E. Linacre and B. Geerts
Sometimes the climate of a place appears to shift for years, decades or centuries from one regime to another. The book documents abrupt changes of Darwin’s annual rainfall in 1898, 1914, 1923, 1967, 1983 (Fig 10.10) and of Sydney’s annual mean temperature in 1912, 1925, 1935, 1942, 1950, 1965 (Fig 15.12).
Other evidence of climatic turning points has been found in records of salmon production in the northwestern USA in 1947 and 1977 (1). These switchings coincided with abrupt changes in the stream flows and surface temperatures on the west coast of the USA and with several changes in the Pacific basin in the late 1970’s, apparently related to ENSO.
About 11.7kBP, at the end of the last glacial, western Europe suddenly turned considerably cooler, by as much as 4K, heralding a return to ice age conditions.This cold period, known as the Younger Dryas, appears to be have been caused by a changed thermohaline circulation as a result of the melting of the North American ice sheet. This circulation reversed itself some 500 years later (Fig 15.9). The beginning of the Younger Dryas came suddenly, within 20 years, and it was felt world-wide, even in Antarctica (2), although most strongly in northwestern Europe.
In conclusion, sudden changes in climate, lasting longer than a few years, are usually related to changes in the ocean circulation.