Why Does Al Gore Refuse to Debate Climate Change?

At a recent economic summit featuring Danish scientist Bjorn Lomberg, Al Gore refused an offer to debate the premises of his worldview on climate change with Mr. Lomberg because:

The scientific community has gone through this chapter and verse.

Lomberg supports Gore's belief that global warming is real and man made, but disagrees that regulating and eliminating CO2 emissions is the best way to help the environment. If global climate change really constitutes a global emergency worse than the war on terrorism, why won't the former vice president debate the validity of his viewpoint?