“Medieval” Cool Period

This interval, extending roughly from 1250 to 1500, corresponds to the Paria Emergence in the eustatic record and has been called one of the “little ice ages” by certain authors. Solar activity records show a decline from 1250 to 1350, a brief rise from 1350 to 1380, and then a phenomenal low that lasted until 1500. Pollen records in northern Europe reveal rather consistently cool conditions, and smoothed mean temperature curves show a cumulative drop during this period. Stalactite studies in a karst cave in France showed a travertine growth peak (indicating cool, moist conditions) in 1450. In North America cool, moist conditions were widespread at first, becoming dry later. The arroyos and washes became filled with the Naha Alluvium, and the human population decreased markedly. There is pollen evidence of a temperature drop of about 1° C. This is the period of the “Great Drought.” In the upper Mississippi valley the Indian cultures began a general decline, accompanied by a transfer from agriculture to hunting. It was similar in the western prairies, and it was this hunting culture that the first Spanish explorers encountered.

In the Canadian north the mean temperatures had dropped about two degrees below the previous high. In the Sierra Nevada, the Rockies, and Alaska there were glacial readvances, with evidence of a 2° C temperature drop. In the Arctic regions, the Eskimo economy underwent a marked change to adjust to these more extreme conditions, which amounted to about 5° or 6° C below the mean of the climatic optimum.

The Norse settlements in Greenland were abandoned altogether as the permafrost advanced. Pollen studies at Godthåb indicate a shift from a maritime climate to a cold, dry continental regime. The sea ice off Iceland reveals an extraordinary growth in severity, from zero coverage before the year 1200 to eight-week average cover in the 13th century, rising to 40 weeks in the 19th century, and dropping again to eight weeks in the 20th century. In Japan there were glacial readvances and a mean winter temperature drop of 3.5° C. Summers were marked by excessive rains and bad harvests.

The equatorial regions now began a marked desiccation, with a drop in level of all the great African lakes. The Nile suffered a decreased flow and alluviation.

South of the equator in the temperate belts there occurred a general return to cooler and wetter conditions that have continued (with oscillations) until the present time in southern Chile, Patagonia, southernmost Africa, southwestern Australia, and New Zealand.

Little Ice Age

Throughout most of what is commonly called the Little Ice Age (1500–1850) the mean solar activity was quite low, but positive fluctuations occurred about 1540–90 and 1770–1800. The main westerly storm belts shifted about 500 kilometres to the south, and for much of the time the northern latitudes came under cool continental conditions. Observed temperature series in Europe from Paris to Leningrad show large fluctuations until 1850.

Glacier advances are recorded in the Alps, in the Sierra Nevada, and in Alaska. Corresponding low sea levels are recorded by early tide gauge records in The Netherlands and Germany. Even in equatorial latitudes there are traces of mountain glacier advances (as in the Andes of Colombia).

The Industrial Age (1850–1950)

The year 1850 started a brief warming trend that persisted for 100 years. It also approximates a critical turning point in climatic, sea level, glacial, and sedimentologic records. In many regions of central and southern Europe “anthropogenic” sediments (or cultural layers) began to appear in Neolithic times (early to mid-Holocene). Elsewhere in the world (e.g., in North America, Australia, South Africa), however, this type of sedimentation began about the middle of the 19th century, depending on soil erosion stimulated by mechanized (disk) plowing, large-scale deforestation, and engineering activity. Thus, independently of natural climatic change, the century 1850–1950, marked by anthropogenic aridification, proved to be a time of man-made deserts.