For the first time in human history, concentrations of carbon dioxide in the atmosphere are expected to pass 400 parts per million across much of the Northern Hemisphere in May, according to scientists who study data from the Mauna Loa Observatory, the world's longest-running CO2 monitoring station.

While crossing the 400 ppm mark isn't a "tipping point" that signals climate catastrophe, scientists told InsideClimate News, it is an important symbolic milestone that underscores government inaction on global warming.

"This is another global emissions target that we've blown past without doing anything," said Jim Butler, director of global monitoring at NOAA's Earth System Research Laboratory. "Stronger storms, droughts, rising seas. We are already seeing the impacts of increased CO2 in the atmosphere ... How much further can we really go?" The NOAA lab operates the Mauna Loa Observatory and dozens of other greenhouse gas monitoring sites across the globe

The record level is expected to be the latest global warming milestone chronicled by the Mauna Loa Observatory, which was started by renowned climate scientist Dave Keeling in 1958. For 55 years, the station has taken hourly atmospheric CO2 readings from atop a Hawaiian volcano two miles up in the air.

The graph produced from its measurements, known as the Keeling Curve, was the first to show the tight relationship between the increase in CO2 in the air and the rise in the burning of fossil fuels like coal, oil and natural gas. The research is considered one of the pillars of the scientific consensus that human activity is the main driver of climate change.

From analyzing air bubbles trapped in icesheets and glaciers, scientists know that global CO2 levels never exceeded 300 ppm during the 800,000 years before the Industrial Revolution. They also know that the buildup of the heat-trapping gas began when humans started burning fossil fuels to power industry in the 18th century.

By the time Keeling set up his autonomous monitoring site in Hawaii in 1958, his instruments showed that concentrations of CO2 were already at 317 ppm.

Five decades later, scientists at the Scripps Institution of Oceanography at the University of California, San Diego—who maintain the Keeling Curve measurements—expect CO2 levels to reach and exceed 400 ppm during much of May.

"This is an important point at which to pause and think about where we've come from and where we're going," said Ralph Keeling, director of the Scripps CO2 Program and the son of Dave Keeling, who died in 2005.

To bring attention to the milestone, the Scripps researchers are publishing daily updates of CO2 concentrations at Mauna Loa on their website and on twitter.

Last week, Rep. Rush Holt (D-N.J.) used the landmark as an opportunity to call for Congressional action to tackle climate change. "As the earth goes barreling past 400 parts per million of carbon dioxide in the atmosphere, let's take action now, this year, this Congress, to address climate change," Holt said on the House floor. "Let's show that this is a turning point, not just a marker of inaction and environmental degradation."

Not Just in Hawaii



In 2012, NOAA-run monitoring sites in the Arctic detected CO2 levels just above 400 ppm, but only for short periods of time. Historically, Arctic CO2 concentrations have always been a few years ahead of the rest of the world because winds tend to blow the gas north and because the area has less vegetation to pull the CO2 out of the air.

Mauna Loa, by contrast, has generally been statistically indistinguishable from the global CO2 average. That means if the amount of CO2 rises above 400 ppm at the Hawaii site this month, the rest of the world is likely experiencing similar concentrations, Keeling said.

Keeling noted that 400-plus ppm levels won't last long this year. As trees regrow their leaves amid warmer spring temperatures in the Northern Hemisphere, they will be able to sop up more CO2 from the air, and concentrations of the global warming gas will dip back below 400 ppm. This natural seasonal variation has been recorded throughout the Keeling Curve's history.

But it will only be a short time before we leave the 300s behind forever, Keeling said.

According to the Mauna Loa data, the yearly low (recorded at the end of summer) usually catches up with the yearly high (recorded at the end of winter) in four or five years, Keeling explained. This means that by around 2017, CO2 concentrations will likely be above 400 ppm throughout the entire year, even in the peak of summer when vegetation is fully grown—levels that scientists haven't seen since the Pliocene era, three to five million years ago.

Butler of NOAA's Earth System Research Laboratory said data from NOAA's dozens of monitoring sites support this finding.

"The next big emissions target is 450 ppm," Butler said. "That's the one the IPCC [The United Nations' Intergovernmental Panel on Climate Change] warns we really don't want to rise above. Based on our current rate of emissions, we could be there in 25 years. That's a scary prospect."

Watch a visual presentation of how CO2 has changed over the last 800,000 years. Video courtesy of Andy Jacobson of the University of Colorado and NOAA's Earth System Research Laboratory:

http://www.youtube.com/watch?v=vA7tfz3k_9A