Former President Bill Clinton told more than 100 mayors Thursday that stopping global warming depends on them demonstrating that it makes economic sense. He said his foundation is teaming up with Wal-Mart Stores Inc. to save cities money on environmentally friendly supplies by buying in bulk.

"We will not get a global agreement on climate change unless you can prove this is not a burden," he said. "This is the greatest opportunity we have had in our lifetimes."

The Clinton Foundation has previously worked with 40 of the world's largest cities to create a buying pool to bring down prices for green supplies such as hybrid vehicles and more efficient street lights. It's the same approach the foundation used to dramatically cut the price of AIDS drugs in Africa.

In addressing a climate summit organized by the U.S. Conference of Mayors, Clinton announced that the 1,100 cities represented by that organization will become part of the purchasing group. Wal-Mart, the nation's largest retailer, said it would work with Clinton's foundation and the cities to bundle orders and product specifications for green technology.

Clinton and many of the mayors present criticized the White House, saying it has done little about global warming and has missed a chance to boost the nascent "green collar" economy — the jobs created by making the U.S. more sustainable, from the people who install solar panels to scientists who develop new technologies.

Wal-Mart, which has embarked on a broad environmental drive to cut costs and burnish its reputation, is offering to help the mayors as it has met resistance in some big cities, including New York and Chicago, to its plans to expand into metro areas from its rural and suburban base.

Wal-Mart has set targets for reducing energy use and packaging waste and selling more environmentally friendly products. Steps include switching to only concentrated liquid laundry detergent that reduces packaging and water use, converting its truck fleet to use less fuel, and asking suppliers to provide data on their greenhouse gas emissions.

But the company isn't the only one whose reputation is at stake. Clinton asked the mayors to think about their legacy and to keep score from an environmental standpoint. "The only way to keep score in public life is whether people are better off when you quit than when you started," he said, telling them the challenge is "your kind of deal."

Cities cover just 2 percent of the planet's land but are responsible for three-quarters of its greenhouse gas emissions — and therefore present the greatest opportunity for reducing those emissions, Clinton said. Much of that progress can be made by picking the low-hanging fruit: replacing wasteful light bulbs with high-efficiency ones, finding the leaks in the water-supply system, capturing the harmful methane produced by landfills and turning it into electricity.

The big orders from cities for more efficient heating and cooling systems for public buildings, ultra-efficient LED lights, or hybrid buses guarantee income for the companies that make them, help bring prices down and create jobs, Clinton said. They also save the cities money on energy.

Earlier in the day, former Vice President Al Gore, winner of this year's Nobel Peace Prize for his climate work, spoke to the mayors by satellite from Tennessee and urged them to continue their work. He told them that his audiences around the world are heartened to learn that while the White House refused to support the Kyoto Treaty, 710 U.S. mayors — led by Seattle's Greg Nickels — have signed an agreement to abide by the treaty's call to reduce carbon emissions.

The two-day summit is designed to allow mayors from around the country to share ideas about how to combat climate change locally. New York Mayor Michael Bloomberg was scheduled to deliver Friday's keynote address.