The global level of carbon dioxide in the atmosphere has topped 400 parts per million for the longest time in recorded history. The National Oceanic and Atmospheric Administration says the grim milestone was reached on average for the entire month of March. The 400 parts per million threshold has been an important marker in U.N. climate change negotiations, widely recognized as a dangerous level that could drastically worsen human-caused global warming. The environmentalist group 350.org takes its name after the 350 parts per million threshold that scientists say is the maximum atmospheric concentration of carbon dioxide for a safe planet.