Is the earth's climate is "tippy?" Recent events -- in geological time -- suggest that the Earth's climate is not very stable, and that highly significant temperature fluctuations can take place over very short periods. Abrupt changes in thermal stratification, convection and the patterns of ocean currents are thought to play an important role.
Billionaire Opens Deep Pockets For Climate-Theory Research
Lands' End Founder Throws Millions Into Hunt for Data Showing Cataclysmic Shifts
By ANTONIO REGALADO
Staff Reporter of THE WALL STREET JOURNAL
In May, billionaire Gary Comer and four climate experts boarded his Cessna Caravan and took off in search of a catastrophe.
Flying low over southwestern Ontario, the group scanned the ground for boulders left behind by an ancient flood. The deluge, involving 2,000 cubic miles of fresh water from a prehistoric lake nearby, sent temperatures over the North Atlantic plummeting about 12,700 years ago, according to a theory advanced by scientists on the flight.
The cataclysm -- triggered by the melting of glaciers at the close of the last ice age -- poses an urgent question for the present: Could global warming also set off unexpected and extreme climate shifts, such as substantial regional drops in temperature or long droughts?
Some scientists think it's a possibility, and now their research is getting a major boost from Mr. Comer, 75 years old. The founder and former chairman of Lands' End Inc. sold the company to Sears, Roebuck & Co. last year, pocketing just over half the proceeds from the $1.9 billion cash deal. Since witnessing unusual ice conditions on an Arctic cruise, Mr. Comer has started handing out millions of dollars to researchers trying to document so-called abrupt climate change.
The idea is that the Earth's climate can sometimes behave more like a switch than a dial, jumping in a matter of years between dramatically different conditions. At the time of the big flood in Ontario, temperatures in Greenland dropped by 18 degrees Fahrenheit. The flood also probably upset ocean currents and changed rainfall patterns as far away as the Asian monsoon.
Abrupt climate change is a wild card in the divisive debate over the causes of global warming. For many, the chief culprits are so-called greenhouse gases formed by the burning of fossil fuels, such as oil and coal. These gases are thought to be insulating the planet like a blanket, causing temperatures to rise. A United Nations report predicts that average temperatures will increase 2.5 degrees to 10.4 degrees by 2100, throwing Arctic ecosystems into turmoil and threatening coastal communities with rising sea levels as glaciers melt and warming oceans expand. (Russia may hold the key to ratification of the Kyoto Protocol, the global-warming treaty, which the U.S. has abandoned.)
While there is broad consensus among scientists that global temperatures are rising because of fossil-fuel use, the extent and consequences of the warming remain uncertain. Such doubts now form the basis of the Bush administration's climate policy, which opposes costly reductions in emissions of carbon dioxide and other greenhouse gases.
For some scientists concerned about the warming, abrupt climate change has become a rallying point. Not only does the theory offer worst-case scenarios, it co-opts one of the arguments favored by skeptics of global warming -- namely that scientists aren't certain about how the climate works.
"What concerns me and a lot of people is that we are provoking a system about which we lack a total understanding," says Wallace S. Broecker, a geochemist at Columbia University who was among the first to outline the abrupt-change theory, in the mid-1980s. A feisty 71-year-old with a reputation for big ideas and for challenging fellow scientists, Dr. Broecker has become Mr. Comer's closest adviser.
Wider Audience
The evidence for sudden climate swings is beginning to find a wider audience. Last January, Robert Gagosian, director of the Woods Hole Oceanographic Institution, on Cape Cod, told the World Economic Forum at its meeting in Davos, Switzerland, that abrupt change could have the perverse effect of lowering temperatures in industrialized parts of the globe. A Senate bill would allocate $60 million to research on ancient ice and mud, and the Bush administration plans to highlight abrupt change in a major new strategic plan for climate-change research, due out this month.
Archaeologists have linked the collapse of several civilizations to large climate changes. A long dry spell may have caused the decline of the Akkadian empire in Mesopotamia around 4,200 years ago. Researchers have unearthed a 180-kilometer-long wall built by a later kingdom to keep out refugees from newly arid regions.
Hollywood is also taking note. News Corp.'s 20th Century Fox is in post-production for "The Day After Tomorrow," a big-budget movie in which global warming sets off a new ice age and Dennis Quaid plays a paleoclimatologist who battles encroaching glaciers. A studio description says the film "revolves around an abrupt climate change that has cataclysmic consequences for the planet."
Critics of such notions -- and there are plenty -- say the yo-yoing of the climate over the millennia simply shows that man's influence may be grossly overestimated. They add that Mr. Comer isn't the first big donor to hand over money to scientists peddling an alarmist message.
