COPENHAGEN - Climate-change heavyweights UN chief Ban Ki-moon and Nobel prize winner Al Gore urged more than 500 business leaders on Sunday to lend their corporate muscle to reaching a global deal on reducing greenhouse gases.

The CEOs of PepsiCo, Nestle, BP and other of the world's major businesses began meeting in Copenhagen, where politicians will gather in December to negotiate a new UN-brokered climate treaty.

Despite the global financial crisis, both Ban and Gore said there was no time for delay in hashing out the specifics of how to cut greenhouse gases that contribute to warming the planet.

"We have to do it this year. Not next year. This year," Gore said. "The clock is ticking, because Mother Nature does not do bailouts."

The three-day World Business Summit on Climate Change is a precursor to the negotiations to determine what will succeed the Kyoto climate treaty that expires in 2012.

"Continuing to pour trillions of dollars into fossil-fuel subsidies is like investing in subprime real estate," Ban said. "Our carbon-based infrastructure is like a toxic asset that threatens the portfolio of global goods, from public health to food security."

A new global warming treaty would build on the Kyoto treaty's mixed success in requiring that 37 industrialized nations reduce greenhouse gas emissions an average of 5 per cent below 1990 levels by 2012.

Gore said any of the ambitious treaty goals being discussed will depend on CEOs working out greener ways of doing business and governments reining in unrestricted pollution.

"The business community and the leaders of the world must go together to safeguard the world," he told a forum that drew even Queen Margrethe of Denmark.

About 300 anti-globalization activists marched Sunday toward a convention center, which was heavily guarded by police. No arrests were reported.

Erik Rasmussen, founder of the Copenhagen Climate Council that sponsored the meeting, said business leaders were discussing specific and binding targets for reducing greenhouse gases within 10 years and 20 years that would be announced at the end of the conference.

Anders Eldrup, CEO of Danish state-controlled oil and gas group DONG Energy, said businesses faced a big choice.

"There are two tracks being discussed now, one a tax on CO2 and a cap-and-trade," he said, leaning toward the carbon tax.

However, Connie Hedegaard, the Danish minister for climate and energy, told The Associated Press the most workable solution would be global limits on the pollution blamed for global warming, rather than an outright tax on carbon dioxide and other major industrial warming gases.

Hedegaard urged businesses to back such limits, called cap-and-trade, which call for governments to issue pollution allowances, or permits, to businesses that could be traded on the open market.

"I would hope that they would sort of agree that some kind of cap and trade will be the most efficient tool to achieve what science tells us what we must achieve," she said. "A carbon tax -- you can just pay that tax -- but you must also have the caps so that you start innovating from there."

An emissions trading plan advanced in the U.S. Congress last week, increasing the likelihood that the full House of Representatives will for the first time address broad legislation to tackle climate change later this year.

Gore predicted it would pass the House, gain Senate approval and be signed into law by President Barack Obama.

The United States has said it is committed to reaching a deal in Copenhagen as long as other major polluters such as China and India do their part as well.