It’s happened: Antarctica has hit 400.

If 300 was a movie about the destructive capacity of a small band of humans, 400 is the same thing only on a much larger scale.

The National Oceanic and Atmospheric Administration announced this week that on May 23 carbon dioxide levels at the South Pole surpassed 400 parts per million (PPM) at the South Pole for the first time in 4 million years, a marker NOAA called “another unfortunate milestone.”

The South Pole has shown the same relentless upward trend in carbon dioxide (CO2) as the rest of world, NOAA said in a statement released on their website, but its remote location meant it was late to register the impacts of fossil fuel consumption, the primary driver of greenhouse gas pollution.

“The far southern hemisphere was the last place on earth where CO2 had not yet reached this mark,” Pieter Tans, NOAA’s Global Greenhouse Gas Reference Network lead scientist, said in the statement. “Global CO2 levels will not return to values below 400 PPM in our lifetimes, and almost certainly for much longer.”

400 PPM “should be a psychological tripwire for everyone,” according to NASA Michael Gunson. “Passing the 400 mark reminds me that we are on an inexorable march to 450 PPM and much higher levels. These were the targets for ‘stabilization’ suggested not too long ago. The world is quickening the rate of accumulation of CO2, and has shown no signs of slowing this down.”

CO2 levels rise during the Northern Hemisphere’s fall and winter and decline during the summer as terrestrial plants consume CO2 during photosynthesis. It’s an AMAZING process, one you can watch on this video from NASA:

But plants only a fraction of emissions. For every year since observations began in 1958 there has been more CO2 in the atmosphere than the year before, according to NOAA. Last year’s global CO2 average reached 399 PPM, meaning that the global average in 2016 will almost certainly surpass 400 PPM.

The question NOAA scientists are now asking is whether even the lowest month of 2016 will have CO2 readings over 400 PPM.

Also concerning, the rate of increase appears to be accelerating. The annual growth rate of atmospheric CO2 measured at NOAA’s Mauna Loa Observatory in Hawaii jumped 3.05 ppm during 2015, according to NOAA’s statement, the largest year-to-year increase in 56 years of monitoring. Last year was the fourth consecutive year that CO2 grew more than 2 ppm – which set another record. This year promises to be the fifth.

Part of last year’s jump was attributable to El Nino, the statement said, referring to the cyclical Pacific Ocean warming that produces extreme weather across the globe and causing terrestrial ecosystems to lose stored CO2 through wildfire, drought and heat waves.

“We know from abundant and solid evidence that the CO2 increase is caused entirely by human activities,” Tans said. “Since emissions from fossil fuel burning have been at a record high during the last several years, the rate of CO2 increase has also been at a record high. And we know some of it will remain in the atmosphere for thousands of years.”

So there’s that…