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Study of Ice Age Bolsters Carbon and Warming Link Study of Ice Age Bolsters Carbon and Warming Link
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A meticulous new analysis of Antarctic ice suggests that the sharp warming that ended the last ice age occurred in lock step with increases of carbon dioxide in the atmosphere, the latest of many indications that the gas is a powerful influence on the earth’s climate.A meticulous new analysis of Antarctic ice suggests that the sharp warming that ended the last ice age occurred in lock step with increases of carbon dioxide in the atmosphere, the latest of many indications that the gas is a powerful influence on the earth’s climate.
Previous research had suggested that as the world began to emerge from the depths of the ice age more than 20,000 years ago, warming in Antarctica preceded changes in the global carbon dioxide level by something like 800 years. Previous research suggested that as the world began to emerge from the depths of the ice age about 20,000 years ago, warming in Antarctica preceded changes in the global carbon dioxide level by something like 800 years.
That relatively long gap led some climate-change contrarians to assert that rising carbon dioxide levels were essentially irrelevant to the earth’s temperature — a side effect of planetary warming, perhaps, but not the cause.That relatively long gap led some climate-change contrarians to assert that rising carbon dioxide levels were essentially irrelevant to the earth’s temperature — a side effect of planetary warming, perhaps, but not the cause.
Mainstream climate scientists have rejected that view and argued that carbon dioxide, while it did not initiate the end of the ice age, played a vital role in the feedback loops that caused a substantial warming. Mainstream climate scientists rejected that view and argued that carbon dioxide, while it certainly did not initiate the end of the ice age, played a vital role in the feedback loops that caused a substantial warming. Still, a long gap between initial increases of temperature and of carbon dioxide was somewhat difficult for the scientists to explain.
Still, a long gap between increases of temperature and of carbon dioxide was relatively hard for the scientists to explain. In the political debate in the United States over global warming, the supposed gap has been invoked repeatedly by climate-change contrarians. A wave of new research in the last few years has raised the likelihood that there was actually a small gap, if any.
In 2007, for example, Al Gore was testifying to Congress about the science in his documentary, “An Inconvenient Truth.” He came under attack by Representative Joe L. Barton, Republican of Texas. The latest paper was led by Frédéric Parrenin of the University of Grenoble, in France, and is scheduled for publication on Friday in the journal Science. Using relatively new, high-precision chemical techniques, his group sought to reconstruct the exact timing of the events that ended the ice age.
“CO2 levels went up after the temperature rose,” Mr. Barton said, citing a scientific paper from 2001. “The temperature appears to drive CO2, not vice versa. On this point, Mr. Vice President, you’re not just off a little. You’re totally wrong.” Scientists have long known that the ice ages are caused by variations in the earth’s orbit around the sun. When an intensification of sunlight initiates the end of an ice age, they believe, carbon dioxide is somehow flushed out of the ocean, causing a big amplification of the initial warming.
But the paper published online Thursday by the journal Science, together with a string of other recent studies, suggests that Mr. Gore was right all along. Since the 1980s, scientists have been collecting a climate record by extracting long cylinders of ice from the ice sheets in Greenland and Antarctica, and from glaciers atop high mountains.
The research was led by Frédéric Parrenin of the University of Grenoble, in France. He and his colleagues took a new stab at sorting out the sequence of events at the close of the last great ice age. Air bubbles trapped in the ice give direct evidence of the past composition of the atmosphere. And subtle chemical variations in the ice itself give an indication of the local temperature at the time it was formed.
Since the 1980s, scientists have been collecting a climate record from those earlier times by extracting long cylinders of ice from the ice sheets in Greenland and Antarctica. The trouble is that air does not get sealed in the ice until hundreds or even thousands of years after the snow has fallen, as it slowly gets buried and compressed.
Air bubbles trapped in the polar ice give direct evidence of the past composition of the atmosphere. And subtle chemical variations in the ice itself give an indication of the local temperature at the time it was formed. That means the ice and the air bubbles trapped in it are not the same age, so it becomes tricky for scientists to put reconstructed atmospheric composition and reconstructed temperature onto a common time scale.
The trouble is that the air bubbles do not get sealed off for hundreds or even thousands of years, as the snow is slowly buried and compressed. Therefore, it is tricky for scientists to put the atmospheric record and the temperature record onto a common time scale. With its improved techniques, Dr. Parrenin’s group sought to clarify the dating of previously recovered ice cores from Antarctica. Instead of the 800-year lag between temperature and carbon dioxide increases found in some previous research, their work suggests that the lag as the ice age started to end was less than 200 years, and possibly there was no lag at all.
