The United States leads the world in total carbon dioxide emissions from fossil fuel combustion, with nearly a quarter of the world’s total. A recent report by the National Academy of Sciences concludes that "human activities are responsible for much of the recent warming" of the planet. With both greenhouse gases and gas prices rising, some of the nation’s biggest companies are being encouraged to embrace environmentally friendly initiatives. Meanwhile, the Kyoto Protocol has come into force, having been ratified by more than 140 countries – although not the U.S. What are your ideas for breaking our dependence on fossil fuels, especially from foreign sources? Your e-mails