Carbon dioxide levels exceeded 400 ppm across the Northern Hemisphere for the entire month of April, a significant climate-change milestone. But this is the first time scientists have seen a whole month over 400 ppm at monitoring stations across the Northern Hemisphere, says Gavin Schmidt, a climate scientist with NASA's Goddard Institute for Space Studies in New York City, who was not involved with the measurements. "Four hundred ppm is not in and of itself particularly important physically," says Schmidt, "but it is emblematic of the fact that we are pushing the climate system into territory that is uncharted." http://news.nationalgeographic.com/news/2014/05/140527-400-ppm-carbon-dioxide-global-warming-climate-science/?utm_source=Twitter&utm_medium=Social&utm_content=link_tw20140528news-nohemco&utm_campaign=Content&sf3080334=1