Early analyses had fairly large error margins. Nonetheless, they produced one of the most striking findings of modern science: an extremely tight association between the temperature and the level of carbon dioxide in the atmosphere. That is consistent with basic physics showing that carbon dioxide is a powerful greenhouse gas. “Before, because of these wrong results of CO2 lagging temperature, people were interpreting it as a weak role for CO2 in the climate variation of the past,” Dr. Parrenin said.
But in several reconstructions based on ice cores, local temperature increases at the poles appeared to slightly precede global increases of carbon dioxide. In the 2001 paper that Mr. Barton cited, for example, Antarctic temperature appeared to lead global carbon dioxide levels by 800 years, give or take 600 years. Indeed, though most climate scientists have never seen the supposed gap as a major conceptual problem, it has been invoked repeatedly by American politicians who want to delay action on global warming.
Using high-precision chemical techniques, Dr. Parrenin and his colleagues have essentially reduced the error margin. Their findings suggest that increases of carbon dioxide lagged temperature increases in Antarctica by no more than about 200 years and may have even preceded the temperature increase. In 2007, for example, former Vice President Al Gore was testifying to Congress about the science in his documentary “An Inconvenient Truth.” He came under attack by Representative Joe L. Barton, a Texas Republican.
“It’s a breakthrough in our concept of how past climate evolved,” Dr. Parrenin said in an interview. “CO2 levels went up after the temperature rose,” Mr. Barton said, citing a scientific paper from 2001. “The temperature appears to drive CO2, not vice versa. On this point, Mr. Vice President, you’re not just off a little. You’re totally wrong.”
It remains to be seen how well the paper will withstand scientific scrutiny. “I’m left with this uneasy feeling that the uncertainties are larger than they claim,” said Eric Steig, a climate scientist at the University of Washington. The emerging evidence suggests that Mr. Gore was right.
Dr. Steig noted that Dr. Parrenin’s paper is the third in recent years to suggest that the gap in the climate records between polar temperature and CO2, if it exists at all, is relatively small. And Jeremy Shakun, a visiting scholar at Harvard, compiled a temperature record for the whole planet, not just Antarctica. He concluded that the carbon dioxide increase preceded the overall planetary warming. Richard B. Alley, a climate scientist at Pennsylvania State University who was not involved in the new work, said by e-mail that it essentially confirmed previous scientific understanding.
A small gap poses no conceptual problems, scientists said. They have long known that the ice ages are caused by variations in the earth’s orbit around the sun. When an intensification of sunlight initiates the end of an ice age, they believe, carbon dioxide is somehow flushed out of the ocean, causing a big amplification of the initial warming. “What this does, again and more clearly than ever, is to show that the large temperature changes are tightly coupled to the large CO2 changes,” he said.
That understanding is one of the cornerstones of the scientists’ warning that modern society is running a big risk by burning fossil fuels and pumping enormous quantities of carbon dioxide into the atmosphere. Dr. Parrenin’s paper is the third in recent years to suggest that the gap in the climate records between polar temperature and CO2, if it exists at all, is relatively small. And Jeremy Shakun, a visiting scholar at Harvard, pointed out in a paper last year that the timing of the temperature increase in Antarctica could not be assumed to be representative of the world as a whole. When he compiled a global temperature record for the end of the ice age, he found that increases of carbon dioxide came first, and rising temperatures came second.
The level has already jumped 41 percent since the Industrial Revolution began in the 18th century, and given the weakness of global efforts to control emissions, scientists say it could eventually double or triple. Even at the current concentration, the evidence suggests that increases in sea level of 25 feet or more may have already become inevitable, albeit over a long period. The tight relationship in past climate between temperature and carbon dioxide is a major reason scientists have warned that modern society is running a big risk by burning CO2-producing fossil fuels.
The level of carbon dioxide in the atmosphere has jumped 41 percent since the Industrial Revolution began in the 18th century, and scientists fear it could double or triple unless stronger efforts are made to control emissions.
Even at the current concentration of the gas, the evidence suggests that increases in sea level of 25 feet or more may have already become inevitable, albeit over a long period.
“We’re just entering a new era in earth’s history,” Dr. Shakun said. “It will be an unrecognizable new planet in the future. I think the only question is, exactly how fast does that transformation happen?”“We’re just entering a new era in earth’s history,” Dr. Shakun said. “It will be an unrecognizable new planet in the future. I think the only question is, exactly how fast does that transformation happen?”