Image: Chris Harrison/Flickr

The monster El Niño of 2015-2016 is finally gone, but scientists are still coming to terms with its impacts on the planet. Among those impacts: charging up the global carbon cycle and pushing atmospheric CO2 levels above 400 parts per million (ppm) for an entire year—a first in human history.

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Humans are constantly adding CO2 to the atmosphere, a reality which has been carefully chronicled at the Mauna Loa Climate Observatory since 1958. Over the past 60 years, the CO2 concentration at Mauna Loa has risen and fallen on an annual basis, owing to the uptake of carbon by plants for photosynthesis, and subsequent release of carbon during decomposition. But we’ve also watched baseline carbon levels climb by approximately 2.1 ppm annually. That’s thanks to the 10 billion-odd tons of fossil carbon our cars and factories spew skyward each year.

The “Keeling Curve,” our best record of modern atmospheric CO2 levels. Image: NOAA

This past year was special. As Gizmodo reported in March, carbon concentrations at Mauna Loa rose 3.76 ppm between February 2015 and February 2016; the single largest jump in recorded history. The previous record rise, of 2.82 ppm, occurred during the 1997-1998 El Niño. In both cases, scientists believe that emissions spiked due to a combination of warming and drying in the tropics, which can accelerate soil carbon decomposition, and large, drought-fueled fires.

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The result is that atmospheric CO2 levels have been hovering comfortably above 400 ppm—a level that was unprecedented in our records until 2013—for months. While CO2 levels may dip below 400 ppm this fall, a study published today in Nature Climate Change finds that 2016 is now on track for an atmospheric average of 404.45 plus or minus 0.53 ppm. In other words, it’ll be the first year in the history of our species that we can truly say we’ve been living in a 400 ppm world.

Atmospheric carbon concentrations measured at Mauna Loa over the course of the last 12 months. Image: NOAA

And it’ll be this way for the rest of our lifetimes, barring large-scale deployment of carbon capture and storage technology, an idea which scientists are starting to take seriously in light of our apparent inability to give a shit about climate change. So, what does it mean to live and breathe a 400 ppm atmosphere? We’re not entirely sure yet, but the geologic past can offer clues. For instance, the last time the Earth was a 400 ppm world—the mid-Pliocene—sea levels were an estimated 50 to 80 feet higher than they are today, meaning Florida, much of the Gulf Coast, and countless other coastlines worldwide did not exist in their current form.

Whether we’re poised to see a repeat of the mid-Pliocene, or something more dramatic, depends largely on our actions this century. Ralph Keeling, the climate scientist who put the first data points on the Mauna Loa CO2 curve in the 50s and 60s, captured both the significance and uncertainty of the 400 ppm milestone in a quote to Climate Central last fall: “400 ppm is not a magic number for climate, but it does nicely symbolize that we are now in a new era of Earth history.”