The wave of tree deaths on Mammoth Mountain, which lies within one of the nation’s largest active volcanic systems, prompted scientists to start monitoring the volcano’s emissions more closely. Now, researchers led by Stanford University geologist George Hilley have made a surprising discovery in the long-running record: The ebb and flow of carbon dioxide emissions from Mammoth Mountain are strongly linked to the weight of snow and ice atop the Sierra Nevada, and to the amount of water that percolates from ground level down into the volcano’s plumbing.