Scientists generally agree that the globe has warmed over the past 40 years, due largely to human activities that raise carbon-dioxide levels in the atmosphere. The Kyoto Protocol mandating limits on carbon emissions took effect in 2005, eight years after it was written. But the United States — the world's biggest carbon emitter — has not ratified the treaty. Debate over global warming has shifted from whether human activities are causing climate change to whether the possible changes will be severe enough to justify the hefty expense of developing cleaner-energy technologies. Economists and even some energy companies have recently proposed taxing carbon as an incentive to consumers and industry to shift to low-carbon fuels. Some multi-state coalitions also hope to issue tradable emissions permits to industry. Congress has begun to show some interest, but the Bush administration still argues strongly against any mandates to cut carbon-fuel use.