The Last Time Atmospheric Co2 Reached 400 ppm

"Today, greenhouse gasses in Earth's atmosphere are at their highest since the Pliocene Era, when sea levels were higher and Earth was warmer... On May 9, 2013, an instrument near the summit of Mauna Loa in Hawaii, United States, recorded a long-awaited climate milestone: the amount of carbon dioxide (CO2) in the atmosphere there had exceeded 400 parts per million (ppm) for the first time in 55 years of measurement—and probably more than three million years of Earth history... The last time the concentration of Earth's main greenhouse gas reached this mark, horses and camels lived in the high Arctic. Seas were at least 9.1 meters (30 feet higher)—at a level that today would inundate major cities around the world... The planet was about 2 to 3 degrees Celsius (3.6 to 5.4 degrees Fahrenheit) warmer. But Earth then was in the final stage of a prolonged greenhouse epoch, and CO2 concentrations were on their way down... between 2.6 million and 5.3 million years ago."