Earth’s Carbon Dioxide Levels Surpass Long-Feared Milestone

Say goodbye to 400 ppm—and hello to Earth’s new atmospheric reality

Mauna Loa Observatory
Observations for the study were taken from the Mauna Loa Observatory atop one of Hawaii Island's five volcanoes. The Hippie Triathlete (Flickr/Creative Commons)

At the top of Hawaii’s Mauna Loa stands an unexpected sight: a cluster of domed buildings standing in stark contrast to the surrounding volcanic landscape. But rather than look down toward the lava-lined slopes or the distant ocean, the observatory peers toward the skies. Now, reports The Washington Post’s Chris Mooney, measurements from the Mauna Loa Observatory have documented something scary: carbon dioxide levels surpassing a critical threshold.

In new research published in the journal Nature, researchers used measurements of carbon dioxide (CO2) from the Mauna Loa Observatory to forecast future levels. They found that, thanks to the recent El Niño event, the monthly average CO2 concentrations will remain above 400 ppm all year long, a long-feared milestone of human impact on the environment.

In a way, a CO2 level of 400 ppm or higher is a symbolic threshold; Earth has been hovering around that level for years. But for scientists, it’s what The Guardian’s Michael Slezak calls “the point of no return”—a tipping point past which plenty of warming will occur, even if humans figure out how to reduce their carbon dioxide output.

Usually, atmospheric CO2 concentrations peak in May, when photosynthesis stokes plant growth. In September, when plants in the Northern Hemisphere die and lose their leaves, CO2 levels generally dip to their lowest annual levels. But El Niño put a wrench in those works. The weather phenomenon warms up water near the Equator, giving Earth what NASA calls "heartburn." As tropical areas get drier, fires begin to burn. Huge amounts of carbon dioxide are released from burning plants.

That's what happened this year: In 2015, scientists at the Mauna Loa Observatory recorded the largest year-to-year increase in CO2 levels since recording began. Not only was it the fourth consecutive year that levels rose to over 2 ppm, but the level shot up to 402.59 ppm for the first time ever. That milestone was mourned by scientists, who greeted it as a reminder of just how much humans have changed their atmosphere—and how much Earth will continue to change as humans keep pumping greenhouse gases into the atmosphere.

Without the El Niño event, say the paper’s authors, that milestone would not have been reached as quickly. They ran models that compared the most recent El Niño with others and used those numbers to simulate future CO2 levels. Given recent events and ongoing human activities like deforestation and the burning of fossil fuels, the team predicts that not only will the level never drop below 400 ppm during their lifetimes, but that CO2 increases will occur at higher levels than ever before—about 3.15 ppm. That’s 0.25 ppm higher than the amount of CO2 documented during the El Niño of 1997-98.

“No matter what the world’s emissions are now, we can decrease growth but we can’t decrease the concentration,” atmospheric scientist David Etheridge told Slezak in March. Humans may not be able to go back below 400 ppm, but they can try to curb the number’s terrifying rise—even as they brace for the effects of Earth’s new atmospheric reality.

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