"We've compared our monthly means to Scripps for many years, and the differences are very small," Tans said. Other CO2 observations taken at Mauna Loa have also been very close, which shows the measurements are credible, he added.
Mauna Loa is a fairly good approximation for what global concentrations are, although it is slightly higher than the global average because it is in the Northern Hemisphere, which generally has higher CO2 concentrations.
The Arctic already experienced a monthly average concentration above 400 ppm, last May. It typically gets to a peak concentration above that of the rest of the world due to winds blowing CO2 up north in the spring.
This year, NOAA monitoring sites in the Arctic have already reached a 400 ppm monthly average, even before that May peak, Tans said.
There is a small chance that this May, the average at Mauna Loa could reach more than 400 ppm. But Tans thinks it is more likely this will happen in May 2014.
And then: "At some point, the global average will go over 400, likely another year after that," he said.
Tans applauded Keeling's effort to create a website with updated daily information, saying the more scientists can educate the public that the increase of CO2 is due to human activities, the better.
"I want us to do more outreach than we typically do. So these websites are one way."
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500