|Mean Central England Temperature for each fiscal year from 1659/1660 to 2014/2015, plus mean and percentiles for period from 1765/1766 to 1973/1974, reproduced from ‘Two “little ice ages” revealed by CET summer data’|
However, as the map below shows, the period from 1903 to 1912 during which all but one of the “record cool” months at a global level occurred was actually hotter than 1885 to 1892 over the northern hemisphere’s land, but cooler over the tropics and oceans.
|Mean 500 millibar (50 kilopascal) May to August geopotential height for 1898 to 1905 vis-à-vis the period since the second “magic gate” identified by Tim Flannery in ‘The Weather Makers’|
- Compiled an unrounded monthly mean CET for every month from 1880 to 1974 via the daily CET data, which can be downloaded online here
- Averaged the CET for each calendar month using only years from 1880 to 1974 inclusive
- Using an Excel file and my dowloaded GISS global temperature maps (which have an anomaly vis-à-vis 1880 to 1974 means to two decimal places), I compiled all the global means as independent variable and plotted CET against it as a dependent variable
- As a final step, I plotted scatter plots of CET anomaly against global temperature anomaly (versus 1880 to 1974 mean in both cases) and calculated the Pearson product-moment correlation.
All Months:Looking at our scatter plot of CET versus global temperature anomaly between 1880 and 1974, we do see some positive correlation both at a glance and with the correlation coefficient of +0.13730109. However, this coefficient is sufficiently small that variation in global temperature can explain a mere two percent of the variation in CET, since percentage explained equals the square of the variation.
It is of course possible that different seasons behave differently. Because the sun is weak or nonexistent, high-latitude winter temperatures (including those of the UK) are controlled almost entirely by air mass. Variations in wind vector could thus overwhelm global temperature trends due to variations in greenhouse gas concentrations or dimming from volcanic aerosols. In the hotter months, however, air masses possess less influence and sunlight of cloudiness has a more critical role: thus, one might expect a better correlation between CET and global temperature anomaly here.
It is also plausible that because the influence of greenhouse gas levels upon CET was shown in ‘Two “little ice ages” revealed by CET summer data’ to peak in the autumn and trough out from late winter to early summer, that we would see a peak in correlation in the autumn. We will look month by month through the fiscal year to see if either thesis can be supported, and I will divide the fiscal year into two halves to avoid having too much memory in one post.
|July CET anomaly versus global temperature anomaly in ˚C, 1880 to 1974|
|August CET anomaly versus global temperature anomaly in ˚C, 1880 to 1974|
However, global temperature anomalies for August still can explain only about one-seventeenth of the variance of CET between 1880 and 1974. 1911 is again a huge outlier:
|Global temperature anomalies for August 1911. Note the alternating patterns of heat and cool across the Northern Hemisphere, and the consistent cool in South America and over the oceans|
|September CET anomaly versus global temperature anomaly in ˚C, 1880 to 1974|
|September 1944 global temperature anomaly – note the cool area centred around the UK amongst an otherwise circumlatitudinal hot anomaly|
|October CET anomaly versus global temperature anomaly in ˚C, 1880 to 1974|
|October 1896 global temperature anomalies. Note the cool over Western Europe and the Great Lakes region and heat over Eastern Europe, northern Russia and Australia|
|November CET anomaly versus global temperature anomaly in ˚C, 1880 to 1974|
|December CET anomaly versus global temperature anomaly in ˚C, 1880 to 1974|
What is actually observed is almost exactly the same correlation coefficient between CET anomaly and global temperature anomaly as for November. There are two major outliers from the general pattern:
|December 1910 global temperature anomalies. Note the hot area of northern Africa and Europe, and the very cold area over Siberia and China|
|Departures from 1880 to 1974 mean of 500 millibar heights (in metres) over the northern hemisphere for December 1910. The block over the Black Sea and Ukraine is decisive in shaping the observed temperature pattern.|
|December 1939 global temperature anomalies vis-à-vis 1880 to 1974 averages. Note the unusual warmth over the interiors of Asia and North America – this was the latter continent’s warmest December of the century.|
|December 1939 500 millibar height anomalies in metres. Note the blocks over Canada and Central Asia driving warm Atlantic and Pacific air into Asia and North America.|