Environmental Engineering Reference
In-Depth Information
THE LITTLE ICE AGE
The commencement of the Little Ice Age has been the subject of much debate. Recent
work suggests that glacial advances in the Swiss Alps starting in the thirteenth century
and reaching an initial culmination in the fourteenth century indicated the
commencement of cooling. Locations around the North Atlantic confirm this
approximate starting point. It was not a steady deterioration but involved many
fluctations, with a decreasing temperature trend. It is the first period in which
instrumental observations can be used to measure climate change. In central England the
mean annual temperature in the 1690s was only 8·1° C - almost 2·1° C below the current
figure. Agriculture in upland areas became more difficult as the growing season
shortened, leading to the abandonment of many farms, whose land often reverted to
moorland or rough grazing. An added problem during this period of cooler temperatures
appears to have been enhanced variability of temperature. It was not merely a swing from
one year to another but a period of several successive years with similar temperature and
precipitation before a change to a period of a markedly different character. Other parts of
Europe were also affected. Glaciers advanced in the Alps, farms had to be abandoned in
Iceland and Scandinavia; in upland Languedoc in southern France there were food
shortages and famines associated with severe winters and wet summers. In Spain
agricultural difficulties arose through increased aridity and temperature variability.
Globally the extent of snow and ice on land and sea seems to have reached its highest
levels since the Loch Lomond stadial period, though the timing of its culmination varies.
It appears to have been earlier in the United States than in Europe or the southern
hemisphere, whilst in China the coldest periods were around 1700 and 1875. Much of the
evidence for cooling is based on ice advances. However, it is the combination of
temperature and precipitation as well as the 'response time' of the glacier which
determines advances rather than temperature alone. Analysis of marine sediments in the
Caribbean has indicated a maximum reduction of sea surface temperature of 2°-3° C in
the eighteenth century.
THE PRESENT CLIMATE
By the middle of the nineteenth century the effects of the Little Ice Age were waning in
most parts of the world and we begin the steady warming of the last century. The first
phase of warming peaked in the 1940s, followed by a slight decline in global mean
temperatures. At that time climatol-ogists were predicting the return of cooler conditions
and perhaps even another Ice Age. From the mid-1970s
