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

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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 highest they’ve been on Earth in at least 800,000 years — as there’s irrefutable proof trapped in the planet’s ancient ice. Though scientists emphasize that air bubbles preserved in ice are the gold carbon standard, there are less direct, though still quite reliable means to gauge Earth’s long-ago carbon dioxide levels. These measurements, broadly called proxies, include the chemical make-up of long-dead plankton and the evidence stored in the breathing cells, or stomata, of ancient plants. Scientists have identified this 15 million number by measuring and re-measuring proxies all over the world. Ancient air stored in ice core bubbles.Image: nasa“It’s a good scientific documentation, but it’s an indirect measure,” Michael Prather, a professor of earth system science at the University of California Irvine, said in