The fears about permafrost carbon losses are based on some simple chemistry. Unlike at warmer latitudes, where microorganisms in the soil constantly break down plant matter and return the carbon it contains to the atmosphere, Arctic soils have been cold enough to preserve the frozen remains of ancient plant life. But as the planet warms, soil microbes become able to break down more and more of this carbon, sending it back into the atmosphere and worsening global warming in a troubling feedback loop.