Four hundred: it’s a number that will go down in climate history while also continuing to rise.
According to the National Oceanic and Atmospheric Administration (NOAA), 400.83 parts per million (ppm) was the average concentration of atmospheric carbon dioxide in March. This news from NOAA marks the first time that the entire planet has surpassed the 400 ppm benchmark for an entire month.
With the rate of growth of atmospheric carbon dioxide concentrations steadily increasing — rising from about 0.75 ppm per year in 1959 to about 2.25 ppm per year in 2015 — this milestone will soon be surpassed. Still, the 400 ppm average has been a long time coming.
“It was only a matter of time that we would average 400 parts per million globally,” Pieter Tans, lead scientist of NOAA’s Global Greenhouse Gas Reference Network, said in a statement. “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 continued. “Half of that rise has occurred since 1980.”
As the climate change activist group 350.org — named after what they deem to be a safe level of atmospheric carbon dioxide — states, at the beginning of human civilization the atmosphere contained about 275 ppm of carbon dioxide. This concentration began to rise during the Industrial Revolution in the 18th century, and the large-scale emissions of greenhouse gases into the atmosphere remains a critical part of human societies across the world.
In the summer of 2014, the NOAA’s Mauna Loa observatory in Hawaii averaged above 400 ppm for several months in a row, marking the longest time in recorded history that that much carbon dioxide had been in the atmosphere. However the global monthly average of 400 ppm was not surpassed until this March. According to NOAA, the global average for CO2 levels in March 2014, one year ago, was 398.10 ppm. The CO2 level at Mauna Loa on May 4, 2015 was 403.90 ppm.
NOAA takes these measurements from 40 isolated sites around the world that are far removed from local pollution sources that could disturb the data.
This year has quickly become the year that the symbolic 400 ppm concentration, which represents a level of CO2 that many scientists associate with a path towards certain irreversible and catastrophic climate impacts, would be definitively surpassed. In January, the the Scripps Institution of Oceanography showed that carbon dioxide levels exceeded 400 ppm on Jan. 1, Jan. 3 and Jan. 7.
The 400 ppm threshold was first crossed at Mauna Loa in May 2013, which is typically the month with the highest carbon dioxide average.
“CO2 levels continue to increase, the amount of heat in the climate system continues to increase, ice continues to melt, and the seas continue to rise,” Ken Caldeira, an atmospheric scientist at the Carnegie Institution for Science’s Department of Global Ecology, told ThinkProgress in January. “We will continue to break through threshold after threshold — unless we stop using the sky as a waste dump soon.”
The symbolism of this number will not be lost on world leaders as they gather at the end of 2015 in Paris in hopes of finalizing the next international climate treaty to mitigate greenhouse gas emissions. These negotiators must not only deal with domestic economic and political pressures, but also a long history of unequal emissions and unequally distributed impacts. One positive note comes from the recent data showing the economic growth and fossil fuel emissions are finally starting to decouple.
“Reaching 400 ppm doesn’t mean much in itself, but the steady increase in atmospheric greenhouse gases should serve as a stark reminder of the task facing politicians as they sit down in Paris later this year,” Ed Hawkins, a climate scientist at the University of Reading, told the Guardian.