Update, May 10: It's official. The National Oceanic and Atmospheric Administration has confirmed that on May 9, the daily average concentration of carbon dioxide in Earth's atmosphere surpassed 400 parts per million for the first time in human history.

Global carbon dioxide levels will likely reach 400 parts per million within days, scientists report, an ominous milestone that's unprecedented in human history. Earth's atmosphere hasn't contained that much CO2 since the Pliocene Epoch, an ancient era that ended more than 2 million years before the first Homo sapiens appeared.

This forecast is based on data from the Mauna Loa Observatory (MLO) in Hawaii, considered the gold standard in CO2 measurements due to its deep data record and isolation from major pollution sources. Located on a 13,000-foot-tall mountain in the Pacific Ocean, the monitoring station recorded a daily average of 399.5 ppm on April 29, and some hourly readings have already surpassed 400 ppm. CO2 levels fluctuate seasonally during the year, and typically peak at Mauna Loa in mid-May.

Although 400 ppm isn't a so-called "tipping point" for climate change, it is a symbolic threshold that illustrates how dramatically humans have altered the atmosphere in just a few generations. Global CO2 levels had hovered between 170 ppm and 300 ppm for thousands of centuries until the Industrial Revolution, then suddenly began to skyrocket. They had reached 317 ppm by 1958, when climate scientist Charles David Keeling established the MLO, and were up to 360 ppm by the end of the 20th century.

"I wish it weren't true, but it looks like the world is going to blow through the 400-ppm level without losing a beat," says Ralph Keeling, a geochemist at the Scripps Institution of Oceanography who has continued the work of his father, the late Charles David Keeling. "At this pace we'll hit 450 ppm within a few decades."

The following two charts demonstrate the speed of this carbon bombardment. The first — a Scripps-produced plot of MLO data dubbed the "Keeling curve" — shows how atmospheric CO2 concentrations have risen by about 25 percent since the late 1950s:

And this one, produced by the National Oceanic and Atmospheric Administration (NOAA), shows a much longer record dating back 800,000 years. Its data come from air bubbles trapped in ancient ice, revealing a roughly 33 percent jump from pre-industrial peaks. It also illustrates how rapid the recent rise has been compared with historical changes:

About 80 percent of human-induced CO2 emissions come from burning fossil fuels, according to NOAA, and about 20 percent come from deforestation and certain farming practices. Since people began widely burning coal, petroleum and other fossil fuels two centuries ago, the Industrial Revolution is generally deemed the starting point for today's ongoing CO2 surge and associated climate change.

The looming milestone at Mauna Loa isn't the first modern 400 ppm measurement — NOAA reported CO2 levels just above 400 ppm at sites in the Arctic last year. But since Arctic CO2 has historically risen faster than in other parts of the planet, it's not necessarily a reliable marker for global concentrations. Mauna Loa, on the other hand, is considered the most accurate place to assess how much CO2 is in the sky worldwide.

The 400 ppm threshold will be fleeting at first, since summertime plant growth in the Northern Hemisphere will soon begin soaking up more CO2 from the air. This phenomenon underlies the seasonal variability seen throughout the Keeling curve's history, but it's a cold comfort. The MLO's late-summer low in CO2 levels tends to catch up with the springtime high after four or five years, so there may be year-round concentrations above 400 ppm as soon as 2017. That hasn't happened since the Pliocene, a warm geologic era that lasted from about 5.3 million years ago to 2.6 million years ago.

Average temperatures were about 18 degrees Fahrenheit warmer in the Pliocene than today, scientists estimate, and sea levels were between 16 and 131 feet higher. The extra warmth trapped by rising levels of CO2 — just one of several greenhouse gases in the atmosphere — is also linked to stronger storms, longer droughts and an array of other climatic and ecological crises. Excess CO2 is also absorbed by Earth's oceans, which are becoming more acidic and thus less hospitable to coral, crustaceans and other wildlife.

Renowned climate scientist James Hansen reported in 2009 that any CO2 level above 350 ppm can spur dangerous warming. But even though U.S. carbon emissions are now at their lowest level since 1994, the U.S. still ranks No. 2 among all countries, behind only China. And the world overall still emits 2.4 million pounds of CO2 per second, making it unlikely we'll be down to 350 ppm anytime soon. The U.N. Intergovernmental Panel on Climate Change estimates 450 ppm is when the worst effects of climate change will begin.

"The 400-ppm threshold is a sobering milestone," says Tim Lueker, an oceanographer and carbon-cycle researcher with Scripps. "[It] should serve as a wakeup call for all of us to support clean energy technology and reduce emissions of greenhouse gases, before it's too late for our children and grandchildren."