Guest: Stephen Schneider.

According to the National Academy of Sciences, the surface temperature of the Earth has risen by about a degree in the last 100 years. Many scientists argue that most of the warming over the last 50 years is due to human activity. Others say we don't have enough data to draw that conclusion. This morning on Focus, we're joined by Stanford biologist Stephen Schneider to explore the question of whether climate change is too uncertain for policy.