After years of inactivity in regulating so-called greenhouse gases (GHGs), U.S. policy may be on the verge of doing so. In April 2007, the U.S. Supreme Court ruled that the federal government was authorized to regulate GHG emissions from human activity, which some believe accelerate warming of the earth’s atmosphere, causing disruptive and costly climate changes. Carbon dioxide is the major source of such GHG emissions, making up 75–80 percent of the total volume. This fall, the U.S. Congress is expected to consider bills to control GHGs. Regionally, state and local governments are already acting to reduce GHGs or curb their growth. Most notably, California proposes to reduce emissions by one-third from 2004 levels by 2020. According to this plan, such reductions will be achieved by requiring more fuel-efficient cars and buildings and by requiring that the state’s electricity is generated from renewable energy sources and less carbon-intensive fuels.