"NOVA: Dimming the Sun" (8 p.m., PBS): The dismal prospect of global warming is overshadowed by global dimming in a "NOVA" investigation. "Dimming the Sun" reports that sunlight reaching smog-shrouded

"NOVA: Dimming the Sun" (8 p.m., PBS): The dismal prospect of global warming is overshadowed by global dimming in a "NOVA" investigation. "Dimming the Sun" reports that sunlight reaching smog-shrouded Earth has been growing dimmer, and this global dimming has led to a cooling that has masked more than half of the greenhouse effect over the past century. Bottom line: Calculations by scientists are thrown out of whack and global warming is even greater than they feared. Many scientists believe we are at the environmental tipping point, "NOVA" reports, and if they're right, we may be at the brink of a climatic catastrophe the likes of which the planet hasn't suffered in its four billion years.
"Journey to Planet Earth" (9 p.m., PBS): A new edition of "Journey to Planet Earth" poses this critical question: Why are nearly half of the world's wildlife species facing extinction within the next few decades? "The State of the Planet's Wildlife" is a global investigation of what scientists call "the sixth great extinction of the world's animals and plants," as a result of such factors as climate change, population and poverty pressures, poaching and the loss of wildlife corridors. The last great extinction happened 65 million years ago, when a giant asteroid crashed into Earth and sealed the fate of dinosaurs along with more than 75 percent of the world's other animals and plants. The next extinction, if it happens, is unlikely to come from outer space -- it will be the result of human deeds. Hosted by Matt Damon.