evidence: White began working on ice cores, the most reliable and accurate way for paleoclimatologists to recreate past climates – and past climate changes. The annual accumulation of snow in the Arctic, Greenland and Antarctica contains isotopic markers of the temperature at the moment it fell, as well as telltale traces of atmospheric components – from carbon dioxide to barium. Year after decade after century after millennium, these compressed snow layers trap air bubbles in them and become buried ice vaults, locking data into retrievable troves of climate and atmospheric information. Think of it as a mile-deep frozen filing cabinet. [...] White's ice–core studies helped reveal two striking facts. The first is that the Earth's great ice ages are bookmarked by a clear fluctuation in carbon dioxide levels: 180 parts per million (ppm) in the glacial periods, 280 ppm in the warmer periods (the level at the beginning of the Industrial Revolution about 150 years ago). A shift of 100 ppm in CO2 concentrations meant the difference between flowers blooming in the Arctic and ice a mile deep over Chicago. [...] At first, White was convinced that humans would understand the obvious implications of his ice-core data: The consequences of human-caused climate change "would basically cripple any kind of modern society." Then they'd quickly figure out alternatives to filling the atmosphere with burned fossil fuels.