Global warming became big news for the first time during the hot summer of 1988 when now-retired NASA climate scientist James Hansen testified before Congress that the trend was not part of natural climate variation, but rather the result of emissions of CO2 and other greenhouse gasses from human activities. By that point we had already passed 350 ppm.
So in spring 2013, the approach of 400 ppm seemed like a significant, new threshold — the first one we’ve reached since climate change started making headlines regularly. But it is both an artificial and fuzzy symbol. Nothing fundamental actually changed in Earth’s climate system when we hit it.
“It’s not that different from 390 or 410,” Tans points out. And even as the instruments in Hawaii indicated that we had reached the milestone, the global average concentration of CO2 was a few points lower. That’s because Hawaii is in the Northern Hemisphere, where most CO2 from fossil fuel burning comes from.
To make things even fuzzier, the CO2 concentration in Hawaii quickly dipped below 400 ppm as plants greened during the Northern Hemisphere’s growing season, soaking up CO2 through photosynthesis.
Over the long term, CO2 will continue to increase, bringing growing risks from sea level rise and disruptions to weather patterns. But it will take a couple of years — until about 2016, Tans estimates — for annual global-average CO2 levels to surpass 400 ppm.
For James White, a paleoclimatologist at the University of Colorado, 400 ppm is “a mile marker you pass on the interstate while flying by at 60 mph.” And he expresses few doubts about the road ahead. “We’re blowing the doors off 400.” He would not be surprised if we get to 800. “That will be the next big milestone, and that’s a fundamentally different world.”
Over centuries, that amount of atmospheric CO2 could cause enough warming to melt all ice on land and bring the sea level up by 80 meters — enough to submerge Bangladesh and almost all of Florida.