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"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.