Such differences could occur particularly between countries that have ratified the Kyoto Protocol, such as Canada, Japan and those in the European Union, and countries that have pulled out of the pact: the United States and Australia.

Other differences include "safety valves," or limits, on the price of a ton of a carbon dioxide equivalent that some plans include, as well as whether emissions targets are outright cuts or "intensity based" limits.

Below is a list of emissions markets already functioning or being planned to trade the gases most scientists believe are warming the earth.

EUROPEAN UNION

The European Union launched the Emissions Trading Scheme (ETS) in January in which outright emissions from about 12,000 industrial plants are capped. Under the mandatory plan, the EU aims to reduce emissions by 8 percent below 1990 levels by the end of 2012. Carbon dioxide equivalent is on the ETS trading at about 20 euros per ton.

NORWAY, SWITZERLAND

Setting up trading plans soon that could link to the ETS.

CANADA

Plans to launch a greenhouse market in 2006 that would have "intensity based" emissions targets.

The plan has a "safety valve" that would cap the price of ton of carbon dioxide equivalent at C$15 until 2012.

JAPAN

Under the first phase of the Kyoto Protocol, Japan is the only Asian country required to cut emissions by 2012. The government currently allows industries to set voluntary targets on emissions cuts.

UNITED STATES

The George W. Bush administration, which pulled out of the Kyoto Protocol, favors voluntary cuts over mandatory cuts. However several emissions trading plans are developing.

1) Regional Greenhouse Gas Initiative. Nine states in the Northeast US planning to launch market by 2009. Would cut power plant emissions of global warming gases by 10 percent by 2020 .

2) US Senator Jeff Bingaman, a New Mexico Democrat, is sponsoring bill that would cut emissions modestly. The bill's "safety valve" would cap the price for a ton of carbon dioxide equivalent at $7.

3) US Senators John McCain, an Arizona Republican, and Joe Lieberman, a Connecticut Democrat, are sponsoring a bill for a national cap and trade program, would cut emissions modestly.

4) Chicago Climate Exchange. Voluntary trade launched in 2003.

AUSTRALIA