"Anyone who studies weather knows that it is variable, but suddenly it is being treated as a boogeyman," says Richard Lindzen, an atmosphere expert at the Massachusetts Institute of Technology. He notes that the biggest shifts, such as the one that occurred 12,700 years ago, happened under ice-age conditions, when mile-thick ice sheets dominated climate processes.
Mr. Comer grew up on the South Side of Chicago, where his father was a railroad conductor, and worked for a time as a copy writer at Young & Rubicam. After quitting to travel to Europe, he decided to turn his hobby of competitive sailing into a business and founded Lands' End. The small mail-order operation grew to employ more than 6,000 people, but battles with his board made the job increasingly unpleasant, Mr. Comer says. A down-to-earth man who drives a six-year-old Lincoln Towncar and plays down his wealth, Mr. Comer concedes that with the gas-guzzling auto, in addition to his fleet of airplanes and boats, his lifestyle is responsible for prodigious amounts of carbon-dioxide emissions. But he doesn't see personal change as the solution.
The former executive brings a degree of political independence to the climate debate. He says he made campaign donations to Bill Bradley and John McCain in the 2000 election, but couldn't bring himself to vote for either of the big-party candidates. He says that prior to his Arctic cruise, he had never given much thought to global warming.
When Mr. Comer steered his 150-foot yacht Turmoil toward the Northwest Passage two summers ago, the crew expected to be blocked by sea ice. Instead, the ship slipped easily through open waters. An experienced Arctic traveler on board said the ice conditions were the mildest he had ever seen. The Turmoil was just the 94th ship to make the transit from the Atlantic to the Pacific through the Arctic islands of Canada since Roald Amundsen first did so in 1905.
"It's obvious something is happening. But no one is really interested in doing anything about it," Mr. Comer said recently over a diner breakfast of bacon and eggs.
After he returned from the Northwest Passage to his home outside Chicago, he typed "global warming" into the Google search engine. A fan of Tom Clancy and Joseph Conrad novels, he had read of 19th-century explorers who died in the passage, and he thought his own trip had been too easy. On the Internet, he found a debate between environmentalists and energy interests -- "one predicting the end of the world and the other saying nothing is happening," he says.
Mr. Comer initially considered launching a Web site of his own to counter the energy industry's arguments, but he decided it would get lost in the noise. Instead, he called the Woods Hole Oceanographic Institution.
"I don't want to go out and tilt at windmills and waste my time, so I have focused on the scientists to help them do their job," he says.
Mr. Comer wanted a splashy news conference, but Woods Hole, the world's largest independent ocean-research center, was more interested in collecting data than in setting off political fireworks. A Woods Hole oceanographer named William Curry came to Chicago and explained to Mr. Comer that researchers weren't sure whether there was actually less ice or if it was being moved elsewhere by wind. Soon the conversation turned to speculation. If the polar ice melted, Dr. Curry said, it could cause abrupt climate change.
The scenario he laid out goes like this: Increasing rainfall and melting ice caused by global warming could lead to a buildup of fresh water in the North Atlantic. That influx could shut down circulating ocean currents that normally draw warm salty water from the tropics along with vast amounts of heat.
Stopping those currents might disrupt the redistribution of heat around the globe. In fact, there is evidence that Atlantic currents may already be under pressure. A few months after the Chicago meeting, British scientists writing in the journal Nature showed that salinity has dropped measurably in the North Atlantic during the past 40 years. The Woods Hole graphics department turned the data into an interactive program that Dr. Curry e-mailed to Mr. Comer.
Shortly afterward, Mr. Comer agreed to give Woods Hole $1 million to seed a program that would place buoys in the Atlantic to monitor changes in salinity, temperatures and ocean currents. According to an internal Woods Hole funding document, Mr. Comer's money came with the proviso that he wanted the research "kicked into high gear."
Paleoclimatic research has exploded in the past several years, thanks to data found in ice cores, tree rings, coral and ocean sediment. The abrupt changes are the most striking feature of that data, but the ocean-currents theory is just one explanation. The atmosphere plays a much bigger role in climate, and many scientists expect tropical air to contain the mechanisms of abrupt change.
Reaching Out
Mr. Comer had been reaching out to other top scientists. He had written to Dr. Broecker at Columbia University, saying he was looking for ways to "make a difference" where he felt the government wasn't. A friend also put Mr. Comer in touch with F. Sherwood Rowland, a professor at the University of California at Irvine, who had shared a Nobel Prize for showing that chlorofluorocarbon gases used in spray bottles and refrigerators could deplete the ozone layer, an important shield against solar radiation. The chemicals were later banned when a huge hole in the ozone layer was detected over the Antarctic.
In May 2002, Dr. Rowland and his wife, Joan, flew to Victoria, British Columbia, for a cruise on the Turmoil. Mr. Comer joined them after closing the sale of his company to Sears. Privately, scientists hope he will provide much more funding than he has. But Mr. Comer, who has also given $40 million for a new children's hospital in Chicago that will bear his name, sees his role as seeding research, not carrying it across the finish line. "The government has really got to step in," he says.
