 Three Democratic senators who are to lead powerful environmental committees in Congress urged President George W Bush on Wednesday to combat global warming by putting mandatory limits on greenhouse gas emissions. "Unfortunately, we have not been satisfied with the level of US participation in the international negotiations or in reducing our own domestic greenhouse gas emissions," the senators wrote in a letter that referred an international conference on global climate change in Kenya this week. The three lawmakers -- Barbara Boxer of California, Jeff Bingaman of New Mexico and Joseph Lieberman of Connecticut, who was re-elected as an independent but has identified himself as a Democrat -- urged Bush to work with them "to signal to the world that global warming legislation is on the way." The letter mentioned the elections last week that delivered Democratic majorities in both houses of Congress. "The recent elections have signaled a need to change direction in many areas, including global warming," the senators wrote. "If we are to leave our children a world that resembles the earth we inherited, we must act now to address GHG (greenhouse gas) emissions. "When the 110th Congress begins in January, we pledge to work to pass an effective system of mandatory limits on greenhouse gases." The Bush administration's representative at the Nairobi talks on global warming, Paula Dobriansky, rejected pleas by UN Secretary-General Kofi Annan on Wednesday for the United States to rejoin the Kyoto Protocol setting limits for participating countries on greenhouse gas emissions. Bush, who was traveling in Asia on Wednesday, withdrew from the Kyoto agreement in 2001, his first year as US president, saying it would cost US jobs and unfairly exempted developing countries from the emissions requirements. Boxer is the incoming chairwoman of the Environment and Public Works Committee, Bingaman is to chair the Energy and Natural Resources Committee, and Lieberman will chair the Senate Homeland Security and Governmental Affairs Committee.