A "voluntary" electronic trading market in greenhouse gas emissions has launched in the US city of Chicago.
The scheme offers an innovative new approach to tackling global warming.
Trading should lower overall emissions
The Chicago Climate Exchange is designed to work like any other market place, but instead of trading in, say, oranges and apples, trades will be in amounts of greenhouse gases.
Companies that have high emission levels will offset these by buying permits from companies with low emission levels.
The overall effect should be lower total emissions - and, supporters of the project say, greater economic efficiency.
A similar trading scheme went live in the UK on 2 April 2002.
Twenty-two companies, including Ford and Motorola, have so far joined the scheme.
Its real significance is that it offers a potential solution to global warming which does not involve government-mandated emissions targets.
Such targets already exist in many countries but in the US the Bush administration has consistently opposed them and is a strong supporter of this market- based solution.
Environmentalists are divided over its validity.
Some welcome it as a positive experiment in applying market forces to environmental problems.
Others say the scheme is a public relations stunt designed to win companies green credentials and an attempt to pre-empt future moves towards regulation.