Thursday, February 15, 2007
More on the subject - 2-15-2007
Global-Warming Report Gets U.S. Emphasis February 3, 2007
By JOHN J. FIALKA
WASHINGTON -- U.S. government scientists Friday said the long-term outlook for global warming may be more dire than suggested by this week's United Nations' report, which they say doesn't fully address the impact of clouds and melting glaciers.
Recent evidence of accelerated melting of glaciers in Greenland and the Antarctic ice cap came too late to be included in the report released Thursday by the U.N.-sponsored Intergovernmental Panel on Climate Change.
Glaciers are among the largest sources of fresh water in the world and are contributing to rising ocean levels. Rising sea levels could expose population centers bordering the ocean to more storm damage and could require evacuation in some areas. But the computer models used for the IPCC report based their predictions only on the results of heating of the existing water in the world's oceans, causing the oceans to expand and sea levels to rise, said Tom Delworth, a climate modeler for the National Oceanic and Atmospheric Administration, the government agency in charge of climate science and weather service.
The IPCC report predicts sea levels will rise by between one to two feet over the next 100 years. Mr. Delworth said there remains "much more uncertainty" over how much accelerated melting of glaciers might add to that.
A second area of continuing uncertainty has to do with the impact of clouds on climate change. Warming the ocean sends more water vapor into the air, and the resulting clouds accelerate global warming by trapping more of the sun's heat in the atmosphere and further warm the ocean. Jim Butler, deputy director of NOAA's global monitoring division, called this "a very scary feedback mechanism."
But, so far, the supercomputers the agency uses to model the effect on the earth's climate -- which were also used for the IPCC report -- aren't detailed or fast enough to predict how much clouds are accelerating the problem. Mr. Delworth said computer models divide the earth's oceans and atmosphere into four million boxes, each about 150 square miles, and that these boxes are too large to model the effects of clouds.
"We could use computers that are one million times faster than they are today and still not be satisfied," Mr. Delworth said.
Further complicating the issue are layers of haze containing pollutants from human activity. Such pollutants, including sulfates, soot, dust and nitrates, tend to make the atmosphere brighter, reflecting more of the sun's heat back into space. The IPCC has found that the net effect of the added pollution is to cool the atmosphere.
A.R. Ravishankara, an atmospheric chemist for NOAA, said this raises a problem for governments attempting to clean the air by removing pollutants. "If you take away this cooling effect, then the heating effect would be exacerbated. It's a highly complex problem."
Write to John J. Fialka at john.fialka@wsj.com
____________________________________________
Ancient Global Warming Flipped Ocean Circulation, May Do So Again Jan 5, 2006
SAN DIEGO, California (ENS) --> For the first time, evidence that global warming triggered a reversal in the circulation of deep ocean patterns around the world has been uncovered by scientists affiliated with the Scripps Institution of Oceanography. While the changes they describe occurred 55 million years ago, the scientists say today's conditions are similar and could have similar drastic effects on ocean circulation. In today's issue of the journal "Nature," scientists Fl via Nunes and Richard Norris describe how they examined a four to seven degree warming period that occurred some 55 million years ago during the closing stages of the Paleocene and the beginning of the Eocene eras.
"The Earth is a system that can change very rapidly," said Nunes. "Fifty-five million years ago, when the Earth was in a period of global warmth, ocean currents rapidly changed direction and this change did not reverse to original conditions for about 20,000 years."
The global warming of 55 million years ago, known as the Paleocene/Eocene Thermal Maximum (PETM), emerged in less than 5,000 years, an instant of geological time. Modern carbon dioxide input to the Earth's atmosphere from fossil fuel sources is approaching the same levels estimated for the PETM period, say the scientists, which raises concerns about future climate and changes in ocean circulation.
They say the Paleocene/Eocene example suggests that changes produced by human activities may have lasting effects not only on global climate, but on deep ocean circulation. Fossil records show that the global warming at the time of the PETM created changes ranging from a mass extinction of deep sea bottom dwelling marine life to migrations of terrestrial mammal species, as warm conditions may have opened travel routes frozen over when climates were colder. This time period is when scientists find the earliest evidence of horses and primates in North America and Europe.
