Gordon McBean drove to Meech Lake, Que., in the summer of 2002 to brief the federal cabinet on global warming.
The Liberals were holding a retreat to debate whether to ratify the Kyoto agreement on climate change. Dr. McBean, one of Canada's top climatologists, told them Kyoto was only the first step, and they would have to make much deeper cuts in greenhouse-gas emissions by 2050.
A hotter world could mean surging sea levels, more frequent violent storms, severe heat waves and droughts, but it would be decades, Dr. McBean told them, before the benefits of cutting emissions would be seen.
One minister -- Dr. McBean won't say who -- seemed appalled.
"And there will be nothing for us between now and the next election?" the minister asked. "Why would we do this?"
Dr. McBean had a guilt-inducing answer ready. "You do it for your grandchildren," he said.
He wouldn't have marshalled this kind of emotional argument earlier in his career, when he was a senior scientist and bureaucrat with Environment Canada. Then, he felt strongly that researchers -- especially those working for the government -- could explain the science of global warming, but not push a course of action.
But by 2002, Dr. McBean had crossed that self-imposed boundary and become an advocate, one of a growing number of scientists whose warnings about the dangers of global warming have become more passionate -- and more personal.
Is it a good thing that they are sticking their necks out, talking publicly about what they see happening and what needs to be done? After all, their work is the foundation for political action on global warming, both in Canada and internationally. Or are they going too far, becoming climate crusaders instead of objective analysts and observers?
Many scientists don't see themselves as advocates, said David Runnalls, president of the International Institute for Sustainable Development. They believe it is their job to warn the world about global warming, he said, just as experts in infectious diseases warn us to get ready for an avian flu pandemic, or atomic scientists warn of the dangers of nuclear holocaust.
They are getting scared by what they are seeing, Mr. Runnalls said, such as the study published last year that suggested Greenland's massive ice sheet is melting faster than expected. "That is why a lot of them are far more outspoken than before," he said.
Dr. McBean, 63, said a number of factors pushed him into advocacy, including the increasingly compelling evidence of the dangers of climate change. He had moved to an academic position at the University of Western Ontario, so he felt more freedom to speak his mind.
He also did it for grandchildren, Amanda, 7, and Stuart, 10. He worried about their future in a warmer world.
So he speaks to church groups, naturalist clubs -- anyone who invites him -- and gives frequent interviews to the media in which he insists that governments must move now. Global warming is an issue of international and intergenerational equity, he says. It is future generations, particularly in the Third World, who will be hit hardest if the rich countries like Canada and the United States refuse to curb emissions. Island nations such as the Polynesian country of Tuvalu could be swamped by rising waves.
"We are talking about whole countries of people who have never had the benefits of burning greenhouse gases basically disappearing."
Burning fossil fuels such as oil, coal and gas keeps our houses warm in winter and our cars on the road. But it also produces carbon dioxide, one of several gases that trap heat in the atmosphere. C02 levels have been steadily building since the industrial revolution. In the late 1980s, scientists started warning that we may be altering the climate in ways that could have serious future consequences.
Those warnings are becoming more dramatic -- and, according to some critics, far more apocalyptic than is warranted by the science.
Perhaps it is the bureaucrat in him, but Dr. McBean is conservative in his language, compared with some of his colleagues, including Andrew Weaver, a professor at the School of Earth and Ocean Sciences at the University of Victoria.
"When you are a scientist and you realize humanity as you know it is being threatened, what is your job?" Dr. Weaver asked. "Is your job to stand by and say nothing, or is your job to say, we are on a precipice, and there is no joking about it.
"I don't get off on saying the sky is falling. I am seriously concerned about the planet that my kids' kids are going to live on. It is not a planet that will look anything like it is today. . . . It is time for people to get scared."
Dr. Weaver, 45, has two children, who are 12 and 8.
