June was the third straight month that scientists measured the concentration of carbon dioxide in the atmosphere at above 400 parts per million. Just hitting that level, as we did back in April, was bad enough. Staying above that level for this long signals that this is not a fluke, but the new norm. And while the threshold itself doesn’t mean much—it’s not like 400 ppm is markedly worse than, say, 399—it’s a sign of how bad things have gotten.
Climate Central’s Andrea Thompson posted a chart on Monday, showing how CO2 concentration peaked over 400 ppm within the last six months. (There’s some seasonal fluctuation to this number, too, and it is expected to decline this summer):
Parts per million is another way of saying how much CO2 there is in the atmosphere, as a ratio of all the other molecules in the air. The level has been rising steadily for the last few decades, as you can see in this graph:
As scientist Michael Gunson explained back in May 2013—when the Scripps Institution of Oceanography in Hawaii reported its first measurements ever above 400—this should be a “psychological tripwire.” It shows “we are on an inexorable march to 450 ppm and much higher levels.”
How big a deal is that? Scientists have analyzed ice cores to get historical data on carbon levels. The data goes back 800,000 years. During that span, it was never anywhere close to this level:
The most plausible path to keeping climate change in check is for the world’s biggest polluters to reach a long-term agreement to keep the planet from warming by more than the agreed-upon 2 degrees Celsius target.
The most plausible path to controlling climate change is for the world powers to reach some kind of long-term agreement that would keep the planet from warming by more than 2 degrees this century. Some experts say that target is already unattainable. To even have a hope of limiting warming to 2 degrees, experts say, the carbon level would have to be lowered significantly, to a level around 350 ppm.
Rebecca Leber is a staff writer for The New Republic.