Scientists know that the earth is warming and most of them acknowledge that it has warmed about one degree in the past hundred years. What many of them find alarming is how quickly that change has occurred. Throughout the planet's history there have been periods of warming and cooling, but the average rate of change has been about one degree per thousand years-so the current global warming appears to be happening ten times faster than ever before. Many scientists believe that this is the result of anthropogenic actions, which are caused by human beings rather than nature. Dr. Stephen H. Schneider, biological sciences professor at Stanford University, is one scientist who believes that people are to blame for the current global warming, as he explains:

Humans are not simply passengers holding a temporary ticket on planet Earth's ride through the galaxy. We are actively...