New Research Shows Smaller Gap Between Warming and Carbon Dioxide at Ice Age's End

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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.

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.

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.

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.

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.

"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."

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.

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.

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.

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.

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.

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.

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.

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.

"It's a breakthrough in our concept of how past climate evolved," Dr. Parrenin said in an interview.

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.

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.

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.

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.

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.

"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?"


This article originally appeared in The New York Times.


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