New York/Washington: Just two years ago, a United Nations (UN) panel that synthesizes the work of hundreds of climatologists around the world called the evidence for global warming “unequivocal".

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

World at stake? A globe at a conference centre in Copenhagen, where the climate change talks are on. Johan Spanner / The New York Times

And the uproar has threatened to complicate a multi-year 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.

In recent days, an array of scientists and policymakers in the US and abroad have said that nothing so far disclosed—the correspondence and documents include references by prominent climate scientists to deleting potentially embarrassing email messages, keeping papers by competing scientists from publication and making adjustments in research data—undercuts decades of peer-reviewed science. Yet the intensity of the response highlights that scepticism about global warming persists, even as many scientists thought the battle over the reality of human-driven climate change was finally behind them.

Also See Mint’s full coverage on the Copenhagen Summit 2009

On dozens of websites and blogs, sceptics and foes of greenhouse gas restrictions take daily aim at the scientific arguments for human-driven climate change. The stolen material was quickly seized upon for the questions they raised about the accessibility of raw data to outsiders and whether some data had been manipulated.

An investigation into the stolen files is being conducted by the University of East Anglia, in England, where the computer breach occurred. Rajendra K. Pachauri, chairman of the UN Intergovernmental Panel on Climate Change, has also said he will look into the matter. At the same time, polls in the US and Britain suggest that the number of people who doubt that global warming is dangerous or caused by humans has grown in recent years.

If the scientific 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 the forecasts are wrong, nations could divert hundreds of billions of dollars to curb the emissions of greenhouse gases at a time when they are struggling to recover from a global recession.

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

Even some who remain sceptical about the extent or pace of global warming say that the premise underlying the Copenhagen talks is solid: that warming is to some extent driven by greenhouse gases spewing into the atmosphere from human activities such as the burning of fossil fuels and deforestation.

Roger A. Pielke Sr, for example, a climate scientist at the University of Colorado who has been critical of the UN climate panel and who once branded many of the scientists embroiled in the email controversy part of a climate “oligarchy", said that so many independent measures existed to show unusual warming taking place that there was no real dispute about it. He continued, “The role of added carbon dioxide as a major contributor in climate change has been firmly established."

The Copenhagen meet is a reflection of the increasing acceptance of the scientific arguments: The negotiations leading to the talks were conducted by high-ranking officials of the world’s governments rather than the scientists and environment ministers who largely shaped the 1997 Kyoto Protocol. Late last week, US President Barack Obama changed the date of his visit to Copenhagen to 18 December, the last day of the talks.

For many, the 2007 report by the Intergovernmental Panel on Climate Change was a marker of a shift in the global warming debate.

In it, the panel—a volunteer network of hundreds of scientists from many disciplines who meet periodically to review climate studies and translate the results into language useful to policymakers—concluded that no doubt remained that human-caused warming was under way and, if unabated, would pose rising risks.

Over the last several decades, other reviews, by the National Academy of Sciences and other institutions, have largely echoed the panel’s findings and said the remaining uncertainties should not be an excuse for inaction.

The panel’s report was built on two decades of intensive scientific study of climate patterns.

Greenhouse gases warm the planet by letting in sunlight and blocking the escape of some of the resulting heat. “The physics of the greenhouse effect is so basic that instead of asking whether it would happen, it makes more sense to ask what on earth could make it not happen," said Spencer Weart, a physicist and historian. “So far nobody has been able to come up with anything plausible in that line."

The atmospheric concentration of greenhouse gases released by humans has risen rapidly in the last century, along with industrialization and electricity use. Carbon dioxide, produced from the burning of coal, oil and natural gas, is the most potent of the greenhouse gases because it can persist in the atmosphere for a century or more.

Methane—from landfills, livestock and leaking pipes, tanks and wells—has recently been found to be a close second. And these gases not only have a heating effect, but also cause the evaporation of water from sea and soil, producing water vapour, another powerful heat-trapping gas.

In reaching its conclusion, the climate panel relied only partly on temperature data such as that collected by the scientists at the Climatic Research Unit at the University of East Anglia, whose circulated email correspondence set off the current uproar. It also considered a wide range of data from other sources, including measurements showing the retreat of glaciers in mountain ranges around the world, changes in the length and character of the seasons, heating of the oceans and marked retreats of sea ice in the Arctic.

Since 1979, satellites have provided another check on surface temperature measurements. Strong disagreements about how to interpret the satellite data were largely resolved after the Bush administration began a review in which competing research groups worked out some of their differences.

Science, by its nature, is about probability, not certainty. And the persisting uncertainties in climate science leave room for argument. What is a realistic estimate of how much temperatures will rise? How severe will the effects be? Are there tipping points beyond which the changes are uncontrollable?

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

Both sides also have at times been criticized for overstatement in characterizing the scientific evidence. Whichever view prevails, the questions will undoubtedly linger well after the negotiators who are trying to work out the complex issues that still stand in the way of an international climate treaty leave Copenhagen.