A record was set in March 2010, though you won’t find any mention of it in the corporate media.
Atmospheric CO2 concentrations measured at Mauna Loa, Hawai’i were the highest for a single calendar month in our history: 391.06ppm.
Monthly data for the past four years and the entire 52-year dataset can be found at NOAA’s ESRL Trends website.
Reputable climatologists have identified 350ppm as a goal that should be formally adopted by the world’s governments. That’s going to take some work since CO2 concentrations increased at ~2ppm/year in the last decade.
The last time atmospheric CO2 concentrations were this high was 15 million years ago. How was the Earth different back then?
“The last time carbon dioxide levels were apparently as high as they are today — and were sustained at those levels — global temperatures were 5 to 10 degrees Fahrenheit higher than they are today, the sea level was approximately 75 to 120 feet higher than today, there was no permanent sea ice cap in the Arctic and very little ice on Antarctica and Greenland,” said the paper’s lead author, Aradhna Tripati, a UCLA assistant professor in the department of Earth and space sciences and the department of atmospheric and oceanic sciences.