America needs a carbon tax – and not just a cap-and-trade system – to force cutbacks in greenhouse gas emissions that could otherwise threaten life on earth, said noted climate scientist James Hansen on Wednesday.

And to make it politically feasible, the money collected under the tax can be given back to low polluters.

"You're going to have to put a price on carbon emissions, or the goals and the caps are just not going to work," Hansen, the head of the NASA Goddard Institute for Space Studies, told scientists Wednesday at the American Geophysical Union's fall meeting in San Francisco.

"In order for people to accept a carbon tax, you're going to have to give the money back to the public on a per-capita basis, so the person who does better than average in reducing his emissions will get a payment," he said.

A tax-and-pay strategy would also prompt entrepreneurs to create more energy-efficient products, he noted.

"If you have a 100 percent dividend on that carbon tax, people will have money in their pockets to buy these new products," he said.

Hansen has described his "tax-and-dividend" plan on his Website, directing his policy recommendations to President-elect Barack Obama, who has said he favors a carbon cap-and-trade system to reduce America's greenhouse gas emissions to their 1990 levels by 2020.

Under cap and trade, like the one in place in Europe and the one being considered for the United States, businesses would buy permits to emit carbon (see Carbon Tax a Better Idea? and U.N. Climate Talks Pose Big Impact on Greentech). Some level of emissions invariably is free and some companies may find it more convenient to pollute, some fear.

Hansen, who has been sounding public alarms over climate change for decades, also said that governments should set a goal of reducing atmospheric carbon dioxide levels to 350 parts per million. That's lower than current atmospheric estimates of about 385 parts per million, and below the 450-parts-per-million goal set by the United Nations' Intergovernmental Panel on Climate Change last year.

To do that, the world will have to cease all emissions from burning coal by 2030, either by abandoning the fuel entirely or finding ways to capture and store the carbon released by its burning (see Canada to Beat U.S. to Carbon Storage and Vattenfall to Trap Carbon Emissions). Left unchecked, carbon emissions could climb above the 500 level causing a 2 plus degree Celsius hike in average global temperatures.

Otherwise, "If we burn all of the coal [on the planet], there is a good chance we will initiate the runaway greenhouse effect," he said. That runaway greenhouse effect could become unstoppable, eventually boiling the oceans and destroying all life on earth in what Hansen called the "Venus Syndrome," after the conditions that exist on the planet next-closest to the sun.

"We already probably have CO2 past the tipping level that would cause some effects like the loss of arctic sea ice," Hansen said (see Global Warming Threatens Arctic Feedback Loops). "But we haven't passed the point of no return."

But to avoid further melting of ice sheets in Greenland and the Antarctic and thus stabilize sea levels – now predicted to rise up to a half-meter over the course of the next 100 years – "clearly CO2 would have to be less than its present amount," he said in defense of his 350-parts-per-million goal.

Hansen has long been critical of government promises to combat climate change. In 2005 he went public with claims that Bush administration political appointees at NASA had sought to restrict his public statements on global warming.

In his Wednesday speech, Hansen said that efforts to reduce greenhouse gas emissions through the Kyoto Protocol have failed, noting that emissions have continued to rise in countries that signed on to the protocol.

"Even our best friends, and the most serious, are not doing anything close to what's needed," he said of government efforts.

Beyond phasing out all emissions from coal by 2030, Hansen said reforestation, better agricultural practices aimed at capturing more carbon, and research into "fourth-generation" nuclear power that can use the waste now produced by light-water nuclear reactors as fuel should also be part of the solution.