SAN FRANCISCO, CA., DEC. 11, 2013 — James Hansen spoke to a packed house on Wednesday at the American Geophysical Union meeting in San Francisco, a day after his scheduled talk was cancelled. He had inadvertently overscheduled, he said, and reassured the audience he was “healthy as a horse.”

“For the fossil fuel people in the audience,” he said, “I intend to be around for a long time.”

Perhaps no one in the world speaks with more authority about climate change than Hansen, recently retired as the Director of NASA’s Goddard Institute of Space Studies. There he discovered much of the science underpinning our modern understanding of man-made climate change, and it was there he fought back against the many forces that sought to blunt and deflect his message.

In a talk here titled “Minimizing the human-made influences of climate change,” Hansen repeated his message that the best the world can do now is to avoid creating dangerous levels of climate change.

That means, in Hansen’s view, keeping the planet’s polar ice sheets from melting, with their attendant changes in sea level that will mean a changing coastline for the next several centuries; avoiding mass extinctions such as those that have accompanied past episodes of climate change like the Paleocene-Eocene Thermal Maximum (PETM); and avoiding a world of weather extremes that would see enhanced frequencies of heat waves, drought, fires, heavy rain, floods, and stronger storms.

Hansen to fossil fuel interests in the audience: ‘I intend to be around for a long time.’

Carbon dioxide is the big issue in climate change, Hansen said, because it lasts for so long — the carbon cycle takes on order of 100,000 years to sequester our emissions into an earth-locked carbonate form. Past climates do not give any reassurance; the Pliocene of about 3 millions years ago was 3 to 4 degrees Celsius warmer than today, with sea levels 15 to 25 meters higher.

The pressure on nonhuman species is unprecedented, Hansen said. Plant and animal species are being constrained by losses of habitat at the same time climate change is forcing these habitats to shift, a combination that has no analog in Earth’s past.

Hansen emphasized the message of his recent paper in PLOS One: Society has already burned most of its allotted carbon, if temperatures are to remain near those where humanity has flourished during the last 10,000 years of the Holocene. In Hansen’s view, this means a necessarily quick phase-out of coal use over the next few decades, and that all unconventional fossil fuels, like shale oil and gas, should stay in the ground.

Hansen’s favored tool for meeting these objectives is a carbon tax-and-dividend, where an escalating price on carbon pollution lets the market decide how to reduce emissions in the cheapest way possible. If all the tax so collected were returned to U.S. citizens on an equal per capita basis, 60 percent of Americans would actually make money, said Hansen, collecting more than they pay. Economic studies estimate that U.S. carbon emissions could be reduced by 30 percent in just 10 years.

Moreover, a carbon tax is “the one viable international approach that only requires the major players” like China and the U.S., Hansen said, since carbon taxes imposed on imported goods at those borders would immediately incent smaller countries to impose an equivalent tax at their own borders.

Hansen stressed that the world cannot make meaningful carbon reductions without increasing its use of nuclear power. In some of the most intriguing minutes of his talk, Hansen played a four-minute video clip from the 1979 “No Nukes Concert” in New York City, where celebrity singers like Jackson Browne and Carly Simon, and later Ralph Nader, sang, drummed, and spoke against the use of nuclear power.

Yet, Hansen said, more people die of coal-related cardiac and respiratory illnesses in one day than have died in the entire history of the world’s use of nuclear power. “You could almost call the anti-nuclear movement a quasi-religion,” Hansen said.

Turning directly to his audience’s strength, Hansen said that “we need to shine the light of science” onto the options that society has chosen to follow, or is looking to follow from here. Renewable energies like solar power and wind power need to be part of the solution, he said, “but are not sufficient.”

Hansen concluded, “We’re running out of time.”

That is not a message that many people at this AGU meeting would doubt. But neither is it a message that many here would think is getting through. Governments, Hansen complained, are still serving the fossil fuel industries, talking out of both sides of their mouth, while the problem gets worse each and every year.

And until that changes, climate will not stop changing either.