In the centuries to come, history books will likely look back on September 2016 as a major milestone for the world’s climate. At a time when atmospheric carbon dioxide is usually at its minimum, the monthly value failed to drop below 400 parts per million (ppm).
That all but ensures that 2016 will be the year that carbon dioxide officially passed the symbolic 400 ppm mark, never to return below it in our lifetimes, according to scientists.
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The carbon dioxide we’ve already committed to the atmosphere has warmed the world about 1.8F since the start of the industrial revolution. This year, in addition to marking the start of our new 400 ppm world, is also set to be thehottest year on record. The planet has edged right up against the 1.5C (2.7F) warming threshold, a key metric in last year’s Paris climate agreement.
Even though there are some hopeful signs that world leaders will take actions to reduce emissions, those actions will have to happen on an accelerating timetable in order to avoid 2C of warming. That’s the level outlined by policymakers as a safe threshold for climate change. And even if the world limits warming to that benchmark, it will still likely spell doom for low-lying small island states and have serious repercussions around the world, from more extreme heat waves to droughts, coastal flooding and the extinction of many coral reefs.
