#3. CO2 Hits 400 ppm — Does It Matter?
On May 9, the atmospheric concentration of carbon dioxide reached 400 parts per million atop Hawaii's Mauna Loa volcano — a level not seen for millennia.
Although there are no real or observable changes as a result of crossing this threshold, CO2 levels are still rising, and fast. In the coming decades, experts predict, those rising numbers will be accompanied by ever more visible impacts.
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