The most depressing number you’ll see all week

400.

That’s how many parts — per million — of carbon dioxide, on average, were concentrated in the Earth’s atmosphere last month, according to the National Oceanic and Atmospheric Administration.

It’s a rather depressing “significant milestone” — it’s the first time the NOAA has measured average levels of 400 parts per million since it began tracking those levels.

What that means is that carbon dioxide makes up about 0.04 percent of the Earth’s atmosphere, a dangerous level given its potency as a greenhouse gas. Half of the rise in the amount of carbon dioxide in the Earth’s atmosphere has occurred since 1980.

“It was only a matter of time that we would average 400 parts per million globally,” said Pieter Tans, lead scientist of NOAA’s Global Greenhouse Gas Reference Network, in a press release. “We first reported 400 ppm when all of our Arctic sites reached that value in the spring of 2012. In 2013 the record at NOAA’s Mauna Loa Observatory first crossed the 400 ppm threshold. Reaching 400 parts per million as a global average is a significant milestone.”

“This marks the fact that humans burning fossil fuels have caused global carbon dioxide concentrations to rise more than 120 parts per million since pre-industrial times,” he added.

That’s not great in the short term: Tans expects those levels to continue through at least May, when carbon-dioxide levels peak due to natural greenhouse gas cycles and rising emissions.

But it’s worse for the long term. First, keeping carbon-dioxide levels the same won’t do anything to reduce the effects of climate change. The parts per million number needs to go down for that to be accomplished. The International Energy Agency reported in March that those levels had stayed the same from 2013 to 2014, but the NOAA threw some cold water (or pushed some hot air) on the cheerleading surrounding that on Wednesday.

That’s why the parts per million number has been called the “most important number on Earth.” There’s even a group, 350.org, that was founded on the basis that parts-per-million levels of carbon dioxide should be at 350 at most.

“Right now we’re at 400 ppm, and we’re adding 2 ppm of carbon dioxide to the atmosphere every year. Unless we are able to rapidly turn that around and return to below 350 ppm this century, we risk triggering tipping points and irreversible impacts that could send climate change spinning truly beyond our control,” the group writes on its website.

But the NOAA isn’t bullish on that happening — at least not in the near future.

“Elimination of about 80 percent of fossil fuel emissions would essentially stop the rise in carbon dioxide in the atmosphere, but concentrations of carbon dioxide would not start decreasing until even further reductions are made and then it would only do so slowly,” said James Butler, the director of NOAA’s Global Monitoring Division.

Brett LoGiurato is the senior national political correspondent at Fusion, where he covers all things 2016. He’ll give you everything you need to know about politics, with a healthy side of puns.