Nunes and Norris base their findings on the chemical makeup of microscopic sea creatures that lived 55 million years ago. The scientists analyzed carbon isotopes, or chemical signatures, from the shells of the one-celled animals called foraminifera, or "forams," that exist in vast numbers in a variety of marine environments.
"A tiny shell from a sea creature living millions of years ago can tell us so much about past ocean conditions," said Nunes. "We know approximately what the temperature was at the bottom of the ocean. We also have a measure of the nutrient content of the water the creature lived in. And, when we have information from several locations, we can infer the direction of ocean currents."
In the study, the scientists looked at a foram named Nuttalides truempyi from 14 sites around the world in deep-sea sediment cores retrieved via the Integrated Ocean Drilling Program, for which Joint Oceanographic Institutions, Inc., manages the U.S. component.
Chemicals from the foram's shells were used as nutrient "tracers" to reconstruct changes in deep ocean circulation through the ancient time period. Nutrient levels tell the researchers how long a sample has been near or isolated from the sea surface, giving them a way to track the age and path of deep sea water. Nunes and Norris found that deep ocean circulation in the Southern Hemisphere abruptly stopped the conveyor belt-like process known as "overturning," in which cold and salty water in the depths exchanges with warm water on the surface. Even as it was shutting down in the south, overturning appars to have became active in the Northern Hemisphere. The researchers believe this shift drove unusually warm water into the deep sea, likely releasing stores of methane gas that led to further global warming and a massive die-off of deep sea marine life.
"Overturning is very sensitive to surface ocean temperatures and surface ocean salinity," said Norris, a professor of paleobiology in the Geosciences Research Division at Scripps. "The case described in this paper may be one of our best examples of global warming triggered by the massive release of greenhouse gases and therefore it gives us a perspective on what the long term impact is likely to be of today's greenhouse warming that humans are causing."
Overturning is a fundamental component of the global climate conditions we know today, said Bil Haq, program director in the National Science Foundation's division of ocean sciences, which funded the research. Haq says overturning in the modern North Atlantic Ocean is a primary means of drawing heat into the far north Atlantic and keeping temperatures in Europe relatively warmer than conditions in Canada. Today, deep water generation does not occur in the Pacific Ocean because of the large amount of freshwater input from the polar regions, which prevents North Pacific waters from becoming dense enough to sink to more than intermediate depths. But in the Paleocene/Eocene, deep-water formation was possible in the Pacific because of global warming, the researchers say, adding that the Atlantic Ocean also could have been a significant generator of deep waters during this period.
_____________________________________________
Warming climate will slow ocean circulation Feb 4, 2006
Later this century, rising concentrations of greenhouse gases in Earth's atmosphere will slow the ocean currents that bring warm waters to the North Atlantic, thereby affecting that region's climate, computer simulations suggest. When the waters of the Gulf Stream and other warm currents of the North Atlantic reach an area just south of Greenland, they cool, become denser, and sink. That, in turn, pulls more surface water northward, says Thomas L. Delworth, a climate scientist at Princeton University. The rate of this so-called thermohaline circulation depends on the temperature and salinity of the surface waters. The warmer and fresher those North Atlantic surface waters are, compared with underlying layers, the more buoyant they are and the slower the circulation becomes.
Using a new computer model, Delworth and his colleague Keith W. Dixon simulated various scenarios for ocean circulation in the North Atlantic from now until 2100. They calibrated the model using weather and ocean-circulation data gathered since 1860. Throughout the 20th century, rising concentrations of greenhouse gases such as carbon dioxide warmed the atmosphere and ocean surface, but not enough to slow the thermohaline circulation. That's because large amounts of air pollutants known as aerosols have scattered sunlight back into space and counteracted the greenhouse effect somewhat, says Delworth. In the remaining years of the 21st century, however, growing concentrations of greenhouse gases will begin to overwhelm the cooling effect of aerosols, Delworth and Dixon suggest. By the year 2040, thermohaline circulation could carry only 80 percent as much warm water to the North Atlantic as it does now. The researchers report their findings in the Jan. 28 Geophysical Research Letters.--S.P.
