Glenn Milne is learning what it means to play in the Super Bowl of climate science.

Like linebackers in a rowdy football match, he and his colleagues are preparing to take the field amid a cacophony of cheers and jeers, knowing environmental advocates will hold up their conclusions to demand action from governments while skeptics take pot shots and search for ways to assail their credibility.

An associate professor at the University of Ottawa, Dr. Milne is one of dozens of lead authors labouring over the next United Nations assessment of climate change. Issued every seven years, each assessment is widely regarded as the most authoritative word on global warming and a guide for policy makers grappling with its potential consequences. That virtually ensures the coming assessment will be a prime target for those who argue that scientists have it wrong.

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"Certainly at the back of our minds was the understanding that this is going to be scrutinized by people that are quite antagonistic," Dr. Milne said. "So any statements that we make have to be carefully worded and supported."

Dr. Milne, who specializes in sea level rise, is among hundreds of researchers tasked with evaluating the current understanding of the physics of climate change. Their report is due at the end of September. Two other reports that make up the assessment, one by a group focusing on the effects of climate change, and another on mitigating its effects, are due next spring.

But the fun is beginning ahead of schedule for Dr. Milne and his co-authors, with the leak of portions of their report a few weeks ahead of schedule. Details from a draft of the group's conclusions were leaked to some media last week. The draft, also obtained by The Globe and Mail, cites human activity as "extremely likely" to be the largest factor behind rising global temperatures. That confidence level, quantified at 95 per cent, is the highest of any assessment to date. Other effects, including warming seas, melting glaciers and an accelerated rise in average sea level, are similarly chalked up to emissions of greenhouse gasses.

The report also puts at better than 90 per cent the chance that continued emissions at or above current rates will produce changes in the global climate system not seen on Earth for hundreds of thousands of years.

The document is under review by national governments, part of the process by which the UN's Intergovernmental Panel on Climate Change garners a degree of political buy-in for its assessments. In a statement on Monday, the IPCC warned the wording of the report is likely to change before its official release, expected after an approval session in Stockholm, Sweden, in late September. Yet experts agree the science behind the report is effectively locked in.

The work, along with the reports coming next year, comprises the fifth assessment from the IPCC. The previous assessment, released in 2007, was harshly criticized for including a statement that glaciers in the Himalayas could disappear by 2035 due to global warming, a conclusion that was found not to be scientifically supported. Calls for the IPCC's overhaul followed, while climate skeptics had a field day.

Dr. Milne is among those participating in the assessment process for the first time as part of an approach that brings in the opinions of a wider range of researchers. The authors are not expected to produce new research, he said, but rather to seek a consensus based on the best science available across the field.

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"You're trying to digest a lot of information and assess things as accurately as you can," Dr. Milne said.

While the report reduces the uncertainty about what is causing climate change – a point on which experts have little doubt – it also remains stubbornly vague on how severe or how rapid the consequences will be.

"Part of it is just the chaos in the system," says Ted Shepherd, a Canadian atmospheric physicist based at the University of Reading. Dr. Shepherd, who was a review editor for the portion of the report dealing with near-term predictability, notes the natural variability of Earth's climate over periods of a few decades still makes it difficult to know exactly how global warming will play out in the short term and on the regional scales that are most relevant to policy makers.

Robert Stavins, a professor of environmental economics at Harvard University, added that while the IPCC assessments are viewed as important works of analysis by those in the climate field, "an analysis can be used as rock or a light bulb." In the politically polarized debate over how governments should respond to climate change, he said, it is more likely that the assessment will become ammunition to support well-entrenched positions rather than change minds.

"I think governments are pretty convinced that the IPCC knows what it's talking about," said Douglas Macdonald, an environmental policy specialist at the University of Toronto. "What's driving the debate now is that this is an incredibly complex, difficult issue which is going to impose costs on all kinds of people who have got the ability to resist."