We've Hit the Carbon Level We Were Warned About. Here's What That Means.

Over the last couple weeks, scientists and environmentalists have been keeping a particularly close eye on the Hawaii-based monitoring station that tracks how much carbon dioxide is in the atmosphere, as the count tiptoed closer to a record-smashing 400 parts per million. 

Thursday, we finally got there: The daily mean concentration was higher than at any time in human history, the National Oceanic and Atmospheric Administration (NOAA) reported Friday.

Don't worry: Earth is not about to go up in a ball of flame. The 400 ppm mark is only a milestone, 50 ppm over what legendary NASA scientist James Hansen has since 1988 called the safe zone for avoiding the worst impacts of climate change, and yet only halfway to what the IPCC predicts we'll reach by the end of the century.

"Somehow in the last 50 ppm we melted the Arctic," said environmentalist and founder of activist group 350.org Bill McKibben, who called today's news a "grim but predictable milestone" and has long used the symbolic number as a rallying call for climate action. "We'll see what happens in the next 50."

Read full text at Mother Jones