http://www.democracynow.org - In the history of the planet, there have been five known mass extinction events. The last came 65 million years ago, when an asteroid about half the size of Manhattan collided with the earth, wiping out the dinosaurs and bringing the Cretaceous period to an end. Scientists way we are now experiencing the sixth extinction, with up to 50 percent of all living species in danger of disappearing by the end of the century. But unlike previous extinctions, the direct cause this time is us — human-driven climate change. In The Sixth Extinction: An Unnatural History, journalist Elizabeth Kolbert visits four continents to document the massive "die-offs" that came millions of years ago and those now unfolding before our eyes. Kolbert explores how human activity — fossil fuel consumption, ocean acidification, pollution, deforestation, forced migration — threatens life forms of all kinds. "It is estimated that one-third of all reef-building corals, a third of all fresh-water mollusks, a third of sharks and rays, a quarter of all mammals, a fifth of all reptiles, and a sixth of all birds are headed toward oblivion," Kolbert writes. "The losses are occurring all over: in the South Pacific and in the North Atlantic, in the Arctic and the Sahel, in lakes and on islands, on mountaintops and in valleys."