The average amount of carbon dioxide in our atmosphere has been above 400 parts per million (ppm) since the beginning of 2015, a number many climate scientists consider a symbolic representation that humans are on the fast-track toward irreversible global warming.
In an article for Climate Central, Andrea Thompson reports on data from the The Scripps Institution of Oceanography showing atmospheric carbon dioxide levels above the 400 pm marker for much of 2015 so far. The first date of 2015 that reached above that concentration was Jan. 1, followed by Jan. 3 and then Jan. 7.
With the start of the year having such high carbon concentrations, some predict that 2015 will likely see many months where carbon levels are, on average, above the 400 pm threshold. “My guess at this point is that January 2015 will be very slightly above 400 ppm, but it’s too early to tell for sure,” Ralph Keeling, the scientist in charge of Scripps’ CO2 monitoring project, told Thompson in an e-mail.
Among watchers of climate change, the 400 ppm level of carbon dioxide is seen sort of like when the temperature reaches 100°F on a hot day. It really isn’t much hotter than 99°F, but somehow it feels more like a landmark of a true scorcher.
For global warming, 400 ppm is a palpable representation of how quickly humans are pumping carbon dioxide — the main driver of global warming — into the atmosphere. A December 2013 report by longtime NASA scientist James Hansen and 16 other scientists says that stabilized carbon levels of below 350 ppm “would be appropriate, if the aim is to stabilize climate without further global warming.”
The first time we reached that 400 ppm threshold was in May 2013. But it was brief — only a small moment where the levels hit the landmark. As ClimateProgress has noted before, it doesn’t make much of a difference if carbon is at 400 ppm for a brief period of time. What does make a difference is a constantly-increasing average level of carbon dioxide trapping more heat in the atmosphere.
Climate scientists have seen that average level of carbon dioxide in the atmosphere steadily increasing. The United Nation’s weather agency said in a September report that more carbon dioxide was emitted into our atmosphere between 2012 and 2013 than in any other year since 1984. And this past summer, scientists observed that June was the third month in a row where, for the entire month, average levels of carbon dioxide were above 400 parts per million (ppm). That marked the longest time in recorded history that so much carbon dioxide has been in the atmosphere.
The finding was troubling to climate scientists, several of whom told ThinkProgress at the time that the levels reminded them that humans are still emitting unsustainable amounts of greenhouse gases. If the trend continues, some worried that carbon levels might soon surpass 450 ppm — a level that many scientists worry would create a level of global warming that would be too difficult for some humans to adapt to.
“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,” said Ken Caldeira, an atmospheric scientist at the Carnegie Institution for Science’s Department of Global Ecology. “We will continue to break through threshold after threshold — unless we stop using the sky as a waste dump soon.”