Carbon dioxide in Earth's atmosphere just reached a major milestone

September 29, 2016, 9:36 a.m.

Change is in the air, or at least the air itself is changing. Earth's atmosphere is shifting to a state unseen in human history, and according to scientists at Mauna Loa Observatory in Hawaii, it just reached a milestone we'd be unwise to ignore.

Our atmosphere now holds 400 parts per million (ppm) of carbon dioxide (CO2), the main greenhouse gas behind human-induced climate change. It has flirted with 400 for about three years, but data from September 2016 suggest it's finally taking the plunge.

"Brief excursions toward lower values are still possible, but it already seems safe to conclude that we won't be seeing a monthly value below 400 ppm this year — or ever again for the indefinite future," writes climate scientist Ralph Keeling, who directs the CO2 monitoring program at Mauna Loa for the Scripps Institution of Oceanography.

Earth's air always has some CO2, which plants need for photosynthesis, but too much creates the heat-trapping effect responsible for climate change. Global CO2 levels naturally fluctuate by season due to plant growth, falling in the Northern Hemisphere's summer and rising in winter. That cycle continues, but with more and more CO2 due to billions of people burning fossil fuels.

On May 9, 2013, CO2 levels at Mauna Loa reached 400 ppm for the first time since the Pliocene Epoch, which ended about 2.8 million years before modern humans existed. (Natural phenomena raised Pliocene CO2 levels gradually, while humans are raising current levels extremely quickly by climatic standards — and with no precedent for how it will affect our species.) CO2 levels fell back to the 390s in the summer of 2013, but not for long. They were above 400 again by March 2014, and Mauna Loa's entire monthly average broke 400 ppm that April, as did a broader global average in March 2015.

And now monthly CO2 levels have stayed above 400 ppm even in September, when they typically reach their annual low. It's clear September 2016 will average more than 400 ppm, Keeling writes, and although September isn't always the lowest month for CO2, he adds it's "almost impossible" for October's value to dip back below 400.

That suggests the 300 ppm era is all but over, given the rate of emissions and CO2's tendency to stay in the sky for centuries. But while it's already too late to stop some effects of climate change, it's also way too early to give up.

Four hundred is a symbolic milestone, as round numbers tend to be, which helps it highlight what's happening more than 380 or 390 could. Like any milestone, it's a reminder of the path we're on — and a natural opportunity to consider changing course.