To prevent runaway climate changes, governments must change fossil fuel policies, said world-renowned climatologist James Hansen to a packed house at Cornell April 19. "Politicians say we have a planet in peril, and we are going to take actions to deal with it, but in fact we are continuing with business as usual," he added.

Hansen, director of NASA's Goddard Institute of Space Studies who has repeatedly testified before Congress on the dangers of climate change, delivered the 2010 Jill and Ken Iscol Distinguished Environmental Lecture in Kennedy Hall. He published a book on climate change, "Storms of My Grandchildren," last year.

Large, wealthy energy companies have resisted changes, yet they don't pay for societal costs from burning these fuels, he said: "As long as fossil fuels are the cheapest forms of energy, then we are going to keep using them."

Hansen recommends taxing carbon when a fuel is first sold and giving the money to the public. For example, a carbon tax equal to $1 per gallon of gas collected at the first sale of a fossil fuel would equal about $3,000 per year in dividends for each legal adult resident. Such dividends would stimulate the economy and could encourage development of cheaper alternative energies and more sustainable practices. Such a "fee and dividend" strategy would make shipping food across the globe more expensive, favoring produce from local farms.

Hansen said that this strategy could substitute for complicated cap and trade schemes, where companies pay for carbon credits; these are designed by companies for their own benefit, he said, and will not change energy systems.

Hansen also is an advocate for "fourth-generation nuclear power," which is nearly ready for use and would burn 99 percent of nuclear fuel and even nuclear waste, compared with current reactors that "leave 99 percent of nuclear fuel unburned" as waste. With the new technology, we now have enough extracted nuclear material from waste and weapons "to power all of our energy needs for a thousand years," he said.

Hansen said that despite growing public skepticism the past year, evidence of human-caused climate change is clearer than ever, and that the Earth's climate is approaching "tipping points, beyond which the dynamics of the system begin to take over and changes proceed that are out of our control," he said.

That evidence includes warming oceans, melting ice shelves and glaciers, rising sea levels, shifting climatic zones and more acidic oceans, which dissolve the shells of tiny but vital sea creatures. He also noted that in the past, shifts in climate zones have led to mass extinctions of half of the planet's species, requiring millions of years for new species to evolve, he added.

Climate change is partly caused by burning fossil fuels, resulting in a buildup of atmospheric carbon dioxide (CO2), which is now at 389 parts-per-million. "We're going to need to get it back below 350 parts-per-million, and probably somewhat lower than that, if we want climate to stabilize," he said. "And that's possible, and there are benefits to doing that, but we are not on such a track."

Climate skeptics often claim that periods of global warming are natural, such as 50 million years ago when "there were alligators in Alaska and there was no ice on the planet," Hansen said. But those changes occurred over several million years; the rate that humans are changing climate is "10,000 times … the natural rate of change. We humans are now in charge of future climate," he said.