Scientists are warning the planet has now reached a grim climate milestone not seen for two or three million years. The National Oceanic and Atmospheric Administration says the amount of heat-trapping carbon dioxide in the atmosphere has topped 400 parts per million. 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 it name after the 350 parts per million threshold that scientists say is the maximum atmospheric concentration of carbon dioxide for a safe planet. In a statement on the parts per million number hitting 400, 350.org co-founder Bill McKibben said: “The only question now is whether the relentless rise in carbon can be matched by a relentless rise in the activism necessary to stop it.”