The costs could also drop significantly if the world reaps other benefits from reduced emissions, including healthier air, greater energy security as reliance on foreign oil drops and export opportunities as newly developed technology is sold and adopted elsewhere, the reports authors said.

"The good news is there's substantial mitigation potential available at reasonable cost," said Jean Bogner, a landfill expert who was one of the contributors to the report. "And the positive message is it's not just one single solution, not just nuclear or reducing coal use. It's that in many sectors there are opportunities for positive contributions to reducing greenhouse gas emissions."

The report, the third in a series of UN climate change studies this year, is the first aimed at analyzing solutions to the problem. It lays out a wide range of options-including behavioral changes-that could be useful in cutting greenhouse gas emissions, and tries to analyze the costs of making the changes.

It suggests major drops in emissions are possible through switching to natural gas rather than coal as source of electricity, using hybrid and fuel-efficient vehicles, incorporating active and passive solar design into buildings, using more insulation and energy-efficient appliances in homes, improving industrial energy efficiency and encouraging the use of renewable energy sources.

Capturing and storing carbon dioxide emissions also could lower atmospheric concentrations of the gas, the authors said.

Nuclear power, a controversial alternative to fossil fuels, probably will not gain wide acceptance as the world searches for ways to cut carbon emissions, if only because of its high cost compared with other options, the report said.

The document also examines a range of policy measures shown to be effective in promoting emissions reductions, including fuel, road and auto taxes to energy efficiency standards for appliances, subsidies for renewable energy, mandatory fuel efficiency standards, investment in public transportation and tax credits.

Voluntary agreements by industries to cut emissions, a favored measure in the United States, for the most part "have not achieved significant emissions reductions beyond business as usual," the report notes, though a few recent efforts have been exceptions.

Probably the easiest and cheapest way to cut greenhouse gas emissions, the report's authors said, is to focus on improving energy efficiency. But other keys will be ensuring that massive expected growth in developing nations, particularly China and India, is cleaner and more sustainable, in part through transfer of clean technology, and that people everywhere take a look at their lifestyles to see how they can cut consumption and emissions without waiting for politicians to act.