Dr. Rowland and Mr. Comer were chatting on the bridge when the billionaire asked, "If I wanted to put $1 million into climate-change research, what should I do?" Dr. Rowland says he had a quick answer: provide 10 two-year fellowships to newly minted Ph.D.s recruited into climate-change science. "One to work with me, and another nine to other scientists I could pick out."
The program soon rose to $6.9 million for 23 research groups, as Mr. Comer huddled several weeks later with Drs. Rowland and Broecker in New York. They gave $300,000 to an expert developing new ice-dating techniques, and an equal sum to Lonnie Thompson, an Ohio State University researcher known as the "Indiana Jones of paleoclimatology," who scales mountains in Latin America in search of rare tropical glaciers.
Last month, Maine Sen. Susan Collins introduced the Abrupt Climate Change Research Act of 2003, a bill that would give the National Oceanic and Atmospheric Administration $60 million in additional funds to implement a major study of ancient climate records. Sen. Collins, a Republican, has parted ways with the Bush administration by calling for a reduction in greenhouse-gas emissions from power plants to 1990 levels.
The administration has opposed mandating limits, arguing that the economic costs aren't justified by available science. The wait-and-see policy assumes that if warming occurs, it will do so gradually over the next century, leaving time to invent new energy sources or to simply adapt.
That assumption could be wrong. In a 2002 report titled "Abrupt Climate Change: Inevitable Surprises," the National Academy of Sciences in Washington concluded that sudden regional climate shifts could be triggered by human activities.
That possibility is starting to influence policy discussions, which have until now focused largely on the threat of steady warming. This month, the Bush administration is expected to release a major report outlining a new national research strategy for climate change. According to Mr. Bush's science adviser, John Marburger, abrupt climate change is identified as a "priority area" in the report, which he has seen. "It is clearly one of the things that needs to be looked at in the short term," says Dr. Marburger.
Before Mr. Comer set out on the expedition to Ontario in May, he had his Dassault Falcon jet collect Dr. Broecker and other members of the team at Chicago's Midway Airport. They gathered for a day of meetings at his Wisconsin home, and later watched the sunset from a five-story, glass-enclosed tower that soars above the estate.
During the three-day field trip, the group couldn't locate the path of the ancient flood. A chagrined University of Manitoba geologist named James Teller explained that he had predicted the flow using topographical maps, as he had never had enough funds or reason to rent a plane. Now Mr. Comer has sent out invitations for a new expedition in September. He thinks the water went north, into Hudson Bay.
Rapid Climate Change
New evidence shows that earth's climate can change dramatically in only a decade. Could greenhouse gases flip that switch?
Kendrick Taylor
This article was published in the July-August 1999 issue of American Scientist.
Over the course of geologic history, the earth's environment has been far from static. Indeed, 600 million years ago the atmosphere lacked sufficient oxygen to support animal life. More recently, as shown by sediments recording conditions over the past 500,000 years, the planet's climate varied between at least two different states.
The record from the past 150,000 years is particularly well preserved, offering details about these repeated climate changes. Between about 131,000 and 114,000 years ago there was a warm period like today's climate, referred to in Europe as the Eemian or globally as Marine Isotope Stage 5e. This was followed by the Wisconsin ice age, which ended about 12,000 years ago when the current relatively warm Holocene period began.
Although the past half-million years constitutes the current-events period in geologic time, on a human time scale the events I just described are in the distant past. Because their time scales are so long, I used to believe that changes in climate happened slowly and would never affect me. After all, a single climate cycle that includes an ice age and a warm period lasts 150,000 years and is controlled by gradually changing orbital parameters of the earth. It did not seem possible that climate cycles that lasted so long could change perceptibly during my lifetime. Even greenhouse-induced climate changes are normally predicted to happen gradually over several generations, allowing an opportunity for society to adapt.
My attitude changed profoundly while I was working on a project funded by the National Science Foundation to develop a climate record for the past 110,000 years. By examining ice cores from Greenland, my colleagues and I determined that climate changes large enough to have extensive impacts on our society have occurred in less than 10 years. Now I know that our climate could change significantly in my lifetime. We are still a long way from being able to predict such a change, but we are getting closer to understanding how it might take place. A pressing concern is whether anthropogenic changes to our planet's atmosphere might perturb the climate's stability.
Ice, the Museum of Climate
One can learn a lot about what controls climate by studying glacial ice. When snow falls, it collects insoluble dust particles, soluble compounds and the water in the snow itself. In some places more snow falls in a year than melts or sublimates away. Annual layers of snow pile up, with atmospheric gases filling the open pores between snow crystals. The weight of accumulating snow compresses the pores in the snow below, turning the snow into ice and trapping the atmospheric gases. The dust, chemicals and gases in the ice reflect the environment along the water's journey from distant sources to the glacier. They record how cold it was, how much snow fell in a year, what the concentration of atmospheric gases was and what the atmospheric circulation patterns were.
