The nation's leading climate scientist had a pointed suggestion for policymakers interested in reducing carbon dioxide emissions: no more coal plants.

"We need politicians with the guts to say that, but I don't see that," said James Hansen, director of NASA's Goddard Institute for Space Studies, who was in town this week for a speech at the Science Museum of Minnesota in St. Paul. They "say the right words and set goals to reduce emissions, but their actions prove they don't mean it."

Coal is the largest remaining source of carbon-based fuel, so the solution is either to leave it in the ground or figure out how to store the gas emissions, Hansen said.

Hansen, who has claimed his findings were tempered by officials in the Bush administration, accepted the invitation to speak from Will Steger, a polar explorer and activist calling attention to the impact of global warming.

In an interview, Hansen said tax- or market-driven efforts to reduce carbon dioxide emissions will be too little and too late to slow the chief cause of global warming. Half of the CO₂ emitted to the atmosphere today will be absorbed, largely by oceans, in 25 years, but it would take 1,000 years to absorb the rest, said Hansen.

Many states and several regional groups, such as the Midwest Governors Association, are working to set up "cap and trade" systems to establish declining emissions limits over decades, primarily for industries. Participants could buy and sell emissions credits.

Minnesota Gov. Tim Pawlenty has been a driving force behind a regional cap-and-trade system, and both major-party presidential candidates also favor the cap-and-trade strategy. Fresh Energy, a St. Paul-based group that advocates cap-and-trade and other market-driven responses to climate change, was one of the sponsors of Hansen's appearance.