From Huffington Post: http://www.huffingtonpost.com/2014/04/22/carbon-dioxide-climate-change_n_5187844.html
In May 2013, it was big news when, for the first time, the concentration of carbon dioxide in the atmosphere hit 400 parts per million. Now, researchers say that number has been consistently above 400 for the last month.
“This is higher than it’s been in millions of years,” said Pieter Tans, a senior scientist with the National Oceanic and Atmospheric Administration’s Climate Monitoring and Diagnostics Laboratory.
Parts per million, or ppm, is a measure of the ratio of carbon dioxide to other gases in the atmosphere. Carbon dioxide is just one type of greenhouse gas that has been found to trap heat, but it is the primary one emitted from human activities and it lingers in the atmosphere for a very long time. There is typically seasonal fluctuation in the parts per million (ppm) of carbon dioxide, according to scientists who track the levels. That explains why, after hitting 400 for the first time in recorded history last May, the levels declined soon after. But they hit 400 ppm again in mid-March, and have stayed above that level for all of April.