BINDING CAPS

Under the UN's Kyoto protocol, about 40 developed nations have agreed to cut emissions of greenhouse gases -- mainly from power plants, factories and cars -- by 5.2 percent below 1990 levels by 2008-12. Developing nations reject such strict goals because their energy use is relatively low.

INTENSITY TARGETS

The United States aims plans to cut the amount of heat-trapping gases it emits per dollar of economic output by 18 percent in the decade to 2012. Some poor nations might accept similar goals, which are less strict than Kyoto-style caps.

SECTOR TARGETS

Big sources of greenhouse gases like the car, steel or cement industries could adopt international emissions standards.

TECHNOLOGY

Rich nations could invest in developing new technology, like ways to filter carbon dioxide from emissions from coal-fired power plants, and promise to share it with poor nations.

HIGHER EMISSIONS TARGETS

Developing nations could get goals for emissions, above current levels. If they managed to keep emissions below target they could sell their surplus greenhouse gases on an international market. If they failed, they would have to buy pollution permits. An alternative is a "no lose" target with no penalty for overshooting a target.

STAGED APPROACH