Short-term climatic change

Climate and weather vary on many scales of time and space. Short-term oscillations are superimposed on long-term ones, and all are governed by the interplay of external influences like solar radiation and the climate system's highly complex internal interactions. Much is known about the grander ice-age rhythms and the short-term seasonal and daily ones, but on the intermediate scale of centuries the picture has been less clear. Historical accounts of climatic episodes and the observed advance and retreat of mountain glaciers in many parts of the world over the last 1,000 years led paleoclimatologists to postulate the two long, sustained warm and cold spells that they called the Mediaeval Warm Period (between about 900 and 1300, when vineyards flourish in Britain and the Vikings colonized Greenland) and the Little Ice Age (from the 15th to the 19th centuries, when mountain glaciers expanded and extreme cold ravaged the northern hemisphere). Under this formulation, the centuries-long Mediaeval Warm Period saw sustained global temperatures perhaps half a degree centigrade warmer than the 20th century, while the global temperature was 1 to 1.5 degrees colder in the Little Ice Age. By comparison earth is 2.8 to 5 degrees warmer now than in the depths of the last ice age.

More precise analysis is now casting serious doubt on this picture of worldwide, centuries-long climatic episodes. Emerging in place of the conventional view is a much more complex picture in which different climatic conditions prevailed in different parts of the world at a given time, and climates fluctuated between warner and colder on a scale of decades rather than centuries. Latest research findings reported in 1993 were that analysis of corings taken from the summit of the Greenland ice sheet showed that while there was no sustained Little Ice Age there, a relatively cold period of greatly fluctuating climate took place after 1700, and a warm period between 620 and 1150 AD, during which conditions were above freezing. But careful analysis of tree rings, mountain glaciers and historical documents fail to show that the Mediaeval Warm Period observed in the North Atlantic region was global in its reach. There is some evidence for a sustained mediaeval warming at high altitudes, like the Greenland summit and the Andes, but not at lower altitudes. A computer reconstruction of the Northern Hemisphere's climate from 1400 to 1970 showed not a single, long-term pattern of any kind but rather a wide range of variations in different parts of the world, with oscillations between warmer and cooler regional climates taking place on scales of a decade or two.