As representatives of about 200 nations converge in Copenhagen, Denmark, today to begin negotiating a new international climate accord, they do so against a background of renewed attacks on the basic science of climate change.

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Only two years ago, a U.N. panel that synthesizes the work of hundreds of climatologists called the evidence for global warming “unequivocal.”

But as representatives of about 200 nations converge in Copenhagen, Denmark, today to begin negotiating a new international climate accord, they do so against a background of renewed attacks on the basic science of climate change.

The debate, set off by the circulation of several thousand files and e-mails stolen from one of the world’s foremost climate-research institutes, has led some who oppose limits on greenhouse-gas emissions, and at least one influential country, Saudi Arabia, to question the scientific basis for the Copenhagen talks.

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And the uproar has threatened to complicate a multiyear diplomatic effort already ensnared in difficult political, technical and financial disputes that have caused leaders to abandon hopes of hammering out a binding international climate treaty this year.

An array of scientists and policymakers in the United States and abroad have said that nothing disclosed so far — the correspondence and documents include references by prominent climate scientists to deleting potentially embarrassing e-mails, keeping papers by competing scientists from publication and making adjustments in data — undercuts decades of peer-reviewed science.

Yet, the intensity of the response highlights that skepticism about global warming persists, even as many scientists thought the battle over the reality of human-driven climate change was behind them. On dozens of Web sites and blogs, skeptics and foes of greenhouse-gas restrictions take daily aim at the scientific arguments for human-driven climate change. The stolen material quickly was seized upon for the questions raised about the accessibility of raw data to outsiders and whether some data had been manipulated.

An investigation is being conducted by the University of East Anglia, in England, where the computer breach occurred. Rajendra Pachauri, chairman of the U.N. Intergovernmental Panel on Climate Change (IPCC), also has said he will look into the matter.

Politics, ideology and economic interests interlace the debate, and the stakes on both sides are high.

If predictions about global warming’s effects are correct, inaction will lead at best to rising social, economic and environmental disruption, at worst to a calamity far more severe. If forecasts are wrong, nations could divert hundreds of billions of dollars to curb greenhouse-gas emissions at a time of global recession.

Yet, the case for human-driven warming, many scientists say, is far clearer now than a decade ago, when skeptics included many people now convinced that climate change is a real threat.

Even some who remain skeptical about the extent or pace of global warming say the premise underlying the Copenhagen talks is solid: Warming to some extent is driven by greenhouse gases from human causes such as the burning of fossil fuels and deforestation.

Roger Pielke Sr., for example, a University of Colorado climate scientist who has been highly critical of the IPCC and who once branded many of the scientists embroiled in the e-mail controversy part of a climate “oligarchy,” said so many independent measures existed to show unusual warming taking place that there was no real dispute about it. “The role of added carbon dioxide as a major contributor in climate change has been firmly established,” he said.

The Copenhagen conference itself is a reflection of the increasing acceptance of scientific arguments: Earlier negotiations were conducted by high-ranking government officials rather than the scientists and environment ministers who largely shaped the 1997 Kyoto Protocol. President Obama will visit Copenhagen on Dec. 18, the last day of the talks.

For many, the IPCC’s 2007 report, built on two decades of intensive study of climate patterns, was a marker of a shift in the debate. In it, the panel concluded that no doubt remained that human-caused warming was under way and, if unabated, would pose rising risks.

Other reviews, by the National Academy of Sciences and others, largely have echoed those findings.

Greenhouse gases warm the planet by letting in sunlight and blocking the escape of some of the resulting heat.

The atmospheric concentration of greenhouse gases released by humans has risen rapidly in the past century, along with industrialization and electricity use. Carbon dioxide, from the burning of coal, oil and natural gas, is the most potent of the gases because it can persist for a century or more. Methane — from landfills, livestock and leaking pipes, tanks and wells — recently has been found to be a close second.

These gases also cause the evaporation of water from sea and soil, producing water vapor, another powerful heat-trapping gas.

In reaching its conclusion, the IPCC relied only partly on temperature data such as that collected by the scientists at the Climatic Research Unit at the University of East Anglia. A wide range of data from other sources was considered, including measurements showing the retreat of glaciers, changes in the length and character of the seasons, heating of the oceans and marked retreats of Arctic sea ice.

Satellites have provided another check on surface temperatures. Strong disagreements about how to interpret that data largely were resolved after the Bush administration began a review in which competing groups worked out some of their differences.

Science, by its nature, is about probability, not certainty. What is a realistic estimate of how much temperatures will rise? How severe will the effects be? Are there tipping points beyond which changes are uncontrollable?

Even climate scientists disagree on many of these questions. But skeptics have been critical of the data assembled to show that warming is occurring and the analytic methods used by scientists, including mathematical models used to demonstrate a human cause for warming and project future trends.

Both sides also have been criticized for overstatement. The contents of the stolen e-mails and documents have given fresh ammunition to the skeptics’ camp.

To skeptics, the purloined files suggest a conspiracy to foist an expensive agenda on nations and to keep inconsistent data from the public. Many scientists, however, deny that any vital data was held back and said the e-mails and documents would prove merely another manufactured controversy.

“There will remain after the dust settles in this controversy a very strong scientific consensus on key characteristics of the problem,” John Holdren, Obama’s science adviser, told a congressional hearing last week.

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