Past Climate Change

2.2.1 Distant past - millions to thousands of years before present (bp)

Climatic change occurs on diverse scales of time and space. The largest changes have occurred on the same time scale as that of drifting continents. However, large variations, such as interglacial periods that have marked the climate record during the past 3 million years or so, occurred in cycles that lasted tens to hundreds of thousands of years (Fig. 2.1a) (Schneider et al., 1990). Thus, climatic change is the normal state of affairs for the earth/ atmosphere system. This suggests that the notion of a stable, stationary climate is an erroneous concept, while that of 'unceasing climatic change' may be a more useful mental model by which to analyse the earth's climate resources through time.

Climates of the past billion years have been about 13°C warmer to 5°C cooler than the current climate (Schneider et al., 1990). Prominent in earth's recent history have been the 100,000-year Pleistocene glacial/interglacial cycles when climate was cooler than at present (Fig. 2.1a). Global temperature varied by about 5°C through the ice age cycles. Some local temperature changes through these cycles were as great as 10-15°C in high latitude regions. During the last major glaciation, ice sheets covered much of North America and northern Europe, and sea level averaged 120 m below current values.

Since the last glaciation, there have been relatively small changes of probably less than 2°C (compared with the current global mean temperature) in

Fig. 2.1. Schematic diagrams of global temperature variations since the Pleistocene on three time-scales: (a) the last million years; (b) the last 10,000 years; and (c) the last 1000 years. The dotted line nominally represents conditions near the beginning of the 20th century. Each unit on the x-axis of all three panels represents 1 °C. (Source: Folland et al., 1990. Reprinted with permission of Hadley Centre for Climate Prediction and Research, Meteorological Office, Bracknell, UK.)

Fig. 2.1. Schematic diagrams of global temperature variations since the Pleistocene on three time-scales: (a) the last million years; (b) the last 10,000 years; and (c) the last 1000 years. The dotted line nominally represents conditions near the beginning of the 20th century. Each unit on the x-axis of all three panels represents 1 °C. (Source: Folland et al., 1990. Reprinted with permission of Hadley Centre for Climate Prediction and Research, Meteorological Office, Bracknell, UK.)

global average temperatures. During the mid-Holocene epoch between 4000 and 6000 years bp, however, temperatures rose significantly, particularly during the summer in the northern hemisphere (Fig. 2.1b) (Folland et al., 1990).

2.2.2 Recent past - medieval optimum and the Little Ice Age

Fluctuations in the distant past are important for analysing the various causes of climatic fluctuations, but substantial fluctuations within recent human history (Fig. 2.1c) are more compelling for humans. For example, the so-called medieval optimum occurred from about the 10th to early 12th centuries. There is evidence that western Europe, Iceland and Greenland were exceptionally warm, with mean summer temperatures that were more than 1°C higher than current ones. In western and central Europe, vineyards extended as much as 5 degrees latitude farther north than today (Gribbin and Lamb, 1978). Not all regions experienced greater warmth; China, for example, was considerably colder in winter.

The most notable fluctuation in historical times was that known as the Little Ice Age, which lasted roughly from 1450 to the mid-19th century. earth's average temperature at one point was 1°C less than that of today (Fig. 2.1c) (Lamb, 1982). The effects of the Little Ice Age on everyday life are well documented: the freezing over of the river Thames in London; the freezing of New York harbour, which allowed citizens of New York to walk to Staten Island; abandonment of settlements in Iceland and Greenland; and crop failures in Scotland (Parry, 1978).

While there has been wide speculation on the cause or causes of the Little Ice Age (e.g. increased volcanism, reduced solar activity), there is no definitive explanation. This fluctuation is significant because it ended before the heavy industrialization of the late 19th century began, and is thus believed to be largely free of human causes. Some have argued that the increased global temperature that has been observed in the 20th century represents a 'recovery' from the Little Ice Age. However, without a definitive cause for the coolness of the Little Ice Age, the concept of a recovery from that anomalous cold period remains dubious.

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