Up, up and away: Global carbon dioxide concentration has surpassed 400 parts per million for the first time since NOAA began to monitor the greenhouse gas in Earth’s atmosphere. (NOAA)

For the first time since we began to track the greenhouse gas in our planet’s atmosphere, carbon dioxide surpassed 400 parts per million in March — a concentration that scientists consider a significant milestone for Earth’s climate and our ability to reverse the trend.

“We first reported 400 ppm when all of our Arctic sites reached that value in the spring of 2012,” said Pieter Tans, lead scientist of NOAA’s Global Greenhouse Gas Reference Network. “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.”

Why does a concentration of 400 ppm matter? “It’s the level that climate scientists have identified as the beginning of the danger zone,” Princeton University Prof.  Michael Oppenheimer told The Washington Post in 2014. “It means we’re probably getting to the point where we’re looking at the ‘safe zone’ in the rearview mirror, even as we’re stepping on the gas.”

[In 2014, the U.N. warned of exponential CO2 growth]

Global carbon dioxide concentration is higher than it has been in more than 800,000 years. That distressing fact is mainly courtesy of our current energy source of choice — the burning of fossil fuels, a process by which Earth’s carbon stores are essentially excavated from their natural position in the global carbon cycle and released into the atmosphere as greenhouse gas.

The rate of that excavation and emission has been particularly staggering in recent years. “Humans burning fossil fuels have caused global carbon dioxide concentrations to rise more than 120 parts per million since pre-industrial times,” said Tans. “Half of that rise has occurred since 1980.”

[The Mauna Loa Observatory record surpassed 400 ppm in 2012]

NOAA, which released a statement marking the CO2 milestone  Wednesday morning, notes that the emission rate in 2014 seemed to have stalled at 2013 levels. But that was the only bright note in an otherwise somber global climate status update. “Stabilizing the rate of emissions is not enough to avert climate change,” it wrote. “NOAA data show that the average growth rate of carbon dioxide concentration in the atmosphere from 2012 to 2014 was 2.25 ppm per year, the highest ever recorded over three consecutive years.”

James Butler, director of NOAA’s Global Monitoring Division, pointed out how far we would need to go in order to see appreciable change in global carbon dioxide levels. “Elimination of about 80 percent of fossil fuel emissions would essentially stop the rise in carbon dioxide in the atmosphere,” said Butler, “but concentrations of carbon dioxide would not start decreasing until even further reductions are made and then it would only do so slowly.”

[Scientists say we are changing our climate in an ‘abrupt and irreversible’ way]

NOAA says that global average CO2 levels are monitored at 40 different sites around the world. NOAA and their partner scientists “collect air samples in flasks while standing on cargo ship decks, on the shores of remote islands and at other locations around the world.”