Monitor: Antarctica recently witnessed an event that last took place four million years ago: Carbon dioxide levels broke through the barrier of 400 parts per million (ppm).
In fact, the frozen continent is a laggard in this respect. The announcement Wednesday by the National Oceanic and Atmospheric Administration (NOAA) gave the date of this milestone as May 23. But the global annual mean concentration of carbon dioxide already surpassed 400 ppm last year, the first time that has happened in human history....