Yesterday and today, in between trips to visit my brother in Balaclava, I have prepared a comprehensive, tabulated analysis of the CET data as a follow-up to the work of Uncharted Territory on the famous anomaly of 1740 – the coldest twelve months in the CET record.
|Mean annual Central England Temperature for each fiscal year from 1659/1660 to 2014/2015|
Uncharted Territory’s work on winter CET has given me the incentive to look at summer CET over time, so we shall do this one next. This is especially significant as long-term ice fluctuations are much more influenced by summer than winter temperature – colder winters actually inhibit the growth of ice sheets as seen in completely unglaciated Siberia and Manchuria. Following on from Uncharted Territory, I have provided a 9-year and 75-year running mean on the graph:
- As with the fiscal year CET graph, there is a general rise in summer CET since the 1690s
- There are numerous clear groups of cool summers such as:
- from 1679 to 1700 (14 of 22 summers in lowest quartile)
- from 1809 to 1823 (nine of fifteen summers in lowest quartile)
- from 1839 to 1848 (seven of ten summers in lowest quartile)
- from 1860 to 1862 (two of three in lowest five percent)
- from 1879 to 1894 (ten of sixteen summers in lowest quartile)
- from 1902 to 1924 (twelve of 23 summers in lowest quartile)
- Before global warming intensified in the 1980s, there were noticeable fewer long runs of hot summers than of cool summers
- there were only two periods (plus a brief one from 1893 to 1901) with five or more “hot summers” (top quartile) in nine years
- these were as noted above all between 1771 and 1783 before the Laki eruption
- or in the late 1720s and early 1730s
We will now look at the spring CET graph:
- The 75-year spring CET peak in the eighteenth century is a little later and does not reach above the virgin mean
- Deep short-term sequences of cool springs occur in the 1740s and 1760s (culminating in the second-coolest spring in 1770) that are not followed by cool summers
- There is no deep trough following the 1810 volcanic eruption due to several notably hot springs in 1811, 1815 and 1822
- The minimum in spring temperature following the eruption of Krakatoa is easily the most pronounced since the 1740s.
- The early-1840s summer temperature trough is a double trough in the spring season due to exceptionally cool springs in 1837 to 1839 and 1855.
Apart from several hot autumns between 1818 and 1828, the sole significant maximum before the 1930s is during the first half of the eighteenth century – the 75-year maximum is centred upon 1743 and falls off rapidly due to a record cool autumn in 1786 and several other very cool autumns between 1782 and 1794.
Although it is also seen in the spring graph, there is a notable 1680s CET autumn upswing that however, relates only to a pair of succeeding hot autumns in 1680 and 1681.
A critical feature of the autumn graph is that it shows a much larger signature from anthropogenic greenhouse gas emissions than the spring, summer or winter graphs. Only three cooler-than-normal autumn seasons have occurred in the past forty years, and two of these are only just below the virgin mean. The distance by which record hot recent autumn seasons have exceeded previous records is also considerably greater than for the summer and spring seasons. This probably reflect the low British insolation and considerable seasonal lag during autumn. Absorption and release of heat by greenhouse gases probably has greatest impact during autumn not only in Britain, but just as importantly in areas – Siberia, the Arctic Ocean – from where cold airmasses must be advected into the UK to produce an unusually cool autumn like 1952, 1919, 1887 or 1786.
I will only briefly discuss the winter and extended winter (November to March) graphs as these have been done elsewhere:
|Note the extreme cold in the US during the extended winter of 1898/1899, when Central England was having one of its mildest seasons since the 1730s with an average of 5.93˚C or 42.67˚F|
Although explaining these peaks and troughs observed in CET data before 1974 is beyond the information I have at present, I do know that rainfall proxy data suggest that before the 1720s and from 1760 to 1820 the Sahel was wet and Southern California dry. This combination of high Sahel rainfall and low rainfall in SoCal implies a strongly positive Atlantic Multidecadal Oscillation (as in the 1950s and 1960s). The mild winters noted by Uncharted Territory for the 1730s suggest a highly negative AMO (perhaps more negative than during the late twentieth century), which is further supported by evidence of Sahel drought and reduced SoCal drought in that decade plus mild British winters during the negative AMO era between about 1905 and 1925. It also suggests a highly negative PNA, which would need verification in western North America (which did have many cold winters between 1896 and 1939) that would be very difficult since winter temperature does not directly influence growth or hydrology in most areas.