Earth’s carbon dioxide levels are likely the highest they've been in 15 million years

We’ve entered some profoundly unfamiliar planetary territory. Amid a backdrop of U.S. politicians still questioning whether the changing climate is attributable to humans (it is), it's quite likely that we’ve actually boosted Earth's carbon dioxide — a potent greenhouse gas — to the highest levels they’ve been in some 15 million years.  The number 15 million is dramatically higher than a statistic frequently cited by geologists and climate scientists: That today's carbon levels are the...