Lucy Perkins

Our series, "Take A Number," is exploring problems around the world — and the people who are trying to solve them — through the lens of a single number.

158,000. That's roughly how many refugees are stuck in limbo in Europe right now.

Updated at 2:30 p.m. ET

President Obama formally unveiled his plan to cut power plant emissions — some two years in the making — calling it the "single most important step that America has ever made in the fight against global climate change."