Years ago, it passed the 350ppm mark that many scientists say is the highest safe level for carbon dioxide. It now stands globally at 395.
So far, only the Arctic has reached that 400 level, but the rest of the world will follow soon.
"The fact that it's 400 is significant," said Jim Butler, the global monitoring director at the National Oceanic and Atmospheric Administration's Earth System Research Lab. "It's just a reminder to everybody that we haven't fixed this, and we're still in trouble."
"The news today, that some stations have measured concentrations above 400ppm in the atmosphere, is further evidence that the world's political leaders – with a few honourable exceptions – are failing catastrophically to address the climate crisis," former vice president Al Gore, the highest-profile campaigner against global warming, said in an email. "History will not understand or forgive them."
Carbon dioxide is the chief greenhouse gas and stays in the atmosphere for 100 years. Some carbon dioxide is natural, mainly from decomposing dead plants and animals. Before the industrial age, levels were around 275 parts per million.