SEA CHANGE IN THE ATLANTIC OCEAN? Apr 26, 2004
By John Carey
One worry about global warming is that the increased concentration of greenhouse gases will upset earth's balance and bring changes in ocean circulation. In an extreme case, scientists say, the flow of warm currents up the Atlantic Ocean to Europe might be shut down. That would cause temperatures to plunge in Western Europe. Such a shift may be unlikely, but anxious researchers have been keeping a keen watch for any variations in ocean flow, using satellites and instruments moored out in the sea. Now, they are starting to spot some potentially worrisome changes. In the Apr. 15 online issue of Science, a NASA-University of Washington team reports that the counterclockwise circulation of surface water in the North Atlantic has become markedly weaker since the early 1990s. "These observations of rapid climate changes over one decade may merit some concern," the authors write. But they also caution that it's not yet clear if the shift in circulation is the result of man-made global warming or part of a natural cycle.
THE GULF STREAM:
PART 3 OF 3: The North Atlantic: hot spot for ice ages
Warm waters linked to glacial eras - By Anthony R. Wood Jan 19, 2006
In this understated harbor village of tight streets and Cape Cod houses, the North Atlantic stirs gentle breezes in summer and tempers New England's harsh winter cold. And yet, only a geologic blip ago, this was a frigid and forbidding place, encased in a mile-thick sheet of ice. Massive ice sheets have advanced and retreated repeatedly over aeons, at a glacial pace. But what researchers have discovered recently is that climate can change in a hurry. Their findings have led to an ultimate irony: In the debate over global warming, one of the hottest issues is ice.
The planet's temperature has warmed robustly in the last 20 years, with 2005 being the second-warmest year on record, and the Arctic polar cap is disappearing. The same melting that has raised concerns about rising sea levels has prompted counterintuitive scenarios that it could produce a fresh and disastrous big chill. Few foresee an imminent glacial outbreak, and some serious scientists insist that one is all but impossible, but ice-core records show clear evidence that rapid coolings and warmings have happened. And that was long before humans started burning the fossil fuels blamed for at least some of the modern warming.
Today, while the debate rages over how much humans are to blame for the planet's indisputable warming, scientists are still trying to figure out what conspired to bring on the flash-frozen ice ages. But a long and tortuous trail of evidence leads to a surprising suspect at the heart of the conspiracy: the Gulf Stream. Logically, it would be an unlikely culprit. It is hundreds of miles from the southern extent of the last ice sheet, and it covers only about 0.2 percent of the world's ocean surface. Yet the mighty stream is a critical piece of something much larger: the North Atlantic current system that moves warmth out of the tropics toward the North Pole and sends cold water back toward the equator, the so- called conveyor belt. It is estimated that the Gulf Stream transports about 20 percent of the heat moved by the oceans. If the Gulf Stream were to slow down or take a more southerly route, the change would disrupt the whole system, the North Atlantic would cool off. Europe and eastern North America might turn colder as the rest of the world heated up. Scientists think that's what happened the last time ice invaded Europe and the United States.
The question is: Could it happen again? In 1992, Richard Alley was in central Greenland, examining ice cores, when he saw something he could not believe. He and his colleagues were looking at the layers that told them about Greenland's temperature year by year, going back millennia. But instead of a gradual change, they saw radical shifts in the layers representing the climate 12,000 years ago. The temperatures had plunged and risen suddenly. He saw a swing of 15 degrees in a matter of 10, or no more than 30, years.
"This was a flipped switch, not a slowly turned dial," he recalls. "Something really dramatic had happened."
