COPENHAGEN, Denmark - World business leaders added momentum yesterday to prospects for a new U.N. climate treaty by agreeing that the world must cut emissions of greenhouse gases in half by mid-century through specific limits on carbon.

Government officials reported little progress in setting specific limits, however, showing that a new treaty remains distant.

About 500 chief executive officers and other top business experts said at the conclusion of the three-day World Business Summit on Climate Change that "immediate and substantial" emissions cuts were needed by 2020. Though the amount of those reductions was not specified, the conference said they should be followed by cuts of at least 50 percent of 1990 levels by 2050.

They said governments should use the marketplace to set a global price on carbon instead of taxing it, according to a statement from conference organizers.

Market-based reductions are generally called cap-and-trade, a system under which a government establishes a market for carbon dioxide emissions by selling credits to companies that emit greenhouse gases.

Companies then have a choice: Invest in technologies to reduce emissions to reach a certain target, or buy credits from other companies that already have met their emission-reduction goals.

But there is widespread dispute over the details of how cap-and-trade would work, with critics saying it would lead to higher costs for some consumers and could hurt bruised economies.

The business leaders said governments' overriding aim at a planned December U.N. meeting in Copenhagen on replacing the 1997 Kyoto Protocol should be limiting the global average rise in temperature to a maximum of 2 degrees Celsius.

Global temperatures have risen 0.22 degrees (0.12 degrees Celsius) since 1990, according to one U.S. government estimate. The United Nation's chief panel on climate change estimates that the risk of increased severe weather will rise if the global average temperature increases between 1.8 degrees Fahrenheit (1 degree Celsius) and 3.6 degrees (2 degrees Celsius) above 1990 levels.

The 1997 Kyoto Protocol's mandatory cuts in greenhouse gases, which have produced mixed results, expire in 2012.

The United States never signed on to Kyoto, citing the costs to the economy and lack of participation by China, India, and other fast-developing countries. But some of those countries have said rich countries are not aggressive enough in cutting their own emissions.