The sky may not be falling, but it's definitely changing. And that transformation — already the most extreme of its kind ever seen by humans — is now on the verge of yet another grim milestone.

Until 2013, Earth's atmosphere hadn't contained 400 parts per million of carbon dioxide at any point in human history, and probably not since the Pliocene Epoch, which ended a couple million years before Homo sapiens appeared. Thanks to a man-made flood of CO2 emissions, though, it crossed this threshold again on May 9, 2013, according to data from the Mauna Loa Observatory in Hawaii.

CO2 levels naturally fluctuate by season, peaking at Mauna Loa in mid-May, so they soon fell back below 400 ppm. But Earth's long-term Pliocene flashback was far from finished. After dipping to 393 ppm last fall, CO2 levels returned to 400 ppm in 2014 — only this time in March instead of May:

MLO data

By mid-March, humans experienced our first week with average CO2 levels above 400 ppm. And now another milestone looms: April 2014 could be the first month in human history with average CO2 levels of at least 400 ppm. Not only would that break a streak of at least 9.6 million months below 400 ppm, but there's a good chance it could happen again in May, when yearly CO2 levels are highest.

"We're already seeing values over 400," said Ralph Keeling, director of the Scripps Institution of Oceanography's CO2 Program, in a March 17 blog post about the 2014 data. "Probably we'll see values dwelling over 400 in April and May. It's just a matter of time before it stays over 400 forever."

There's nothing magical about 400 molecules of CO2 in every 1 million molecules of air, and their greenhouse effect is about the same as that of 399 or 401. But 400 is a round number, and round numbers serve as natural milestones, whether it's a 50th birthday, a 500th home run or the 100,000th mile on an odometer. And when it comes to CO2, even a merely symbolic milestone is important if it can draw more public attention to the nonstop catastrophe of global climate change.

"On some level, watching these milestones be passed is a lot like watching paint dry," Columbia University climatologist Jason Smerdon tells Climate Central. "The upward march is neither surprising nor unexpected as a direct consequence of human activities; it is only alarming in the sense that it keeps happening unabated." And since "it keeps happening" doesn't necessarily sound like breaking news, numerical landmarks can help make the invisible CO2 buildup feel more tangible and urgent.

So can graphs like the one below. It plots monthly average CO2 levels, as measured at Mauna Loa, between late 2009 and early 2014. That includes several seasonal fluctuations, illustrating natural variability as well as the longer-term trend toward more and more CO2 in the air:

MLO data

Mauna Loa's data record goes back to 1958, so here's a more zoomed-out look at the whole thing:

MLO data

But how do we know CO2 levels before 1958? Ice-core data, which come from CO2 bubbles trapped in ancient ice sheets. That's how scientists can make graphs like the one below, which shows sub-300 ppm CO2 levels dating back 800,000 years (humanity is about 200,000 years old). For a reminder why ppm of CO2 matters so much, a second graph shows how temperatures mirror CO2:

CO2 and temperature data

And, finally, this graph illustrates the human contribution to Earth's recently surging CO2 levels:

CO2 emissions

A day or even a month at 400 ppm may be mostly symbolic, but the effects of that much CO2 are all too real. The last time CO2 levels were this high, Earth's surface was about 11 degrees Fahrenheit warmer and sea levels were about 100 feet higher. Global temperatures are projected to rise by up to 7.2 degrees in the next 80 years, based on current rates of CO2 emissions, and oceans have already risen 8 inches in the past century alone. They could rise another 3 feet by 2100, joining a barrage of climatic disasters like longer droughts, fiercer wildfires, more strong storms and less food security.

CO2 lingers in the sky for centuries, and with emissions still rising, it won't be long before CO2 levels top 400 ppm every day. According to the United Nations' Intergovernmental Panel on Climate Change, the most severe environmental effects will likely arrive when CO2 levels are around 450 ppm.

"The 400-ppm threshold is a sobering milestone," Scripps scientist Tim Lueker said after last year's first foray into the 400s. "[It] should serve as a wakeup call for all of us to support clean energy and reduce emissions of greenhouse gases, before it's too late for our children and grandchildren."

Russell McLendon is science editor at MNN. Follow him on Twitter and Google+.

Related climate change stories on MNN:

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.

Earth is about to reach another terrible CO2 milestone
Last year, CO2 in Earth's air hit 400 ppm for the first time in human history. Now it's poised to average 400 ppm for a month.