This made the Little Ice Age look like a snow flurry. Alley had come across a phenomenon described in 1985 by Wallace Broecker, a chemical oceanographer and paleontologist with Columbia University's Lamont-Doherty Observatory. Broecker called it the Younger Dryas period, for an Arctic shrub that mysteriously appeared throughout Europe. But whereas Broecker drew upon a variety of research sources, Alley was looking at direct physical evidence. The science of climate change was itself changing. Until the 1950s, climate was viewed as essentially a stable system, said Spencer Weart, head of the history center for the American Institute of Physics. That view was stood on its head when researchers saw evidence that big swings could occur in just a couple of millennia. By 1980, scientists came across further clues that such changes could happen in a few centuries. Broecker tightened the possible time frame in 1985 by publishing a paper on the Younger Dryas era. In the process, he indicted the North Atlantic and gave global warming an icon. The article, which appeared in the journal Natural History, posited that tundra conditions overspread Europe as the Gulf Stream and the North Atlantic heat-transport system broke down. Europe turned arctic.
That the North Atlantic would be so important underscores the complexity of oceanic circulation. The Pacific is triple its size, yet the Atlantic, Broecker explains, does a better job of moving heat northward than the wind- driven currents of the Pacific. And the mighty stream is the engine driving it. Critical ingredient A critical ingredient in the recipe for climate change is one of the most plentiful substances on the planet: salt. The key to keeping the conveyor belt in motion is the sinking action of the water. Salt adds weight to water, so the more saline it is, the better it sinks; the better it sinks, the faster the conveyor moves. Why is the Atlantic saltier than the Pacific? In part, said Broecker, it's because more fresh water from rain and snow drains into the Pacific than into the Atlantic. The differences are subtle but important. Every quart of ocean water has between 1.1 and 1.2 ounces of salt. Add a mere 0.03 ounces of salt to the water, and there is the same sinking effect as cooling the water by several degrees, by Broecker's calculation.
This is why any buildup of freshwater is so troubling: It could dilute the ocean subtly but critically. In the case of the Younger Dryas era, Broecker theorized that a mighty pulse of freshwater from melting glaciers stopped the sinking action. Freshwater accumulated in the far North Atlantic, and it froze. The conveyor suddenly slowed, interrupting the northward flow of warm water and warm air. The Gulf Stream couldn't do its job. What Alley found in his Greenland ice cores was that such a cosmic change could happen suddenly. In Weart's view, it marked a sea change in scientific opinion. "The whole notion of rapid climate change was very hard for science to accept," he said. "The guys who said there could be rapid climate change had to drag the rest of the climate community kicking and screaming." The chances of a shutdown of the conveyor are remote, if not out of the question. But any significant changes in the oceanic circulation would likely have major, and wholly unpredictable, effects on climate. At the peak of the Little Ice Age, the Gulf Stream did not shut itself down. Researchers think, however, that it slowed down, or maybe wandered from its usual trek. If that happened again, they don't want to be caught by surprise.
Today, concerns about the state of the ocean run so deep that an unprecedented international effort is under way from the Straits of Florida to Greenland to track changes in the flow of the North Atlantic. So far, at least two new studies suggest that concerns about the freshwater buildup in the North Atlantic are warranted. Satellite data have detected a slowing of the circulation from Ireland to Labrador, according to a research team led by NASA's Sirpa Hakkinen. The team said that if the slowing continues, it might lead to large-scale ocean and, eventually, climate changes.
In a second study, 1,500 miles to the south, a group of British scientists reported in December 2005 a 30 percent slowdown in the movement of Atlantic deepwater. Hakkinen and Henry J. Bryden, the head of the British team, cautioned that their results weren't conclusive. Bryden looked at measurements taken at five intervals from 1957 to 2004. Hakkinen said it was impossible to predict whether the slowing in the so-called North Atlantic gyre would continue or was part of a natural cycle. Herein is a basic problem of oceanic research: the period of record is minuscule. Carl Wunsch, an oceanographer at the Massachusetts Institute of Technology, said researchers are just beginning to build a baseline to track the movements of the North Atlantic conveyor. Right now, they have little basis for comparison. Climate evidently obeys the first rule of weather, only on a grander scale: What might happen is almost always more interesting than what is happening. If anything, however, the uncertainty makes it even more important to find out what the conveyor belt is up to. For the volatility of climate is inarguable. "These scenarios are conceivable," Wunsch said, "and we sure as hell want to know what's going on out there." ¶ 8:32 PM
By JOHN J. FIALKA
WASHINGTON -- U.S. government scientists Friday said the long-term outlook for global warming may be more dire than suggested by this week's United Nations' report, which they say doesn't fully address the impact of clouds and melting glaciers.
