Hansen told a nuclear industry conference on Thursday that massive amounts of heat are being absorbed by the planet's oceans, the daily equivalent of 400,000 atomic bombs like the one dropped on Hiroshima, Japan.
"That's going to have consequences," said the former NASA scientist, one of the first to recognize global warming in the 1980s.
"The situation is that we actually have an emergency and yet we're not acting as though we understand that," Hansen told the Canadian Nuclear Association's annual conference and trade show.
"There's a real danger that we're going to hand young people a situation that's out of their control, with enormous consequences."
Yet despite his dire prognosis, Hansen argues a rapid revolution in energy production is possible, driven by the very capitalist system sometimes vilified by anti-oil campaigners.
Putting a price on carbon, starting at $10 a tonne and increasing to $100 over a decade, would unleash market forces that would transform the energy market starting immediately, he said.
"Economics work better if prices are honest," said Hansen.
The Columbia University professor has previously drawn the ire of senior cabinet ministers in the Harper government for his insistence that Canada's oilsands need to stay in the ground as part of the global fight against climate change.
Former natural resources minister Joe Oliver, now the finance minister, blasted Hansen in 2013 for his "exaggerated rhetoric" and said the scientist should be ashamed of writing what Oliver called "frankly nonsense."
Hansen, for his part, seems unconcerned about causing offence.
He took a poke Thursday at environmental groups for their blanket opposition to nuclear power, joking that their major pre-occupation appears to be turning off anti-nuclear donors.
Hansen told the conference his next research project will be a frank examination of the pros and cons of nuclear power generation, but made it clear he believes at least some of the opposition to nuclear power is ill-informed.
Global energy needs are going to increase and people need to intelligently weigh risks.
"Outdoor air pollution, mostly from fossil fuels, is killing — according to the World Health Organization —3.7 million people per year. That's 10,000 per day," said Hansen.
"If there was nuclear power accident that killed 10,000 people, you would close every nuclear plant, yet we're killing that many every day by burning fossil fuels."
"There's a communication problem and an understanding problem."
The current generation of global leaders, however, can no longer pretend it doesn't understand what the burning of fossil fuels is doing to the planet, he said.
During a question-and-answer session, Hansen said more research needs to be done on the effects of low-level radiation.
"Life developed on a planet bathed in low-level radiation," he said.
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