association and the owner of the recreational angling boat the Salty Lady.

"It's true there's not much krill, but there're lots of anchovies and sardines," Thomas said, "and the salmon are filling up on those."

Thomas acknowledged that the bait fish wouldn't benefit many coastal and offshore birds.

"Sardines are too big for the auklets, and even for other species like common murres," he said. "They rely on smaller prey species."

In fact, say scientists, krill are the keystone forage species for almost everything that swims off Northern California.

"It's the krill that drive the food web dynamics off this coast," said Ellie Cohen, the executive director of the Point Reyes Bird Observatory. "Their absence has tremendous implications for everything out there, right up to the humpback and blue whales. We don't know if this is a result of global warming or some natural cycling, but without the krill, you could be looking at a food web collapse."

E-mail Glen Martin at glenmartin@sfchronicle.com.

Page A - 1

Global warming makes sea less salty

Researchers predict effects on 'conveyor belt' of ocean currents

Updated: 6:15 p.m. ET June 29, 2005

You won't want to drink water straight from the ocean anytime soon. But the salt content is on the decline, a sign of potentially worrisome consequences that scientists can't accurately predict.

Since the late 1960s, much of the North Atlantic Ocean has become less salty, in part due to increases in fresh water runoff induced by global warming, scientists say.  Now for the first time researchers have quantified this fresh water influx, allowing them to predict the long-term effects on a "conveyor belt" of ocean currents.

Climate changes in the Northern Hemisphere have melted glaciers and brought more rain, dumping more fresh water into the oceans, according to the analysis.

One of the expected high-profile consequences is a rising sea that will swamp coastal communities. But there are other possible effects.

"Precipitation and river runoff at high latitudes have been increasing," said Ruth Curry of the Woods Hole Oceanographic Institution (WHOI).  "In the last decade, fresh water has been accumulating in the Nordic Seas layer (the upper 1,000 meters) that is critical to the ocean conveyor, so it is something to watch."

What's going on
Curry and Cecilie Mauritzen of the Norwegian Meteorological Institute calculated that an extra 19,000 cubic kilometers of water flowed into and diluted the northern seas between 1965 and 1995.

For comparison, the Mississippi River releases about 500 cubic kilometers of freshwater into the Gulf of Mexico each year, while the Amazon, the Earth's largest river, discharges roughly 5,000 cubic kilometers annually. 

Because water with lower salinity is less dense, adding fresh water may affect ocean flows like the conveyor belt – a system of Atlantic currents that exchanges cold water in the Arctic region for warm water from the tropics.

The top part of this conveyor is made of warm ocean currents, like the Gulf Stream, flowing northward along the surface.  At high latitudes, this water cools and sinks – releasing its heat to the atmosphere and making for moderate winter climates in places like England.  

Deep, cold currents return some of the water to the south.

Slight changes in the currents -- both seasonal and longer-term variations -- affect everything from hurricane formation to droughts and heat waves.

Future uncertain
No significant change in the conveyor belt has yet been observed, however.  Curry and Mauritzen estimate that it would take another century to slow the ocean exchanges if the current rate of fresh water inflow continues.

Scientists disagree over whether the planet is warming and how much humans might be contributing. But most climate experts see a clear warming trend that they expect will continue for at least a century.

"Given the projected 21st Century rise in greenhouse gas concentrations and increased fresh water input to the high-latitude ocean, we cannot rule out a significant slowing of the Atlantic conveyor in the next 100 years," Curry said.

She emphasized, however, that effects will be gradual.  "We are not suggesting that the Gulf Stream will shut down," she said.

A study last year concluded that an altered conveyor belt could actually plunge the planet into a global cooling event.

The new research was published in the June 17 issue of the journal Science.

© 2005 LiveScience.com. All rights reserved.

Clue to sudden climate change found in Arctic: study

Climate could change extremely rapidly with very dramatic effects

June 28, 2005

The sudden deep freeze of the northern hemisphere that occurred 13,000 years ago has been traced to events originating in northern Canada, according to University of Toronto research. The findings could shed light on the future of climate change due to greenhouse gases.

The study, published in the June 2 issue of Nature, pinpoints the exact location where freshwater generated by the melting of the massive Canada-wide Laurentide ice sheet entered the global ocean and caused the Younger Dryas cold reversal, a frigid period where the planet temporarily plunged into ice age conditions. Contrary to previous thinking, the study shows that this meltwater entered the Arctic Ocean rather than the Atlantic and the point of entry was through the MacKenzie River. As the freshwater - lighter due to its lack of salt content - flowed into the ocean it was transported across the pole into the North Atlantic where it shut down the process whereby heavy surface water sinks into the abyss and leads to a warming of the northern hemisphere.

While the Younger Dryas cold reversal occurred just as the Earth was emerging from the most recent ice age, a rapid meltback of the Greenland ice sheet - another large accumulation of land ice adjacent to the North Atlantic Ocean - could theoretically contribute to another such shutdown.

"Greenland contains enough ice to raise sea level by about seven metres if it were all to melt," says the study's co-author University Professor Richard Peltier of U of T's Department of Physics. "If it were to melt very quickly we could easily have a similar event, so the question is just how Greenland will react to the ongoing warming due to the increasing concentration of atmospheric greenhouses gases. How probable this is remains an open question."

To pinpoint the location of where the Younger Dryas event occurred, Peltier and his co-author, physic's research associate Lev Tarasov, used the University of Toronto Glacial Systems Model (GSM) - a model that produces a three-dimensional view of the evolving ice-sheet as it expands and contracts over the North American continent in response to climate variations. The model also analyses how the shape of the Earth is affected by the evolution of the heavy ice loads. As the continental ice melted, a huge amount of deglaciation derived freshwater was added to the oceans. At the time of Younger Dryas onset the routing of this meltwater was into the Arctic Ocean.

"In considering the issue of climate change, many people imagine that this could only happen very gradually," says Peltier. "This event shows that our climate could change extremely rapidly and with very dramatic effect."

Peltier stresses that climate changes, such as a massive Greenland melt, are very difficult to predict as Earth's climate system is highly non-linear, involving the interactions between a number of distinct and individually complex components such as sea ice and land surface processes as well as the atmosphere and oceans. "These systems are capable of responding in a way that is out of proportion to the stimulus," he says. "You can push them just a little bit and cause them to cross a threshold, such that the response is extremely surprising. From a physics standpoint, the climate system of the planet is a beautiful example of such non-linear systems."

The research was supported by the Natural Sciences and Engineering Research Council of Canada and by the Canadian Foundation for Climate and Atmospheric Science through a new collaborative research network called Polar Climate Stability which is led by Peltier. The network involves researchers from seven different Canadian universities.

Source: University of Toronto