Climate Central: April set a carbon dioxide milestone by averaging 400 parts per million for the entire month. That's uncharted territory over the course of human history, and a new data visualization makes clear just how high and fast it has risen.
The march to 400 ppm might seem slow by human standards, rising just one or two parts per million each year, but it's a veritable sprint by geological standards. We know this from ice cores, which contain air bubbles that give snapshots of carbon dioxide levels over...