We can identify annual layers in the ice because the concentration of sea salts, nitrate and mineral dust and the gas content in winter snow are different than in summer snow. We count the annual layers to determine the age of the ice, and by measuring the thickness of the annual layers we can determine how much snow fell each year. The gas trapped between ice crystals offers a sample of the ancient atmosphere, and we can use it to determine what the concentrations of greenhouse gases such as carbon dioxide and methane were long before human beings measured the atmosphere directly. General patterns of atmospheric circulation can be reconstructed by using tracers such as soluble chemicals (for example, nitrate, ammonium, sodium and calcium) and rare earth elements in insoluble dust particles to determine how wind moved air and dust from the source regions for these compounds to the drilling site.
Ice as Thermometer
Air temperature is naturally of primary interest when we talk about climate, and fortunately we have three ways to determine what it was in the past. First, we can measure the isotopic composition of the oxygen and hydrogen in the ice. When water vapor in clouds condenses, the ratio of oxygen-18 to oxygen-16 and the hydrogen-2/hydrogen-1 ratio are affected by the ambient temperature; the colder the cloud, the lower the ratio. Measuring how the ratios of these isotopes changes along an ice core gives us a good idea how the air temperature changed over time.
The second way to determine prehistoric temperatures is to measure the isotopic composition of the nitrogen gas trapped in the ice. At depths between about 5 and 50 meters in an ice sheet, air can move in interconnected pores but is sheltered from mixing by the wind. Nitrogen-15 slowly moves toward colder locations, and nitrogen-14 slowly moves toward warmer locations. This process creates a near-surface gradient in the nitrogen-15/nitrogen-14 ratio that depends on the near-surface temperature gradient. The resulting isotopic composition of the nitrogen trapped in the ice depends on the difference between the surface temperature and the temperature at depth at the time when the ice overburden pressure closes the pores and traps the nitrogen gas in the ice. Variations in the isotopic composition of the nitrogen along a core show when and by how much the surface temperature changed.
Finally, because of the large thermal inertia of an ice sheet, the current temperature distribution in an ice sheet is strongly influenced by what the surface temperature was in the past. The physics is similar to cooking a large frozen turkey. If we move the turkey directly from the freezer into the oven, the outside of the turkey will be done before the inside even defrosts. By modeling the current thermal state of the turkey, or an ice sheet, we can determine the history of the turkey's, or ice sheet's, surface temperature. The physics of these three approaches is well understood; together they allow us to reconstruct how the surface temperature changed during the past several hundred thousand years.
The Greenland Weather Report
In Greenland, annual ice layers are stacked up like thousands of annual weather reports. In 1982, a European and American team made the first attempt to read that record, by recovering an ice core from southern Greenland. Measurements on the ice core indicated that about 11,700 years ago the climate of the North Atlantic region changed from a dry and cold ice age to the current warmer and wetter Holocene. Altogether it took 1,500 years for the climate transition to be complete and a few thousand more years to melt most of the ice, but the surprise was that most of the transition occurred in only 40 years. This was only one record, and it came from a single 10-centimeter-diameter ice core. Still, this finding was impossible to ignore and too puzzling to comprehend.
In 1993, Americans and Europeans led by Paul Mayewski of the University of New Hampshire and Bernhard Stauffer of the University of Bern in Switzerland finished recovering two new ice cores from the summit of the Greenland ice sheet. More than 40 university and national laboratories participated in the projects. We shared samples, spent time in one another's labs, replicated one another's results, proposed ideas, tore them apart and then jointly proposed better ones. One of the justifications for these new cores, located 30 kilometers apart, was to verify and learn more about the 40-year change in climate, an event observed in both cores. The records stored in these cores were more detailed than before and showed that within a 20-year period at the summit of Greenland, where ice is thickest, the amount of snow deposited each year doubled, average annual surface temperature increased by 5 to 10 degrees Celsius and wind speeds increased. The same ice cores also showed that the spatial extent of sea ice decreased, atmospheric-circulation patterns changed, and the size of the world's wetlands increased. Many of these shifts in parameters, including at least a 4-degree Celsius increase in the average annual air temperature, happened in less than 10 years. These changes were not restricted to Greenland; the global nature of many of these ice-core records showed that low-latitude, continental-scale regions rapidly got warmer and wetter. The most dramatic change occurred 11,700 years ago. But we also found comparable anomalies every several thousand years during the Wisconsin ice age (see Figure 6). Further, Antarctic ice cores also show comparable climate transitions at these times.
