Carbon dioxide in the atmosphere has reached a critical record, halfway to doubling preindustrial levels. Click here to read the full story.

For the first time in recorded history, the concentration of atmospheric carbon dioxide was measured at more than 420 parts per million at the Mauna Loa Observatory on the Big Island of Hawaii. It’s a disconcerting milestone in the human-induced warming of the planet, around the halfway point on our path toward doubling preindustrial CO2 levels.

When the station began collecting CO2 measurements in the late 1950s, atmospheric CO2 concentration sat at around 315 parts per million. On Saturday, the daily average was pegged at 421.21 parts per million — the first time in human history that number has been so high. Previously, it had never exceeded 420 parts per million.

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