Drilling and core sampling carried out on ocean floors and residual icecaps in Greenland and Antarctica have allowed scientists to reconstruct climate changes. Oxygen isotope ratio cycles provide information about local temperatures and the total volume of ice on the continents. Foraminifera, molluscs and insects are also reliable climate indicators.
The last glacial period of the Pleistocene reached its maximum extent some 20,000 years ago (Pleniglacial), a time when vast areas of the Earth were covered in a thick layer of ice. The Middle Magdalenian period (ca. 13,000–15,000 years BP) marked the beginning of the Tardiglacial, a time of climatic instability which gave rise to the relative stability of the Late Glacial Maximum (fig. ). Sea levels were some 120 metres lower than the current shorelines. The variations connected with Heinrich Event 1 (H1) were swift and large-scale. One of the key consequences was that huge quantities of fresh cold water from melting icebergs were released quickly, which cooled surface waters and triggered desertification. The prevailing climate during the Middle Magdalenian was thus very cold and dry.