The Most Important Number on Earth

Sooner or later, you have to draw a line. We've spent the last 20 years in the opening scenes of what historians will one day call the Global Warming Era-the preamble to the biggest drama that humans have ever staged, the overture that hints at the themes that will follow for centuries to come. But none of the notes have resolved, none of the story lines yet come into clear view. And that's largely because until recently we didn't know quite where we were.