General Considerations

In this final chapter we examine the variation of climate on different timescales, related climatic forcing factors and projected future changes. Global climate change in the twentieth century has reinforced the recognition that climatic conditions are non-stationary and that human activities have major impacts on the climate system.

Realization that climate is far from being constant came only during the 1840s, when indisputable evidence of former ice ages was obtained. Yet, in many parts of the world, the climate has altered sufficiently within the past few thousand years to affect the possibilities for agriculture and settlement. Study of past climate began with a few individuals in the 1920s and more actively in the 1950s (see Box 13.1). Weather records for most parts of the world span only the last hundred years or so. However,proxy indicators of past conditions from tree rings, pollen in bog and lake sediments, ice core records of physical and chemical parameters and ocean foraminifera in sediments provide a wealth of paleoclimatic data.

It is first worthwhile to consider the nature of climate variations. The standard interval for climatic statistics adopted by the World Meteorological Organization is thirty years: 1971 to 2000, for example. However, for historical records and proxy indicators of climate, longer, arbitrary time intervals may be used to calculate average values. Tree rings and ice cores can give seasonal/annual records, while peat bog and ocean sediments may provide records with only 100- to 1000-year time resolution. Hence, short-term changes and the true rates of change may not be identifiable.

The present climatic state is usually described in terms of an average value (arithmetic mean, or the median value in a frequency distribution), a measure of

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