Hybrid car sales by year: Alternative Fuels and Advanced Vehicle Data Center; U.S. Department of Energy.
Number of electric bicycles in China: “An Electric Boost for Bicyclists,” The New York Times, January 31, 2010.
Increase in wind energy generated in the United States: U.S. Energy Information Administration, U.S. Department of Energy.
Annual savings from the use of compact fluorescent bulbs: Energy Star, U.S. Environmental Protection Agency.
Acres of arable land per person in the United States and China: World Development Indicators, The World Bank.
Amount of solid waste by product (recovered and not recovered): Municipal Solid Waste Generation, Recycling, and Disposal in the United States: Facts and Figures for 2011, U.S. Environmental Protection Agency.
Average gasoline tax in the United States: American Petroleum Institute.
Average gasoline tax in Britain: Fuel Duty Rates, HM Revenue & Customs.
European Union Atlantic cod catch by year: European Commission, EU Market Observatory for Fisheries and Aquaculture Products.
Wind generation capacity by state: AWEA 4th Quarter 2012 Public Market Report, American Wind Energy Association.