The petroleum age began about 150 years ago. Easily available energy has s- ported major advances in agriculture, industry, transportation, and indeed many diverse activities valued by humans. Now world petroleum and natural gas s- plies have peaked and their supplies will slowly decline over the next 40 50 years until depleted. Although small amounts of petroleum and natural gas will remain underground, it will be energetically and economically impossible to extract. In the United States, coal supplies could be available for as long as 40 50 years, depending on how rapidly coal is utilized...