Viewpoint: Yes, there is strong evidence that most of the global warming observed over the last 50 years is due to human activities.

Viewpoint: No, present global warming is not due more to human activity than to natural geologic trends.

Of all the issues relating to earth science, none is more politically volatile than that of global warming. Even the name is controversial, since it is not entirely clear, from the scientific data, that Earth really is warming. And even if it is, the causes are open to debate. Typically, when the term global warming is mentioned in the media, the implication is "global warming due to human activities," but this may only be part of the picture.

In the debate over whether...