By Timothy Gardner

NEW YORK (Reuters) - More than 20 U.S. cities, including New York, Las Vegas and Denver, have agreed to measure their carbon footprints, with a system some 1,300 companies have been persuaded to use, in an attempt to find ways to curb emissions blamed for warming the planet.

"If you don't measure these emissions, you cannot manage them," said Paul Dickinson, the chief executive of the UK- based Carbon Disclosure Project, which joined forces with the cities. Urban traffic, buildings and manufacturers emit 70 percent of the world's greenhouse gases.

Each of the 21 cities will gather emissions data for their municipal functions, such as their fire and police departments, government buildings and waste services, which will help cities compare how they are doing. They will also assess emissions from the city as a whole.

"Working together, and with the best data, we can manage this problem," New York Mayor Michael Bloomberg said in a release.

The CDP, which represents 385 global institutional investors that manage a total of more than $57 trillion in assets, has gathered corporate emissions data through surveys since 2000. It says it has collected the largest corporate greenhouse gas emissions database in the world.

CDP also assists multinational organizations to collect climate change data from their suppliers.

Earlier this year, more than 20 of the world's largest companies, including IBM, Nestle SA, and Tesco, with a combined purchasing power of about $1 trillion, found that only a quarter of their suppliers had greenhouse gas reduction targets, according to a survey coordinated by CDP.

Dickinson said once the cities discover their biggest sources of emissions, emerging energy-efficiency companies should swoop in and find ways for them to save emissions and money by slowing the waste of fuel. "The process should really lead to the beginnings of a fundamental restructuring of how cities consume energy," he said.