If the coal and oil companies keep misleading the public about their climate-wrecking pollution, he said, his grandchildren would inherit a planet that is dangerously out of control.
Of all people, Hansen should know. Based at the Nasa Goddard Centre for Space Studies and Columbia University in New York, he is one of the world's leading climate scientists, and has played a major role in alerting governments to the risks over the last three decades.
"In the last few years the fossil-fuel industry and its supporters have done a very good job in confusing the public," he told the Sunday Herald. "With their resources, it's not too difficult. Money talks."
He pointed out that, globally, governments were subsidising the fossil fuel industry to the tune of between $400 and $500 billion a year (£250-£320 billion). "The public is unaware of the situation," he said.
Unless the world acted now to start rapidly reducing fossil-fuel emissions, he warned, future generations would be hit by huge cyclonic storms wreaking havoc over large parts of the globe, as well as increasingly severe droughts and floods.
Hansen, who has just turned 71, is coming to the Edinburgh International Science Festival this week to receive the festival's annual award for major contributions to the "well-being of humanity".
He will talk about a study he has recently completely with 17 other scientists setting out the case for urgent action to cut climate pollution. It was due to be published by the US National Academy of Sciences, but has run into a last-minute hitch because of an objection to its conclusions.
The study concluded that tackling climate change was a matter of morality and "intergenerational justice" rather than economics. "As with the earlier great moral issue of slavery, an injustice done by one race of humans to another, so the injustice of one generation to all those to come must stir the public's conscience to the point of action," it said.
Oil companies denied they had misled people, and stressed the transition to a low-carbon economy "cannot happen overnight".
"The world is paying lip service to the climate-change issue," Hansen said in a telephone interview from New York. "We have to make fuels pay for their true cost, rather than subsidising them."
Perhaps surprisingly, he is optimistic the truth about the fossil fuel industry will win out. "If the public realise what is happening, they will see through the attempts to mislead them," he argued. "But we're running out of time."