According to Sherlock Holmes "detection is, or ought to be, an exact science". Sometimes the reverse is also true: science often involves detective work.
Meteorologists turn the most unlikely stones in search of their climatic past, and rummage endlessly in nature's bin for bits and scraps of information to complete their theories. Recapturing the weather of centuries gone by can be, as Holmes put it, "quite a three-pipe problem".
Organised instrumental records are available from the mid-19th century onwards, and the pattern of the weather can be clearly seen. But in the case of previous eras, numbers are hard to come by, and the searcher must look elsewhere. Contemporary writings are a fruitful source, but the problem becomes more difficult when the search concentrates on epochs when there were no written records - the early days of civilisation and prehistory. The evidence is less obvious but it can be found.
A fruitful source of data, for example, is dendrochronology. The growth-rings of trees vary in width depending on the rainfall and the temperature in the year in which they form; a rotting prehistoric log can tell a long and useful story to the climatologist.
Another technique is radioactive dating, which emerged during the late 1940s. The bombardment of the Earth by cosmic rays from outer space results in the formation in the atmosphere of tiny quantities of a radioactive element called Carbon 14 (C14). Traces of this substance find their way into the tissues of all living organisms. As long as a plant or animal lives, it continues to absorb C14, and the substance is maintained in its system at a more or less constant level. But when the organism dies, the C14 within it begins to decay by radioactive breakdown.
The age of any piece of preserved bone, or any fragment of a long-dead tree, can be dated by measuring its C14 content, and then relating the residual quantity of this substance which it contains to the known rate of radioactive decay of the element.
The benefit of this technique to climatologists lies in the fact that the fauna and flora of a region tell a great deal about the local climate. Long-dead plants and animals are a signature of the climatic conditions which existed at the time they were alive, and radioactive dating allows this time to be identified with great accuracy. It can be used to date samples up to 40,000 years old, and proved to be such a valuable addition to so many branches of science that its discoverer, Willard Frank Libby, received the Nobel Prize for Chemistry in 1960.