Mongabay: Atmospheric carbon dioxide levels are higher than any point in the last 2.1 million years, report researchers writing in the journal Science. Analyzing the shells of single-celled plankton buried under the Atlantic Ocean off the coast of Africa, Bärbel Hönisch, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory, and her colleagues dtermined that peak CO2 levels over the last 2.1 million years averaged only 280 parts per million. By comparison current CO2 levels …