In the Central England Temperature (CET) series, which extends continuously back to 1659, the coldest year is 1740 with a mean temperature of 6.8 °C, nearly 3 degC below the mean of the 363-year record. The probability of such a value being reached in 300 years is about 1 in 1000 (Probert-Jones, 1984), which could be translated into the statement that such a value would be expected to occur on average once in every 300 000 years, although that does assume a non-changing climate. Nevertheless, the preceding decade was warm with a mean CET of 9.9 °C, and the following decade (1741–1750) had a mean of 9.1 °C.
The exceptionally anomalous year of 1740 is just what would be expected if there had been a large volcanic eruption. Yet the only possible eruption was in Kamchatka, in the far east of Russia, and was almost certainly irrelevant (Lamb, 1970). As far as I am aware, no explanation has been advanced for this unique event. Can it be that on very rare occasions the climate simply ‘goes mad’?