Guest post by Tony Brown
This is the third examination of Central England Temperatures (CET) in a series that commenced in 2015 and which has charted the recent decline in temperatures from their highest values. The two previous articles in this series are referenced here;
The Rise and Fall of Central England Temperatures; Part 1 covering 2000 to 2015
The Rise and Fall of Central England Temperatures; Part 2 covering 2000 to 2017
When referencing any ‘decline’ we need to put that into context against CET’s overall substantial rise in recent decades. The official CET dataset used in this article, which is compiled by the UK Met office is linked here and shown in Figure A);
It should be noted that the values between 1538 and 1658 are my own reconstruction and are not used at all in this current paper.
Note: Weather comprises the day to day events that we all experience. Climate is officially the trend of the weather (often temperature and rainfall) taken over a continuous thirty year period. The two terms have sometimes been used in an interchangeable manner here, when a period of more than a year is being examined.
According to the Met office; ‘Since 1974 the data have been adjusted to allow for urban warming: currently a correction of -0.2 °C is applied to mean temperatures.’
Over the last couple of years an exercise has been carried out within the Met Office to re-evaluate urban warming values affecting England, bearing in mind the 25% increase in population and many additional buildings and infrastructure since 1974. I understand the results are now in the public domain so have summarised and paraphrased a series of email conversations conducted with the Met office over the last few weeks.
“ The new version of HadCET will be fully documented in a scientific journal paper to appear in due course. Meanwhile, the existing CET software (has been used) to include ‘official’ values for January 2020. (You may need to refresh your browser to see them)
Although the current CET revisions are not yet finalised, I do have enough data put together to be able to examine the likely urbanisation corrections. In principle, the urbanisation corrections are applied from 1974 onwards but, because our daily and monthly CET values are only provided to one decimal place, it must take until 1980 for any of them to exceed 0.05 degC and hence make a difference when just one decimal place is quoted. (Accordingly) 1980 appears to be the earliest year for which a correction will be applied.
The corrections vary by calendar month, but by 2000 some of them are as large as 0.3 degC for mean temperature; they do not continue to get larger after that. Note also that the adjustments for minimum temperature are one-and-a-half times as large as those for mean temperature, and for maximum temperature the adjustments are only half those for mean temperature – this is because the urban fabric tends to hold on to daytime heat through the night, especially in summer, so urban minimum temperatures remain further above the rural equivalent than is the case for maximum temperature.”
CET uses three weather stations to record temperatures, contained in a triangle roughly in the centre of England, bounded by London, Bristol and Lancashire. As far as possible stations are placed away from urban areas but the country as a whole is small and crowded and as the Met office has recognised for many years, has been affected by urbanisation. No doubt there will be much discussion over the changes to HadCET once the scientific paper has been published, so no further reference will be made to it in this article.
‘Climate’ over the last 30 years;
The evolution of the CET temperatures over the last three decades can be seen in the series of graphics below. Figure 1 represents a scientifically derived 30 year climate period. In this respect it can be seen that the data during this period shows an upwards trend of 0.7c per century. This is a snapshot, and over a century temperatures don’t generally adhere to the relatively short term trends used to calculate them. This can be seen in the other graphics, where the shorter periods used will result in exaggerated periods of rise or fall.
When does the modern decline in temperatures commence?
The graphs shown in Figures 2-8 were compiled in order to see where the inflexion point came, when the overall annual temperature stopped rising and instead went into reverse. As can be seen it was becoming close to that state in the mid 90’s but the inflexion point seems to have been 1998 (Figure 6). This was the year of the large El Nino, described here, although that year was by no means the warmest in the recent CET record.
The remaining two graphics after 1998, (Figures 7 and 8) were compiled to ascertain whether the trend continues, if calculated from dates later than 1998.
There are several features we can observe in order to provide context, by stepping back and looking at the longer record shown in Figure A). The first is that the UK decadal weather through the centuries is highly variable, with numerous peaks and troughs and this has an impact on the full 30 years that represent a climate period.
