In the first quarter of this year, U.S. carbon dioxide (CO2) emissions reached a 20-year low, down 8 percent from the previous year, in part because of the increased use of natural gas instead of coal. Coal emissions alone fell 18 percent, the lowest level since 1986. Many power producers are shifting to natural gas, which is cheaper and more available. Natural gas emits about half the amount of CO2 that coal does. The Renewable and Sustainable Energy Institute warns against an energy strategy that relies on natural gas, however, saying that renewable forms of energy are critical solutions in addressing climate change (Worldwatch Institute, September 5).