When he started research on how the oceans affected climate, in the late 1980s, he was skeptical about the idea of man-made global warming. But his work soon convinced him that climate change posed a threat, one he has a moral and ethical responsibility to talk about.
But he isn't an advocate, he said, because he isn't telling governments how to cut greenhouse-gas emissions, only that cuts are necessary. One of the reasons he speaks out, he said, is that global-warming skeptics, or "denialists" as he calls them, get so much attention from the media even though they are a minority -- perhaps 5 per cent -- among climate experts.
Experts who disagree about the threat posed by global warming write letters to the editor, post comments on websites and hold news conferences. They make a number of arguments, including that warming temperatures may be part of the natural variation in the Earth's climate. Last spring, 20 or so Canadian scientists, economists and others skeptics, and roughly 40 more from other countries, sent a letter to Prime Minister Stephen Harper, asking for an independent review of climate science before Canada squandered billions implementing the Kyoto Protocol.
That letter prompted Dr. Weaver, Dr. McBean and a third scientist, Ken Denman, to organize their own missive to Mr. Harper. The project was riskiest for Dr. Denman, who works for the federal government on climate models.
He is free to talk about science but not policy, he said. The border between the two, however, is grey, not black and white.
"Yes, I'm a government scientist. I try and walk the line," he said. "But I still feel I have to speak out. . . .
"I have a grandchild, I am about to have another one. I think in the past 15 years, the idea of what my generation has done, which is use up most of the readily available fossil fuels, is that we are robbing from future generations."
The letter the three helped draft and circulate was signed by 90 scientists and sent to Mr. Harper in April, calling on him to develop a national climate-change strategy.
Dr. Denman said he heard that there was some unhappiness among senior bureaucrats that the letter advocated action -- even the vaguely worded request for a national strategy. But he wasn't reprimanded or disciplined.
The duelling missives are a sign of how political the scientific debate over global warming has become.
Ross McKitrick, a University of Guelph economist who signed the skeptic's letter, doesn't think it is wrong for experts on either side of the issue to air their views. He said it is important that those pushing for action don't exaggerate the risk or play down the uncertainty.
All four men -- Dr. Weaver, Dr. Denman, Dr. McBean and Dr. McKitrick -- are involved in the Intergovernmental Panel on Climate Change report that will be released Friday in Paris. The IPCC, established in 1988, brings together climate experts from around the world to assess the risks posed by greenhouse gases. They have issued three previous reports on the science of global warming, each more definitive than the previous one.
Dr. Weaver and Dr. Denman helped write chapters. Dr. McKitrick was asked to review some of the work, and make comments. Dr. McBean was a review editor, assigned to make sure that the critical comments of people such as Dr. McKitrick were taken into account.
The language in the report will be a carefully chosen. You won't see words like catastrophic or chaotic, said Mike Hulme, director of the Tyndall Centre for Climate Change Research in the U.K. Researchers should stay away from apocalyptic terms when they talk about global warming, he said, and not ignore the uncertainties over what kind of changes global warming will bring, and when they will occur.
Dr. McBean tries not to use the word catastrophe, even though he chairs the Institute for Catastrophic Loss Reduction at the University of Western Ontario, which is partly funded by the insurance industry.
"I don't think the Earth is going to be unlivable in 100 years."
But he does believe some of the impacts of global warming will be irreversible, and that Canada, a greenhouse laggard, must start reducing emissions.
The Liberals, under former prime minister Jean Chrétien, agreed to ratify Kyoto, but did nothing to meet Canada's ambitious commitment to cut emissions by 6 per cent from 1990 levels. Instead, the country has increased its greenhouse-gas output by about 30 per cent.
The Conservatives announced a plan that would require no absolute reductions until after 2020, but earlier this year Mr. Harper said he would do "a lot more" about climate change.
Dr. McBean said we need aggressive targets for 2050, well beyond 50 per cent, and legislation on emission reductions that can't be revoked by a change of government.
He says it feels good to speak his mind.
"It feels like the right thing to do."