Not so many years ago, schoolchildren were taught that carbon dioxide is the naturally occurring lifeblood of plants, just as oxygen is ours. Today, children are more likely to think of carbon dioxide as a poison. That's because the amount of carbon dioxide in the atmosphere has increased substantially over the past one hundred years, from about 280 parts per million to 380. But what people don't know, the IV scientists say, is that the carbon dioxide level some 80 million years ago--back when our mammalian ancestors were evolving--was at least 1,000 parts per million. In fact, that is the concentration of carbon dioxide you regularly breathe if you work in a new energy-efficient office building, for that is the level established by the engineering group that sets standards for heating and ventilation systems. So not only is carbon dioxide plainly not poisonous, but changes in carbon-dioxide levels don't necessarily mirror human activity. Nor has atmospheric carbon dioxide been the trigger for global warming historically: ice-cap evidence shows that over the past several hundred thousand years, carbon dioxide levels have risen after a rise in temperature, not the other way around.