Upon arriving in office in 2001, President Bush withdrew United States support of the then-pending Kyoto Protocol, a UN Convention seeking to impose mandatory targets for reducing "greenhouse gas" emissions. Bush stated that human activity had not been proven to be the cause and cited concerns about the treaty's impact on the U.S. economy and pointed out that China and India had not signed on. The Protocol entered into force on 16 February 2005. As of September 2011, 191 states have signed and ratified the protocol. The only remaining signatory not to have ratified the protocol is the United States.