In "What's Up with the Weather?" NOVA and FRONTLINE join forces to investigate the science and politics of one of the most controversial issues of the 21st century: the truth about global warming.

The program charts how scientists agree on many key issues underlying global warming: so-called "greenhouse gases" such as carbon dioxide (CO₂) and methane trap radiation from the sun, thereby warming the earth. Worldwide measurements -- taken from trees, ice cores, coral reefs and air--show the concentration of these gases has increased by one-third since the Industrial Revolution. And, scientists also agree that the earth's surface temperature has increased one degree Fahrenheit over the 20th century (see graphs).

But a fierce debate centers on whether this warming is a natural phenomenon or the result of man's burning of fossil fuels like coal and oil, which increase the amount of CO₂ in the atmosphere. This NOVA/FRONTLINE report interviews leading scientists, policymakers and fuel industry representatives with sharply different viewpoints.

Those who believe human activity is contributing to global warming say the problem is compounded because most people are unaware of their own role in global warming--due to their dependency on fossil fuels for energy. It's a dependency which could affect the planet's fate, say greenhouse proponents.

"If the planet warms too much, we are going into unknown territory," says Tom Wigley, senior scientist for the National Center for Atmospheric Research (NCAR). "If we were to warm the world by five degrees, I strongly believe that large parts of the Antarctic ice sheet would flow into the ocean, melt, and cause sea level to rise by many meters."

That could mean disastrous flooding of coastal areas. Other scientists predict intense storms, droughts, famines, the spread of infectious diseases, and the wholesale destruction of species and habitats.

But others aren't so sure. A direct connection between man and the earth's warming has yet to be proven, say scientists such as Fred Singer, an atmospheric physicist. He suggests that the earth's increased temperature is due to natural causes and argues that increased levels of CO₂ in the atmosphere are actually a good thing.

Who's right? Scientists on both sides of the global warming debate are building sophisticated computer models to predict various climatic scenarios. But, the slightest change in one variable produces a range of possible climatic outcomes, resulting in even more speculation about global warming's real impact.

This scientific uncertainty fuels political uncertainty, as the United States struggles to decide what, if any, action to take. The 1997 Kyoto Protocol commits the United States and other industrial nations to significant cutbacks in greenhouse gas emissions. Critics argue that the cutbacks will harm the U.S. economy, particularly since developing nations have not agreed to cut their own growing use of fossil fuels.

"What Up With the Weather" concludes with a look at the possibilities--and challenges--in turning to non-carbon energy sources, including nuclear power and "renewable energies." Scientists also hold out hope for developing unimagined new technologies that will replace fossil fuels.

"There comes a point when you can't escape the idea that you're having serious climatic consequences," says Richard Somerville of the Scripps Institution of Oceanography. "The issue becomes one of whether we get wise before that day or whether we have to wait for some perhaps quite unanticipated climate surprise that wakes us all up."