In Chapter 2, we detailed the overwhelming evidence that the Earth's climate is changing – evidence so overwhelming, in fact, that virtually no one disputes this anymore. Instead, much of the most heated argument is over the cause of the warming: Is it caused by human activity, or is it natural? In this chapter, we address this question.

Attributing the cause of a trend is more difficult than identifying the trend. Our strategy here is to examine the mechanisms that have changed climate in the past and examine each of them to determine if they could be the cause of the recent warming. You will see that a careful review of all of the possible causes of the recent warming yields the conclusion that the most likely explanation is the increase in greenhouse gases in our atmosphere, which we learned in Chapter 5 is due to human activity.