The average carbon dioxide concentration in May was 407.7 parts per million, or ppm, which is how molecules in the atmosphere are measured. In May, for every 1 million molecules in the air, 407.7 of them were carbon dioxide. This was a 3.76 ppm increase since May 2015, and the largest year over year increase on record.

On April 9, a daily record of 409.44 ppm was set, although scientists tend to not follow the daily averages too closely, since they often fluctuate too wildly to be representative of the atmosphere as a whole.

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One thing is for certain — we are in an unprecedented era. “Carbon dioxide levels are increasing faster than they have in hundreds of thousands of years,” said Pieter Tans, lead scientist of NOAA’s Global Greenhouse Gas Reference Network. “It’s explosive compared to natural processes.”

The last time our planet saw such a sustained increase in carbon dioxide was between 17,000 and 11,000 years ago, NOAA says, when levels increased by just 80 ppm. Tans says today’s rate of increase is 200 times faster than that.

Carbon dioxide in the atmosphere is higher now than it has been in more than 800,000 years. We know that because we can literally measure the carbon dioxide in ancient air that is trapped deep inside glacial ice. The increase comes courtesy of our preferred energy source: the burning of fossil fuels, a process by which Earth’s carbon stores are essentially excavated from their natural position in the global carbon cycle and released into the atmosphere as heat-trapping greenhouse gas.

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Perched on top of the second-largest volcano in the world, the Mauna Loa Observatory in Hawaii has been measuring atmospheric carbon dioxide for nearly 60 years. The resulting carbon dioxide levels create the iconic Keeling Curve, shown above, named for the scientist who began the measurements in 1958, Charles David Keeling.

The first carbon dioxide measurement recorded by Keeling was just 316 ppm. Since then, it has shot through the roof. In 2013, our atmosphere’s carbon dioxide concentration hit 400 ppm — a significant milestone, because it’s the level at which climate scientists identify as “the beginning of the danger zone,” said Princeton University’s Michael Oppenheimer.

“It means we’re probably getting to the point where we’re looking at the ‘safe zone’ in the rear-view mirror, even as we’re stepping on the gas,” Oppenheimer said.

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Though the vast majority of the increase in carbon dioxide is due to the burning of fossil fuels, part of this year’s very large spike can be attributed to El Niño.

The tropics tend to dry out during an El Niño, which kills off a lot of plant life. Plants absorb atmospheric carbon dioxide, which is necessary for photosynthesis. Fewer plants means more CO2 in the atmosphere. El Niño also increases the likelihood of extreme wildfires, which also inject large amounts of CO2 into the air.