Mayors from around the world have signed a voluntary pact in Mexico City to reduce greenhouse gas emissions at a meeting meant as a precursor to next week's s UN-sponsored talks in Cancun.The gathering in one of the world's most polluted cities assembled some 3,000 local and regional leaders to discuss a wide range of economic and social issues, including climate change.Participants from some 135 cities and urban areas on Sunday signed a pact committing them to adopt a slate of measures to stem climate change. The pact will be presented at next week's UN climate talks in Cancun, Mexico from November 29 to December 10.Top climate scientists from around the world hope in Cancun to break the deadlock on reducing greenhouse-gas emissions and channeling aid to poor, vulnerable countries after the widely regarded failure of the last climate summit in Copenhagen.Mexico City Mayor Marcelo Ebrard and Paris Mayor Bertrand Delanoe, the current president of the United Cities and Local Governments opened the mayoral gathering, set to last four days. Mexico City last week pledged to reduce its annual greenhouse gas emissions by around 14 per cent.Meanwhile, a new study released on Sunday found that fossil-fuel gases edged back less than hoped in 2009, as falls in advanced economies were largely outweighed by rises in China and India. The global decrease was less than half that had been expected, because emerging giant economies were unaffected by the downturn that hit many large industrialized nations.