ONE OF THE mightiest El Nino patterns in memory brought unprecedented drought and flooding in 2016, the hottest year on record. The planet also passed a grim milestone: an atmospheric carbon dioxide concentration of 400 parts per million, including remote Antarctica, which hasn't seen that much CO_2 for 4 million years. Climatologists target 350 ppm as the uppermost threshold before we see dire consequences, like catastrophic sea level rise. Some locations previously had topped 400 ppm on a seasonal cycle, but scientists say this time it's permanent - and global.
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