Atmospheric concentrations of CO2 are set to pass 400 ppm, far faster than scientists would have predicted. That shows how difficult it's been to reduce carbon emissions—and points the way towards dangerous warming in the future

Jonathan Kingston Mauna Kea Observatory in Hawaii

Climate change is, first and foremost, a consequence of the addition of carbon dioxide into the atmosphere. We emit carbon dioxide, through burning fossil fuels or forests, and some of that carbon stays in the atmosphere, intensifying the heat-trapping greenhouse effect and warming the climate. What kind of global warming we’ll see in the future will largely be due to how much carbon dioxide—and to a lesser extent, other greenhouse gases like methane—we add to the atmosphere. And to fully understand the future, we need to understand the present and the past, and track the concentration of CO2 in the Earth’s atmosphere.

The fact that we can and have been tracking that very important number is due largely to the efforts of the geochemist Charles David Keeling. As a postdoctoral fellow at the California Institute of Technology in the 1950s, Keeling developed the first instrument that could accurately measure the CO2 levels in the entire atmosphere through sampling. When he got to the Scripps Institution of Oceanography a few years later, Keeling began taking regularly CO2 measurements at the Mauna Loa Observatory in Hawaii. Keeling discovered that atmospheric CO2 underwent a seasonal cycle, as plants bloomed and decayed in the Northern Hemisphere, and more importantly, that CO2 was rising fast. In 1958, CO2 levels recorded at Mauna Loa were about 316 parts per million (ppm). By 2005, when Keeling died—and his son, Ralph Keeling, took up the project—CO2 levels were just under 380 ppm. Plotted on a graph, the readings over time curve upwards sharply as humans added more and more CO2 to the atmosphere—which is why the readings came to be known as the Keeling Curve.

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Now, thanks to Keeling’s successors at Scripps, we know that global CO2 levels are about to pass a major threshold: 400 ppm. It’s a momentous enough occasion, at least for scientists, that Scripps has begun releasing daily readings—today the level is 399.50 ppm—on a website and via a Twitter account. We should pass 400 ppm any day now—possibly, by the time that you read this. And that’s not good.

The fact that we’re going to cross 400 ppm doesn’t mean that much by itself. It’s not like the sound barrier—the difference in warming between 399 ppm and 400 ppm would likely be minute. But the sheer rate of increase over just the past 55 years shows how fast global warming could hit us in the future—and the present—and underscores how much we’ve failed as a planet to slow down carbon emissions. As Ralph Keeling put it in a statement:

I wish it weren’t true, but it looks like the world is going to blow through the 400-ppm level without losing a beat. At this pace we’ll hit 450 ppm within a few decades.

400 ppm may be high enough. The last time CO2 levels were this high was likely during the Pliocene epoch, between 3.2 million and 5 million years ago. The Earth’s climate was warmer during the Pliocene than it is today—perhaps by 2 to 3 C—and sea levels were much higher. It was a very different planet than the one we’ve lived on so successfully for thousands of years.

There’s no guarantee that we’d experience the same levels of warming in the future if CO2 levels stay that high, but it doesn’t look good. Nor will CO2 levels stop at 400 ppm—barring a virtually impossible immediate turn away from fossil fuels, CO2 emissions will keep growing globally, and CO2 concentrations will keep rising. The U.N.’s official goal is to keep CO2 levels below 450 ppm, and as Ralph Keeling indicated, we’re rapidly running out of time to make that happen. CO2 can stay in the atmosphere for centuries, which means that we’ve already baked in far more warming than we’ve yet experienced. But we will soon enough. The Keeling Curve tells us our past, but it’s also a roadmap for our future—a future that will almost certainly be hotter and wilder.

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