For most of the period we call human civilization, the amount of carbon dioxide in the atmosphere hovered at about 275 parts per million. In the late 18th century, we started burning fossil fuel, and that number started to rise. The first time we actually measured it, in the late 1950s, it was already about 315. Now it's at 385, and growing by more than 2 parts per million annually- well past the tipping point. |