Recent evidence of accelerated melting of glaciers in Greenland and the Antarctic ice cap came too late to be included in the report released Thursday by the U.N.-sponsored Intergovernmental Panel on Climate Change.
Glaciers are among the largest sources of fresh water in the world and are contributing to rising ocean levels. Rising sea levels could expose population centers bordering the ocean to more storm damage and could require evacuation in some areas. But the computer models used for the IPCC report based their predictions only on the results of heating of the existing water in the world's oceans, causing the oceans to expand and sea levels to rise, said Tom Delworth, a climate modeler for the National Oceanic and Atmospheric Administration, the government agency in charge of climate science and weather service.
The IPCC report predicts sea levels will rise by between one to two feet over the next 100 years. Mr. Delworth said there remains "much more uncertainty" over how much accelerated melting of glaciers might add to that.
A second area of continuing uncertainty has to do with the impact of clouds on climate change. Warming the ocean sends more water vapor into the air, and the resulting clouds accelerate global warming by trapping more of the sun's heat in the atmosphere and further warm the ocean. Jim Butler, deputy director of NOAA's global monitoring division, called this "a very scary feedback mechanism."
But, so far, the supercomputers the agency uses to model the effect on the earth's climate -- which were also used for the IPCC report -- aren't detailed or fast enough to predict how much clouds are accelerating the problem. Mr. Delworth said computer models divide the earth's oceans and atmosphere into four million boxes, each about 150 square miles, and that these boxes are too large to model the effects of clouds.
"We could use computers that are one million times faster than they are today and still not be satisfied," Mr. Delworth said.
Further complicating the issue are layers of haze containing pollutants from human activity. Such pollutants, including sulfates, soot, dust and nitrates, tend to make the atmosphere brighter, reflecting more of the sun's heat back into space. The IPCC has found that the net effect of the added pollution is to cool the atmosphere.
A.R. Ravishankara, an atmospheric chemist for NOAA, said this raises a problem for governments attempting to clean the air by removing pollutants. "If you take away this cooling effect, then the heating effect would be exacerbated. It's a highly complex problem."
Write to John J. Fialka at john.fialka@wsj.com
____________________________________________
Ancient Global Warming Flipped Ocean Circulation, May Do So Again Jan 5, 2006
SAN DIEGO, California (ENS) --> For the first time, evidence that global warming triggered a reversal in the circulation of deep ocean patterns around the world has been uncovered by scientists affiliated with the Scripps Institution of Oceanography. While the changes they describe occurred 55 million years ago, the scientists say today's conditions are similar and could have similar drastic effects on ocean circulation. In today's issue of the journal "Nature," scientists Fl via Nunes and Richard Norris describe how they examined a four to seven degree warming period that occurred some 55 million years ago during the closing stages of the Paleocene and the beginning of the Eocene eras.
"The Earth is a system that can change very rapidly," said Nunes. "Fifty-five million years ago, when the Earth was in a period of global warmth, ocean currents rapidly changed direction and this change did not reverse to original conditions for about 20,000 years."
The global warming of 55 million years ago, known as the Paleocene/Eocene Thermal Maximum (PETM), emerged in less than 5,000 years, an instant of geological time. Modern carbon dioxide input to the Earth's atmosphere from fossil fuel sources is approaching the same levels estimated for the PETM period, say the scientists, which raises concerns about future climate and changes in ocean circulation.
They say the Paleocene/Eocene example suggests that changes produced by human activities may have lasting effects not only on global climate, but on deep ocean circulation. Fossil records show that the global warming at the time of the PETM created changes ranging from a mass extinction of deep sea bottom dwelling marine life to migrations of terrestrial mammal species, as warm conditions may have opened travel routes frozen over when climates were colder. This time period is when scientists find the earliest evidence of horses and primates in North America and Europe.