Climate, from the Bottom Down
One can also learn a lot about what controls climate by studying sediments on the ocean floor. These sediments contain the decayed remains of ocean organisms and inorganic material from the erosion of rocks. Ocean organisms assimilate chemical compounds from the water as they grow, and the compounds they incorporate are partially determined by the environment in which they live. Thus the decayed remains of the organisms that fall to the ocean floor contain a record of what chemical compounds were available and the temperature of the water in which they lived.
For example, consider an ocean-sediment core collected at Bermuda Rise, a place where ocean currents deposit a lot of sediment. The oxygen-18/oxygen-16 ratio of seawater varies through time depending on how much water is locked in ice sheets and how much water is in the ocean (see Figure 7). The near surface�dwelling foraminiferan Globigerinoides ruber uses seawater to make its shell. By measuring the oxygen isotopic composition of the shells recovered from an ocean core, we can determine how much water was locked up in ice sheets when the foraminiferan was living. Likewise, the bottom-dwelling foraminiferan Nutallides umbonifera incorporates cadmium and calcium in its shell. By measuring the ratio of cadmium to calcium in the shells recovered from an ocean core, we can tell where the bottom water came from when the foraminiferan was living. High values of the cadmium-to-calcium ratio indicate that the water near the bottom came to the Bermuda Rise from the south, whereas a low ratio indicates that the bottom water came from the north.
Ocean sediments also contain ground-up rock, which is transported and deposited by ocean currents, just as wind carries airborne dust to be deposited on ice sheets. The mineralogy of the ground-up rock can be used to identify where it came from. For example, a layer of hematite-rich sediments in ocean cores near Bermuda indicates that ocean currents were transporting material from the east coast of Canada to Bermuda when the sediments in the layer were deposited.
To determine what the temperature of the ocean surface was in the past we can use organic compounds made by phytoplankton. Phytoplankton live near the ocean surface where there is light for photosynthesis. Some phytoplankton produce compounds know as alkenones, which are straight chains of carbon atoms. Along these chains of carbon there can be two or three double bonds. The number of double bonds depends on the water temperature. The double bonds are thought to keep the cell membrane pliable in cold water. When the phytoplankton die, the alkenones fall to the bottom and become incorporated into the sediment. By measuring the ratio of different types of alkenones we can determine what the surface water temperature was when the phytoplankton were living.
By collecting cores of the ocean sediments at different locations, we can determine a lot about how the ocean circulated water and heat in the past. The rapid climate changes recorded in the ice cores encouraged a search for ocean sediment records with high time resolution. In the past few years locations have been identified in the ocean where sediment accumulates rapidly, and the sediment cores from these locations have comparable time resolution to the ice cores. Coring projects off the coast of Bermuda by Konrad Hughen, Julian Sachs and Scott Lehman with the University of Colorado, in conjunction with Lloyd Keigwin of Woods Hole Oceanographic Institution and Ed Boyle of the Massachusetts Institute of Technology, found the same rapid changes in climate as were recorded in the ice cores. Other groups have found similar records near Santa Barbara, California and off the coast of India.
Paleoclimatic evidence worldwide shows that a global change in climate took place 11,700 years ago, and in the North Atlantic a large part of the change took less than 20 years. It was a few thousand years before the completion of the transition from ice age to warm period; still, in just a 20-year period the climate of a large part of the earth changed significantly. There was no warning. A threshold was crossed, and the climate in much of the world shifted abruptly from cold to warm. This was not a small perturbation; our civilization has never experienced a climate change of this magnitude or speed. To get an idea of what happened, imagine that over a 20-year period the weather at your home became that typical of a place 400 to 600 miles farther south. What might be the mechanism for so rapid and large a climate change?
Climate's Control Mechanism
Like the atmosphere, the oceans are far from static. Currents, of which the Caribbean-Atlantic Gulf Stream is just a small part, continually exchange water among all the oceans and between the surface and the depths. For the sake of convenience, we shall start this journey in the Gulf Stream, where water moves northward along the East Coast of the U.S. toward Iceland. Along the way, the water exchanges heat with the air, warming the air and cooling the water in the process. Water evaporates from the surface and leaves behind dissolved salt. The combination of chilling and evaporation makes surface water denser as it moves north. In the vicinity of Iceland, the surface water becomes denser than the water below it and sinks. This dense, cold water then moves south along the bottom of the Atlantic, around the Horn of Africa and, still near the bottom, continues to the North Pacific, where it upwells to the surface. Surface water in the North Pacific makes room for the upwelling bottom water by moving south, passing between Asia and Australia and finally catching the tail of the circulation pattern at the beginning of the Gulf Stream in the Atlantic off Central America (see Figure 8). For most of its journey, the surface water collects heat and freshwater, which makes the surface water more buoyant than the water underneath it. But in the North Atlantic, the combination of cold temperatures and evaporation makes the water dense again and it sinks.
