“Our forecast supports the suggestion that the Mauna Loa record will never again show CO2 concentrations below the symbolic 400 ppm within our lifetimes,” write the researchers, led by Richard Betts of the U.K. Met Office’s Hadley Center, in Nature Climate Change. The study was conducted with colleagues from the Hadley Centre and Ralph Keeling of the Scripps Institution of Oceanography in La Jolla, Calif.

Carbon dioxide concentrations in the pre-industrial atmosphere were around 280 parts per million. But concentrations began to rise with the early growth of industry and continually climbed throughout the 20th century, as documented by the famous Keeling curve, based on observations taken at Mauna Loa dating back to the late 1950s.

This record is referred to often as a “saw-toothed curve,” because every year, concentrations go up and down somewhat (because of the life cycles of plants across the globe, which draw in carbon dioxide through the process of photosynthesis). But nonetheless, the long-term trend is steadily upward because humans are putting more carbon dioxide into the atmosphere than plants and other natural “sinks” can pull back out again.

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Concentrations have crossed 400 parts per million on a temporary basis. It began as brief excursions, and last year the annual average concentration at Mauna Loa was more than 400 parts per million for the first time (it was 400.9). Nonetheless, during the course of the yearly cycle in carbon dioxide concentrations, there were still some parts of the year last year when they remained below that level.

What the new study suggests is that those days are over — carbon dioxide will never fall below 400 ppm this year, nor the next, nor the next. The reason is that the strong 2015-2016 El Niño event has pushed concentrations upward more than usual for a given year — El Niños tend to do that, because they dry out tropical regions, lessening tree growth and sparking vast wildfires. That means that even in September of this year, when annual concentrations are typically at their lowest (as northern hemisphere trees lose their leaves and vegetation growth declines heading into winter), they’ll likely still be slightly over 400 parts per million, scientists forecast.

“I don’t think anything sort of special will happen just because we’re going past 400,” Betts said. “But I do think that these numbers are important for awareness, really. … It’s a reminder of the long-term effects we’re having on the system.”

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The paper also predicts that this El Niño will drive a year-to-year rise in average atmospheric concentrations of 3.15 parts per million, exceeding the single-year change caused by the last major El Niño, from 1997-1998, of 2.9 parts per million. On June 12, concentrations were at 407.26 parts per million, according to NOAA’s Earth System Research Laboratory, which monitors the data. But they should start to decline soon, according to the seasonal cycle, which reaches a peak in May and a low in September and is driven by the growth of plants in the northern hemisphere (where there is much more total land area).

Because it presents a forecast that is naturally subject to some uncertainty, the study does note that there is still a possibility of a day this year that dips somewhat below 400 parts per million at Mauna Loa — but adds that it’s not that likely. “Daily values will be most likely to stay above 400 ppm, although values slightly below remain a small possibility,” the researchers note.

Atmospheric carbon dioxide concentrations will continue to rise even though global greenhouse gas emissions from industry may be leveling off somewhat, the study adds — because each year still represents a net addition to the atmosphere, and carbon dioxide is a very long-lived greenhouse gas. Thus, even by 2050, the researchers don’t think we’ll find any way of getting back below 400 ppm.

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“In the longer term, a reduction in CO2 concentration would require substantial and sustained cuts in anthropogenic emissions to near zero,” they write. Barring dramatic cuts in fossil fuel emissions, combined with the development of some kind of “negative emissions” technology that actively withdraws carbon dioxide from the atmosphere, we’ll see rising carbon dioxide concentrations for some time, they note.

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