Nunes and Norris base their findings on the chemical makeup of microscopic sea creatures that lived 55 million years ago. The scientists analyzed carbon isotopes, or chemical signatures, from the shells of the one-celled animals called foraminifera, or "forams," that exist in vast numbers in a variety of marine environments.
"A tiny shell from a sea creature living millions of years ago can tell us so much about past ocean conditions," said Nunes. "We know approximately what the temperature was at the bottom of the ocean. We also have a measure of the nutrient content of the water the creature lived in. And, when we have information from several locations, we can infer the direction of ocean currents."
In the study, the scientists looked at a foram named Nuttalides truempyi from 14 sites around the world in deep-sea sediment cores retrieved via the Integrated Ocean Drilling Program, for which Joint Oceanographic Institutions, Inc., manages the U.S. component.
Chemicals from the foram's shells were used as nutrient "tracers" to reconstruct changes in deep ocean circulation through the ancient time period. Nutrient levels tell the researchers how long a sample has been near or isolated from the sea surface, giving them a way to track the age and path of deep sea water. Nunes and Norris found that deep ocean circulation in the Southern Hemisphere abruptly stopped the conveyor belt-like process known as "overturning," in which cold and salty water in the depths exchanges with warm water on the surface. Even as it was shutting down in the south, overturning appars to have became active in the Northern Hemisphere. The researchers believe this shift drove unusually warm water into the deep sea, likely releasing stores of methane gas that led to further global warming and a massive die-off of deep sea marine life.
"Overturning is very sensitive to surface ocean temperatures and surface ocean salinity," said Norris, a professor of paleobiology in the Geosciences Research Division at Scripps. "The case described in this paper may be one of our best examples of global warming triggered by the massive release of greenhouse gases and therefore it gives us a perspective on what the long term impact is likely to be of today's greenhouse warming that humans are causing."
Overturning is a fundamental component of the global climate conditions we know today, said Bil Haq, program director in the National Science Foundation's division of ocean sciences, which funded the research. Haq says overturning in the modern North Atlantic Ocean is a primary means of drawing heat into the far north Atlantic and keeping temperatures in Europe relatively warmer than conditions in Canada. Today, deep water generation does not occur in the Pacific Ocean because of the large amount of freshwater input from the polar regions, which prevents North Pacific waters from becoming dense enough to sink to more than intermediate depths. But in the Paleocene/Eocene, deep-water formation was possible in the Pacific because of global warming, the researchers say, adding that the Atlantic Ocean also could have been a significant generator of deep waters during this period.
_____________________________________________
Warming climate will slow ocean circulation Feb 4, 2006
Later this century, rising concentrations of greenhouse gases in Earth's atmosphere will slow the ocean currents that bring warm waters to the North Atlantic, thereby affecting that region's climate, computer simulations suggest. When the waters of the Gulf Stream and other warm currents of the North Atlantic reach an area just south of Greenland, they cool, become denser, and sink. That, in turn, pulls more surface water northward, says Thomas L. Delworth, a climate scientist at Princeton University. The rate of this so-called thermohaline circulation depends on the temperature and salinity of the surface waters. The warmer and fresher those North Atlantic surface waters are, compared with underlying layers, the more buoyant they are and the slower the circulation becomes.
Using a new computer model, Delworth and his colleague Keith W. Dixon simulated various scenarios for ocean circulation in the North Atlantic from now until 2100. They calibrated the model using weather and ocean-circulation data gathered since 1860. Throughout the 20th century, rising concentrations of greenhouse gases such as carbon dioxide warmed the atmosphere and ocean surface, but not enough to slow the thermohaline circulation. That's because large amounts of air pollutants known as aerosols have scattered sunlight back into space and counteracted the greenhouse effect somewhat, says Delworth. In the remaining years of the 21st century, however, growing concentrations of greenhouse gases will begin to overwhelm the cooling effect of aerosols, Delworth and Dixon suggest. By the year 2040, thermohaline circulation could carry only 80 percent as much warm water to the North Atlantic as it does now. The researchers report their findings in the Jan. 28 Geophysical Research Letters.--S.P.