Wally Broecker of Columbia University likens this circulation pattern to a long conveyor belt that moves water, salt and heat. He was among the first to recognize that alterations in the path of the ocean conveyor belt would change climate in much the same way that turning off the furnace fan changes the temperature distribution in a house. He proposed that the large oscillations in climate observed in the geologic record were caused by different patterns of ocean circulation.
Three Climate Modes
Because the oceans must abide by the constraints of geography and the laws of physics, there are only a few patterns in which the oceans can circulate. At a recent conference organized by Robert Webb of NOAA and Peter Clark of Oregon State University, climate scientists identified three modes of ocean circulation, each of which is associated with a different climate. The current mode produces the warmest conditions in the North Atlantic. Surface water sinks in two regions of the North Atlantic, and a large volume of surface water and heat is drawn from the tropics to replace the sinking North Atlantic water. The heat carried north by the northward-moving surface water warms eastern North America, the North Atlantic and most of Europe. The second mode of ocean circulation occurs when surface water sinks in only one area of the North Atlantic. Less surface water sinks to the bottom, so smaller amounts of warm surface water and heat are drawn north to replace the sinking water. This mode was in place during the warmest times of the Wisconsin ice age, when climate was only slightly colder than current conditions. In the coldest mode, no water sinks in the North Atlantic; hence no warm water is drawn north. This was the condition during the coldest portions of the Wisconsin ice age.
Each of these modes of ocean circulation is associated with a small range of prevailing environmental conditions. Weather anomalies such as 10-year-long droughts or wet periods, as significant as they may seem to human affairs, are a reflection of the small range of environmental conditions associated with a single mode of ocean circulation. If, however, environmental conditions are externally forced to be inconsistent with the existing mode of ocean circulation, the circulation will switch to a mode that is more consistent with those environmental conditions. An example of this forcing would be a change in the amount of solar heat reaching, and retained at, the earth's surface. Such a change could be the result of alterations either in solar output or in the way the atmosphere regulates the exchange of heat between the earth's surface and space. The transitions between different modes of ocean circulation are abrupt. The ocean-sediment cores and ice cores tell us that they frequently take only several decades or less.
Numerical models of ocean circulation developed by Thomas Stocker of the University of Bern and Syukuro Manabe of Princeton University show that each circulation mode is stable for a particular range of environmental conditions. For example, if the discharge of a river changes, altering the density of the surface water in the adjoining ocean, the ocean-circulation pattern will change only if it is unstable under this new set of conditions. As long as the climate system stays within the stable-mode range, river discharge and greenhouse-gas concentration can vary without having much influence on climate.
Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research has used numerical models to show how surprisingly sensitive ocean circulation can be to changes in freshwater discharge. His numerical models show that if the climate system is near the threshold between stable modes, a small change in the amount of freshwater entering the North Atlantic will force a large and rapid shift to a different ocean-circulation pattern. Like a coin on edge, which topples with only a breath of air, an unstable pattern quickly assumes a new position where it becomes quite stable. The climate changes recorded by the ice and ocean-sediment cores appear to have taken place when some crucial threshold was crossed, resulting in large and rapid switches�in geologic time, like the flip of a switch�in ocean circulation.
Unfortunately, no one knows yet what caused this switch to flip. We know of external forcing mechanisms, but their time periods do not match the record. For example, the distribution of solar energy reaching the earth varies according to the relative positions of the sun and earth. These variations, called Milankovitch cycles, have periods of tens of thousands of years and are thus too slow to explain the rapid changes seen every couple of thousand years during the Wisconsin ice age. The Milankovitch cycles define the big picture and determine when changes could occur, but some smaller, quicker-acting mechanism triggered the more frequent switches during the Wisconsin.
The leading idea, which has been simulated in computer models, is that increased discharges of freshwater glacial ice and river runoff into the North Atlantic reduced the density of the surface water enough that it could not sink. This slowed the ocean conveyor, forcing it to switch to another circulation pattern. Other emerging concepts place the source of the disruption of the conveyor in the tropical Pacific. Variability in the sun's output is another possible cause of the climate variations, but the record of solar output is not good enough to adequately investigate this idea. Furthermore, the dynamics of ocean circulation around Antarctica are too poorly understood to completely exclude the possibility that they may play a role. Whatever the cause may be, it is worrisome that the phenomenon that has repeatedly triggered major changes in ocean circulation and the earth's climate is so subtle that we have not been able to identify it. This emphasizes how large changes in the interaction of the oceans, atmosphere and ice sheets have been triggered by small perturbations of the environment.
Tampering with Our Stable Mode?
