In another climate milestone, a study projects levels of carbon dioxide in the atmosphere will smash the threshold of 400 parts per million this year and not fall below it again in our lifetimes. The 400-parts-per-million threshold has been an important marker in U.N. climate negotiations, widely recognized as a dangerous level that could drastically worsen 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.