Beneath our advanced 21st-century economy lies a smoke-belching 18th-century economy. For all our sophistication, we still depend on fossil fuels dug from the earth to power our homes and offices. And it is now abundantly clear that this dependence is becoming a lethal threat. From the burning of coal and gasoline, we release into the atmosphere toxic mercury, acidic sulfur and nitrogen oxides, and particulate matter that produces choking smog and causes asthma and other respiratory sicknesses. But more dangerous, because less noticeable, is the invisible gas carbon dioxide, which is released in vast quantities, billions of tons per year, by the burning of all fossil fuels.