Human beings have made major modifications to the earth's environment in little more than a century, increasing the concentration of carbon dioxide in the atmosphere to its highest level in 260,000 years. Numerical models can be used to estimate what will happen when anthropogenic increases in the atmospheric concentration of gases such as carbon dioxide and methane block heat from leaving the earth. The increased concentration of these gases acts like a greenhouse, and the average temperature of the earth gets warmer. But the numerical models of Stocker, Rahmstorf and others suggest there may be surprises in the greenhouse.
When the greenhouse effect warms the earth, it accelerates the hydrologic cycle, more water moves around in the atmosphere, and rainfall increases in many places. Some models suggest that this will slowly decrease the salinity of the North Atlantic, making the surface water less dense. Were a critical density threshold to be crossed, ocean circulation would abruptly switch to a new stable mode.
This would be more than just a switch in ocean circulation; it would be a switch in the way tropical heat is transported to the North Atlantic. At the least, Northern Europe and Scandinavia would be 2 to 5 degrees colder on average than they are now, and the amount of precipitation would decrease dramatically. It would not necessarily be a rapid return to an ice age, but it might be a start in that direction. The orbital parameters of the earth are such that we are due for another ice age, and a cooling in the north Atlantic at a time when orbital parameters favor a return to a much colder climate could be the trigger that initiates such a change.
A switch in climate from a warm period (like the current Holocene epoch) to an ice age has happened before. Ocean and lake cores tell us that the warm Eemian period from about 131,000 to 114,000 years ago�when the distribution of ice sheets was similar to what it is today�switched to the Wisconsin ice age in no more than 400 years, the minimum time resolution of the record from these ancient sediments. Unfortunately, we have yet to recover an ice core that shows in sharp detail how the Eemian Period ended. This is old ice. It is difficult to find a place where it snowed enough to produce a high time-resolution record but not so much as to smear the record against the bedrock. An international project, led by Claus Hammer of the University of Copenhagen, has identified the most likely location in Greenland for this ice to be found and is collecting a core.
Many arctic ice cores tell us that 8,200 years ago the climate approached ice age conditions for a 400-year period before returning to conditions similar to today. This excursion was most likely caused by the one-time draining of lakes left behind by the melting of the Canadian ice sheets. This change in freshwater flux to the oceans was large but not that much different from what greenhouse-induced changes may produce in the future. The fact that it took place when climate conditions were similar to today demonstrates that large and rapid climate switches do not happen exclusively when there are extensive northern hemisphere ice sheets. It is ironic that greenhouse warming may lead to rapid cooling in eastern North America, Europe and Scandinavia, and it is possible that altered ocean circulation could lead to much larger changes. We have no experience predicting climate switches between stable modes, so it would be wise to expect surprises.
Climate and Choices
Climate is the result of the exchange of heat and mass between the land, ocean, atmosphere, ice sheets and space. As long as changes to the land, ocean, atmosphere and ice sheets stay below the thresholds I have just described, climate changes will happen slowly. But the climate will change rapidly if those thresholds are crossed. So rapidly that it would be impossible to rearrange agricultural practices quickly enough to avoid stressing world food supplies. So rapidly that many species would not be able to adapt, because their habitat, already greatly reduced by human activities, would be eradicated.
Human ingenuity would most likely allow us to adapt to a rapid change in climate, but we would pay a larger price than our civilization has ever known. Imagine the economic and social cost of moving, in a 20-year period, most of our agricultural activities 500 miles south of their current locations. Imagine the social cost and famine if agriculture could not be relocated quickly enough. Even a short-duration event such as the Dust Bowl years in the 1930s had a large influence on American society. The Little Ice Age, which caused major resettlement in Europe in the 15th and 16th centuries, is a more likely analogue of where we might be headed.
Some have proposed that we could counterbalance the greenhouse effect by manipulating the global exchanges of heat and mass. Methods that have been discussed include blocking the Strait of Gibraltar to change the salinity of the North Atlantic, using airplane-distributed particles or large orbiting sunshades to shade the earth, and fertilizing the ocean with iron to promote production of carbon dioxide-consuming biomass. But we have a poor record of managing even small ecosystems and lack a complete understanding of the ocean-atmosphere interactions that govern our climate. Intentionally manipulating climate would not only be costly and imprecise; it would also be impossible to benefit some regions without adversely effecting others. It would be a risky experiment on the only planet we can call home.
Although we do not know the critical level of greenhouse-gas concentration, we do know that reducing the rate of greenhouse emissions would help in two ways. First, the atmospheric concentration of greenhouse gases would increase more slowly. Second, numerical models by Thomas Stocker and Andreas Schmittner of the University of Bern and others predict that the climate threshold will occur at a higher concentration of greenhouse gases if the concentration of greenhouse gases increases slowly. Slowing the rate of greenhouse-gas emissions would buy us more time to understand the consequences of our actions and might allow us to increase greenhouse-gas concentrations to a higher level before reaching the critical threshold.