SEA CHANGE IN THE ATLANTIC OCEAN? Apr 26, 2004
By John Carey
One worry about global warming is that the increased concentration of greenhouse gases will upset earth's balance and bring changes in ocean circulation. In an extreme case, scientists say, the flow of warm currents up the Atlantic Ocean to Europe might be shut down. That would cause temperatures to plunge in Western Europe. Such a shift may be unlikely, but anxious researchers have been keeping a keen watch for any variations in ocean flow, using satellites and instruments moored out in the sea. Now, they are starting to spot some potentially worrisome changes. In the Apr. 15 online issue of Science, a NASA-University of Washington team reports that the counterclockwise circulation of surface water in the North Atlantic has become markedly weaker since the early 1990s. "These observations of rapid climate changes over one decade may merit some concern," the authors write. But they also caution that it's not yet clear if the shift in circulation is the result of man-made global warming or part of a natural cycle.
THE GULF STREAM:
PART 3 OF 3: The North Atlantic: hot spot for ice ages
Warm waters linked to glacial eras - By Anthony R. Wood Jan 19, 2006
In this understated harbor village of tight streets and Cape Cod houses, the North Atlantic stirs gentle breezes in summer and tempers New England's harsh winter cold. And yet, only a geologic blip ago, this was a frigid and forbidding place, encased in a mile-thick sheet of ice. Massive ice sheets have advanced and retreated repeatedly over aeons, at a glacial pace. But what researchers have discovered recently is that climate can change in a hurry. Their findings have led to an ultimate irony: In the debate over global warming, one of the hottest issues is ice.
The planet's temperature has warmed robustly in the last 20 years, with 2005 being the second-warmest year on record, and the Arctic polar cap is disappearing. The same melting that has raised concerns about rising sea levels has prompted counterintuitive scenarios that it could produce a fresh and disastrous big chill. Few foresee an imminent glacial outbreak, and some serious scientists insist that one is all but impossible, but ice-core records show clear evidence that rapid coolings and warmings have happened. And that was long before humans started burning the fossil fuels blamed for at least some of the modern warming.
Today, while the debate rages over how much humans are to blame for the planet's indisputable warming, scientists are still trying to figure out what conspired to bring on the flash-frozen ice ages. But a long and tortuous trail of evidence leads to a surprising suspect at the heart of the conspiracy: the Gulf Stream. Logically, it would be an unlikely culprit. It is hundreds of miles from the southern extent of the last ice sheet, and it covers only about 0.2 percent of the world's ocean surface. Yet the mighty stream is a critical piece of something much larger: the North Atlantic current system that moves warmth out of the tropics toward the North Pole and sends cold water back toward the equator, the so- called conveyor belt. It is estimated that the Gulf Stream transports about 20 percent of the heat moved by the oceans. If the Gulf Stream were to slow down or take a more southerly route, the change would disrupt the whole system, the North Atlantic would cool off. Europe and eastern North America might turn colder as the rest of the world heated up. Scientists think that's what happened the last time ice invaded Europe and the United States.
The question is: Could it happen again? In 1992, Richard Alley was in central Greenland, examining ice cores, when he saw something he could not believe. He and his colleagues were looking at the layers that told them about Greenland's temperature year by year, going back millennia. But instead of a gradual change, they saw radical shifts in the layers representing the climate 12,000 years ago. The temperatures had plunged and risen suddenly. He saw a swing of 15 degrees in a matter of 10, or no more than 30, years.
"This was a flipped switch, not a slowly turned dial," he recalls. "Something really dramatic had happened."
