Last year atmospheric carbon dioxide passed the 400 ppm milestone. Now it's on track for another next month.
“We’re already seeing values over 400. Probably we’ll see values dwelling over 400 in April and May. It’s just a matter of time before it stays over 400 forever,” said Ralph Keeling in a blog post.
The graph of those concentrations is known as the Keeling Curve, one of the most iconic images in science. In addition to showing a steady rise in carbon dioxide, the graph also shows the seasonal variations in the curve. In Northern Hemisphere spring, plants burst to life and suck carbon dioxide out of the air until they die off in the fall.
Atmospheric carbon dioxide usually peaks in May. If levels continue to rise in the next few months — and there’s no reason to believe they won’t — April or May will likely be the first time the monthly atmospheric carbon dioxide average will be above 400 ppm. Estimates for when the atmosphere last contained this much carbon dioxide range from 800,000 years ago all the way to 15 million years.
While 400 ppm is mostly a symbolic number, the climate changes it could cause are not. Among other impacts, increased carbon dioxide contributes to heating the planet’s surface and ocean temperatures, which in turn melts ice and raises ocean levels.