It is true that computer models are not perfect; they indicate general patterns. And we need to improve our understanding in many areas before the models can pinpoint thresholds. For example, our understanding of the details of ocean circulation is poor, and the physics of cloud formation and their influence on heat exchange is elusive. When we model previous switches in climate, we can compare the model to the results of real-world experiments recorded in ocean sediments and ice cores. But when we model the future, we have no empirical basis to judge the model's accuracy. If we take no action until we are completely confident the models are correct, then the only use for the models will be to explain what happened. Our insistence on a tested model is part of the reason society is continuing to conduct the largest experiment ever done, the experiment of increasing the atmospheric concentration of greenhouse gases.
It will be another 20 years before the climate changes that are predicted to be associated with the greenhouse effect become large enough to be unambiguously differentiated from naturally occurring variations in climate. As a society we have the choice of ignoring the warning signs that our studies have uncovered or taking some action.
I think we should spend the next 20 years aggressively investigating our options. We should continue to focus on improving our ability to predict climate change. At the same time, we should test the technologies and polices we might need to reduce greenhouse-gas emissions, implementing them on a small scale where there would be minimal economic and social disruption. I am not alone among scientists in anticipating that 20 years from now our society may have to choose between disruptions associated with our current approach to energy use and disruptions associated with adopting an approach to energy use that produces fewer greenhouse gases. Procrastination will prevent making an informed decision and will increase the social and economic costs.
Bibliography
Broecker, W. S. 1997. Thermohaline circulation, the Achilles heel of our climate system: Will man-made CO2 upset the current balance? Science 278:1592�1588.
Hughen, K. A., et al. 1996. Rapid climate changes in the tropical Atlantic region during the last deglaciation. Nature 380:51-57.
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Rahmstorf, S. 1997. Bifurcations of the Atlantic thermohaline circulation in response to changes in the hydrological cycle. Nature 378:145�149.
Stocker, T. F., and A. Schmittner. 1997. Influence of CO2 emission rates on the stability of the thermohaline circulation. Nature 388:862�865.
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American Geophysical Society Meeting, San Francisco, December 2001
Abrupt climate change likely
Report calls for research and policy to cope with volatile climate.
13 December 2001
TOM CLARKE
Humans increase the risk of sudden climatic surprises
Abrupt changes in global climate, common in the past, will become more so in future, thanks to man's impact on the environment, according to a new report presented at the American Geophysical Union meeting in San Francisco, California.
The report from the US National Research Council, to be published next year, calls for more research into rapid climate change, and for policy to equip societies to deal with it better1.
"Realization has been growing over the past decade that climate change is not always gradual," says Richard Alley of Pennsylvania State University in University Park, chair of the committee that produced the report.
For example, data increasingly suggest that about 11,500 years ago, during a period called the Younger Dryas, global temperatures fell by up to 16 degrees within a decade and rainfall halved. Things stayed that way for more that 1,000 years.
Studies of this and other swings indicate that climate is flexible up to a point, but that beyond certain thresholds, change can be rapid and long-lasting. It can be like turning the dial on a thermostat, says Alley. "But now we know that climate has switches as well as dials," he says.
And human influences increase the risk of sudden climatic surprises: "Although chaotic processes can flip the switch, forcing the climate increases the probability of abrupt climate change."
Slow-moving ocean currents that remove heat from the tropics and take them to the poles, and other ocean-driven cycles such as the Arctic Oscillation, could be key to future abrupt shifts in global temperatures, the report states. These processes are particularly important, being vulnerable to human-induced changes such as global warming.
Rain check
But future sudden climate changes may be of a different flavour from those of the Younger Dryas and its ilk. In colder phases such as the ice age, the Earth's climate is susceptible to radical swings in temperature. In warmer phases, such as today, rainfall is more likely to alter dramatically, argues Dorothy Peteet at the Lamont-Doherty Earth Observatory in Palisades, New York.
"Since the ice age there have been few rapid temperature switches, but instead rapid changes in precipitation," she says.
If and when it will get hotter or cooler, dryer or wetter is difficult to predict, says Alley. But abrupt climate change "has happened in the past and will happen again", he warns.
With that in mind, the report calls for new climate models to accommodate threshold-crossing. It also underlines the need for more research into past abrupt events and ways in which the oceans and the atmosphere interact with one another.
No climate model is ever perfect, so abrupt changes will never be predictable, argue some climatologists. "It's like being blindfolded and walking towards the edge of a cliff," according to Wallace Broecker, also at Lamont-Doherty.
Society should therefore be prepared for change, the report suggests. From ecosystem and water management to developing economic institutions that are more resilient to change, "we need to stay flexible", says Alley.
References
Abrupt Climate Change: Inevitable changes. National Academy Press, Washington DC (2002).
