evidence: Man is also pumping smaller quantities of other greenhouse gases into the atmosphere. Nitrous oxide, the chlorofluorocarbons—which are notorious for their ability to destroy the planet’s ozone layer—and several more all trap warmth with greater efficiency than carbon dioxide. Methane and the rest of these gases, even though their concentrations are small, will together account for fifty per cent of the projected greenhouse warming. They are as much of a problem as carbon dioxide. And as all these compounds warm the atmosphere it will be able to hold more water vapor—itself a potent greenhouse gas. The British Meteorological Office calculates that this extra water vapor will warm the earth two-thirds as much as the carbon dioxide alone. Most discussion of the greenhouse gases rushes immediately to their future consequences, without pausing to let the simple fact of what has already happened sink in: the air around us—even where it’s clean, and smells like spring, and is filled with birds—is significantly changed. We have substantially altered the earth’s atmosphere.