The Half Degree That Will Change Earth

Something not so funny happened on the road to the United Nations climate summit in Paris last year.

After trying and failing for decades to reach an accord that limits global warming to less than 2 degrees Celsius (3.6 degrees Fahrenheit), world officials finally struck a deal in December cementing that goal. But the accord took an extra half step, "encouraging" an even lower target of 1.5C in the future.

As representatives of more than 150 nations gather tomorrow at the UN's headquarters to sign the agreement, it's become clear to many scientists that even the required 2C goal is aspirational at best. And that's a depressing prospect, it turns out, because a new report Thursday shows why the safer goal of 1.5C is the outer limit of what the world needs to avoid an unprecedented shift in human history. As it stands, the pact to be signed in New York implicitly acknowledges that the planet is headed into decidedly choppy waters.