When there has been a peak there has eventually been a decline and this can mean a 30 year ‘climate’ period can include a sharp decadal rise, a considerable decadal fall and a relatively static decade, which is then averaged out, and the nuances of the shorter periods of more extreme weather become less apparent.
The second feature is that the current modest decline since 1998 (Figure 6) is from a historically high plateau and covering only 22 years does not represent a 30 year period that can be scientifically termed ‘climate’. Rather it falls into the category of ‘interesting,’ as this observed decline does not correspond with the volume of scientific and media attention that has led to the UK Parliament and numerous local councils declaring a ‘climate emergency.’ Intriguingly it can be seen that no one living in England during the 21st century has experienced an overall warming trend.
The long term upwards trend
There has been a generally upwards trend in temperature since 1690, typified by notable jumps, then a fall back to some extent, with the period since around 1880 being generally more warmly benign and less extreme, as the trend continues upwards. At this stage it is impossible to predict if Figure 6) from 1998 represents merely a hiatus in the longer term warming or is part of a genuine trend to cooler values.
Does CET have a wider significance?
England is a geographically small area, but because three sets of records are used to compile CET and due to the UK’s geographical location, it is said to be representative of a much wider area. See the ‘Long Slow Thaw’ Section 6, a study by the author in 2011. The section was headed “Can CET represent a wider geographic area and establish the existence of a Hemispherically significant cooling period?”
As Mike Hulme remarks
“ (CET) is also quite well correlated with land temperatures over the entire Northern Hemisphere. At an annual level this correlation is about 0.4, but when average values over 10-year periods are compared this correlation rises to about 0.75.”
A variety of other science luminaries including the UK and Dutch Met office also see a reasonable correlation with the Northern Hemisphere. CET does therefore appear to have some broader relevance for ascertaining broad modern and historic trends in areas outside of England.
Looking at Figures 1 to 8, it was intriguing to note the manner in which changes in the character of one or two seasons (warmer to colder or vice versa) impacted on overall annual trends. Nowhere is this better illustrated than Figure 1, whereby Autumn suggests a sharp upwards trend of well over 2 degrees Centigrade per century. By 1995 (Figure 3) this had become a negative figure and at the turn of the 21st century (Figure 7) it had become minus 1.71C per century.
The Autumn figures from 2004 (Figure 8) show an even steeper decline, but whilst again it is ‘interesting’, it can be recognised that it has no scientific basis as in any 15 year period a couple of exceptionally cold or mild autumns will have a considerable impact and the century trend is highly unlikely to continue.
CET topics for future article
In the second part of this article we intend to examine the evolution of CET since 2004 (Figure 8) and also examine the past 350 years of CET records in order to put the current period into historical context. A detailed examination will be made of previous peaks and troughs and also of the apparent ever changing nature of the seasons, from warm to cold and back again,which appears to be a feature of the extended temperature record.
These variations last for varying amounts of time but appear frequent enough to warrant querying the view that the UK has an equitable and rather unchanging climate, with a certain constancy to the seasons.
Indeed at first sight, what can be considered as a ‘normal’ Spring, Summer, Autum or Winter appears to depend on which decades and which century you are looking from, and the more variable they are, the more impact individual seasons may have on overall annual trends.
Consequences of continued temperature decline
England has a large and growing population dependent on ever increasing amounts of food from its farmers and who need to keep warm, consequently a short term decadal decline of even half to one degree would have serious consequences for this country, as could adverse changes in seasonality.
Tony Brown February 2020
Acknowledgements; With grateful thanks to Ed Hoskins for compiling the many graphics used in this article. Ed has a great site which covers numerous aspects of Climate, Co2 emissions, Energy and much more. https://edmhdotme.wordpress.com/
All data used in this article is derived from statistics maintained by the Met Office and available here; https://www.metoffice.gov.uk/hadobs/hadcet/
Figure A) comes directly from it.
Credit: Source link