This made the Little Ice Age look like a snow flurry. Alley had come across a phenomenon described in 1985 by Wallace Broecker, a chemical oceanographer and paleontologist with Columbia University's Lamont-Doherty Observatory. Broecker called it the Younger Dryas period, for an Arctic shrub that mysteriously appeared throughout Europe. But whereas Broecker drew upon a variety of research sources, Alley was looking at direct physical evidence. The science of climate change was itself changing. Until the 1950s, climate was viewed as essentially a stable system, said Spencer Weart, head of the history center for the American Institute of Physics. That view was stood on its head when researchers saw evidence that big swings could occur in just a couple of millennia. By 1980, scientists came across further clues that such changes could happen in a few centuries. Broecker tightened the possible time frame in 1985 by publishing a paper on the Younger Dryas era. In the process, he indicted the North Atlantic and gave global warming an icon. The article, which appeared in the journal Natural History, posited that tundra conditions overspread Europe as the Gulf Stream and the North Atlantic heat-transport system broke down. Europe turned arctic.
That the North Atlantic would be so important underscores the complexity of oceanic circulation. The Pacific is triple its size, yet the Atlantic, Broecker explains, does a better job of moving heat northward than the wind- driven currents of the Pacific. And the mighty stream is the engine driving it. Critical ingredient A critical ingredient in the recipe for climate change is one of the most plentiful substances on the planet: salt. The key to keeping the conveyor belt in motion is the sinking action of the water. Salt adds weight to water, so the more saline it is, the better it sinks; the better it sinks, the faster the conveyor moves. Why is the Atlantic saltier than the Pacific? In part, said Broecker, it's because more fresh water from rain and snow drains into the Pacific than into the Atlantic. The differences are subtle but important. Every quart of ocean water has between 1.1 and 1.2 ounces of salt. Add a mere 0.03 ounces of salt to the water, and there is the same sinking effect as cooling the water by several degrees, by Broecker's calculation.
This is why any buildup of freshwater is so troubling: It could dilute the ocean subtly but critically. In the case of the Younger Dryas era, Broecker theorized that a mighty pulse of freshwater from melting glaciers stopped the sinking action. Freshwater accumulated in the far North Atlantic, and it froze. The conveyor suddenly slowed, interrupting the northward flow of warm water and warm air. The Gulf Stream couldn't do its job. What Alley found in his Greenland ice cores was that such a cosmic change could happen suddenly. In Weart's view, it marked a sea change in scientific opinion. "The whole notion of rapid climate change was very hard for science to accept," he said. "The guys who said there could be rapid climate change had to drag the rest of the climate community kicking and screaming." The chances of a shutdown of the conveyor are remote, if not out of the question. But any significant changes in the oceanic circulation would likely have major, and wholly unpredictable, effects on climate. At the peak of the Little Ice Age, the Gulf Stream did not shut itself down. Researchers think, however, that it slowed down, or maybe wandered from its usual trek. If that happened again, they don't want to be caught by surprise.
Today, concerns about the state of the ocean run so deep that an unprecedented international effort is under way from the Straits of Florida to Greenland to track changes in the flow of the North Atlantic. So far, at least two new studies suggest that concerns about the freshwater buildup in the North Atlantic are warranted. Satellite data have detected a slowing of the circulation from Ireland to Labrador, according to a research team led by NASA's Sirpa Hakkinen. The team said that if the slowing continues, it might lead to large-scale ocean and, eventually, climate changes.
In a second study, 1,500 miles to the south, a group of British scientists reported in December 2005 a 30 percent slowdown in the movement of Atlantic deepwater. Hakkinen and Henry J. Bryden, the head of the British team, cautioned that their results weren't conclusive. Bryden looked at measurements taken at five intervals from 1957 to 2004. Hakkinen said it was impossible to predict whether the slowing in the so-called North Atlantic gyre would continue or was part of a natural cycle. Herein is a basic problem of oceanic research: the period of record is minuscule. Carl Wunsch, an oceanographer at the Massachusetts Institute of Technology, said researchers are just beginning to build a baseline to track the movements of the North Atlantic conveyor. Right now, they have little basis for comparison. Climate evidently obeys the first rule of weather, only on a grander scale: What might happen is almost always more interesting than what is happening. If anything, however, the uncertainty makes it even more important to find out what the conveyor belt is up to. For the volatility of climate is inarguable. "These scenarios are conceivable," Wunsch said, "and we sure as hell want to know what's going on out there." ¶ 8:32 PM
