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The reading from the Mauna Loa Observatory in Hawaii finds that concentrations of the climate-warming gas averaged above 410 parts per million throughout April. The first time readings crossed 410 at all occurred on April 18, 2017, or just about a year ago.

Carbon dioxide concentrations — whose “greenhouse gas effect” traps heat and drives climate change — were around 280 parts per million circa 1880, at the dawn of the industrial revolution. They’re now 46 percent higher…….

The rate of growth is about 2.5 parts per million per year, said Ralph Keeling, who directs the CO2 program at the Scripps Institution of Oceanography, which monitors the readings. The rate has been increasing, with the decade of the 2010s rising faster than the 2000s. ……

“As a scientist, what concerns me the most is not that we have passed yet another round-number threshold but what this continued rise actually means: that we are continuing full speed ahead with an unprecedented experiment with our planet, the only home we have,” Katharine Hayhoe, a climate scientist at Texas Tech University, said in a statement on the milestone.

Planetary carbon dioxide levels have been this high or even higher in the planet’s history — but it has been a long time. And scientists are concerned that the rate of change now is far faster than what Earth has previously been used to.

In the mid-Pliocene warm period more than 3 million years ago, they were also around 400 parts per million — but Earth’s sea level is known to have been 66 feet or more higher, and the planet was still warmer than now.

As a recent federal climate science report (co-authored by Hahyoe) noted, the 400 parts per million carbon dioxide level in the Pliocene “was sustained over long periods of time, whereas today the global CO₂ concentration is increasing rapidly.” In other words, Earth’s movement toward Pliocene-like conditions may play out in the decades and centuries ahead of us.

……Keeling said that the planet, currently at 1 degree Celsius (1.8 degrees Fahrenheit) above preindustrial levels, is probably not yet committed to a warming of 1.5 or 2 degrees Celsius, but it’s getting closer all the time — particularly for 1.5 C. “We don’t have a lot of headroom,” he said.“It’s not going to be a sudden breakthrough, either,” Keeling continued. “We’re just moving further and further into dangerous territory.”https://www.washingtonpost.com/news/energy-environment/wp/2018/05/03/earths-atmosphere-just-crossed-another-troubling-climate-change-threshold/?noredirect=on&utm_source=rss_energy-environment&utm_term=.650775eb0646