EISRIESENWELT, AUSTRIA—Tracing his glove along a chalky layer in a house-size block of ice that lines this cave in the Austrian Alps, Michael Behm can feel all that is left of an ancient warm spell. The ice, likely formed over the decades or centuries as calcium-enriched rainwater trickled deep in to the cave and froze, must have once warmed enough on top to melt and release a few years’ worth of the mineral, the Vienna University of Technology geophysicist explains.