The last time global CO2 levels were 400 parts per million was 800,000 years ago.
Now, the Washington Post reports, monitoring stations in the Arctic, as well as Mongolia, are recording readings of 400ppm or higher. The global average is now 395ppm and continues to climb.
For now it will remain below 400ppm globally, as well in those areas currently recording these (recent) record highs, but as the numerous scientists quoted by the Post say, it's "significant," "depressing," a "troubling milestone, " and "an indication that we're in a different world"—one that, I hope it shouldn't come as a surprise, human activity is responsible for.
Near universal political inaction, or lack of sufficient action universally, means that returning atmospheric CO2 to a safe level of 350ppm or below, and keeping average temperature rise below 2°C is increasingly looking out of reach and very dangerous, potentially catastrophic climate change all the more certain.
A timely example of the type of changes we will increasingly be seeing, via Jeff Masters' Wunderblog:
The record books for Greenland's climate were re-written on Tuesday, when the mercury hit 24.8°C (76.6°F) at Narsarsuaq, Greenland, on the southern coast. According to weather records researcher Maximiliano Herrera, this is the hottest temperature on record in Greenland for May, and is just 0.7°C (1.3°F) below the hottest temperature ever measured in Greenland. The previous May record was 22.4°C (72.3°F) at Kangerlussuaq (called Sondre Stormfjord in Danish) on May 31, 1991.