UK WARMING
VARIABILITY is a defining feature of the UK's weather and climate and the UK
is fortunate to have several long term weather datasets which have recorded these
variations. The
Central
England Temperature record (CET graphs:
annual;
seasonal) which begins in 1659 is probably the
best known and is considered to be the longest temperature series in the world.
The early part of this temperature record lies within the period of the Little
Ice Age (LIA) 1560-1830 which accounts for the overall
warming trend although temperatures were almost as high around 1730 as they were
in the 1990s. Prior to the CET, the Medieval
Warm Period (MWP) 900-1300 AD in which the Vikings
temporarily colonised Greenland is
considered to have been warmer than any period since. "The
Hockey Stick: A new low in climate science" summarises a number of
scientific publications which document evidence from all over the world showing that
temperatures were lower than today in the LIA and higher than today in the MWP.
That the MWP was warmer than the twentieth century has long been an orthodoxy
acknowledged by both scientists and historians and of course, accounts for its
name. In contrast, this article
about the surface temperature record claims that 1998 was the warmest year in
the last millennium, the 1990s the warmest decade and the twentieth century the
warmest century!
ALARMIST rhetoric and dramatic claims are a characteristic
of the climate change/global warming industry. A recent Countryfile program on
the BBC on climate change was faithful to this tradition. One claim was that
"average summer temperatures are significantly higher than they were fifty years
ago". Table 1 compares average summer temperatures (Jun-Aug) in Central England (using the CET)
in the period 1989-2002 with three different fourteen year periods around 1950
using t-tests and this
graph shows summer
temperatures and 10 year running means from 1659 to 2002.
TABLE 1 Comparison of mean summer temperatures
(Jun-Aug) in central England around 1950 with those of 1989-2002 using t-tests
Period |
Mean Temperature C |
t value |
Significance |
1989-2002 |
15.95 |
- |
- |
1940-1953 |
15.64 |
1.37 |
not significant |
1945-1958 |
15.45 |
1.75 |
not significant |
1950-1963 |
15.27 |
2.60 |
p< 0.05 |
THE GRAPH shows that average summer temperatures have
varied throughout the 332 year record and the ten year running mean was slightly higher around 1780 than it has been in recent years. Only 1975, 1976, 1983 and 1995 have
been exceptionally warm in the last 50 years. The results in Table 1 indicate that the first two
comparisons gave a non-significant result while the period 1950-63 was
significantly cooler than 1989-2002 at a low level of significance. Table
1 also shows that average summer temperatures were falling in the 1950s as
indeed were
annual temperatures and this cooling continued in the 1960s. This twenty year
cooling in the 1950s & 1960s is also a characteristic of the global temperature
record and is surely inconsistent with the script that rising CO2 concentrations result
in higher global temperatures.
DRAMATIC claims about snowfall reduction on the Cairngorms were a feature of the second part of the Countryfile
program apparently suggesting there would be no snow patches in the summer
months within the next 20-50 years.
Climate Change & Changing Snowfall Patterns in Scotland is a recent
study which does predict fewer days with snow lying in Scotland
but the higher elevations where most summer snow patches
occur are expected to be less affected than elevations around 400m. This
report does not mention a complete absence of snow patches in summer on
the Cairngorms in the next 20-30 years although a reduction is implied.
ANOTHER theme of the program was current and predicted
rates of temperature change. One method of
assessing the rate of temperature change is to calculate ten year running means
from annual temperature values and to subtract the previous year's value from
the next
year's to give annual values of temperature change. This method was applied to
the CET record and the temperature
record for Aberdeen/Dyce.
ANNUAL
CHANGES in the ten year running mean for central England show that the
annual rates of increase in some years in the 1990s have been the highest since 1899
but that there were several occasions between 1659 and 1899 when annual values
were higher than the highest values of the 1990s.
Decadal averages of the
annual values show the 1990s had an average annual increase in the ten year
running mean of 0.07 C per year which is lower than the comparative value for
the first decade of the 18th Century (0.1 C per year) and equal to that of the
1820s. Recent annual values have been much lower and the value
for 2001 was negative indicating a decrease in the ten year running mean. Annual
changes in winter CET temperature also show a similar pattern of reaching a
peak in the 1990s and dropping off a little in recent years.
ANNUAL CHANGES in the ten year running mean for Aberdeen/Dyce show that an
annual increase of 0.1 C per year has been exceeded on three occasions since
1988 but also on four occasions in the 1970s and on two occasions in the 1930s.
The highest annual value in the 130 year record was 0.181 in 1989 and higher
annual values occurred in 1889, 1898 and 1976 than in any year in the 1990s.
Decadal averages of
the annual values show the 1990s to have an average annual increase of 0.05 C
per year which is slightly above the averages of both the 1890s and 1930s which
both had 0.04. As in Central England, recent annual values show that the ten year
running mean has been falling.
OTHER long term UK weather records are: the England & Wales Rainfall series
(EWR) (graph) which begins in
1766 and shows that recent wet years are not unprecedented in the 235 year rainfall record with the ten year running mean higher in both the 1770s & 1880s; the Lamb Daily Weather types (LDWT) from
1861 which are classifications of
daily UK weather charts which we are all familiar with from television and
newspapers. There are eight directional types (westerly, northwesterly etc) each subdivided into three categories (e.g westerly is subdivided into cyclonic (low pressure) westerly, straight westerly and anticyclonic (high pressure) westerly), and two non-directional types, (cyclonic & anticyclonic). A small number of days are unclassifiable under any of these types and are termed unclassifiable. These LDWTs provide a different method of looking at UK weather variations and for example, show that northerly weather types have been unsually rare during the 1990s and this will have contributed to the high temperatures during this decade. Although strictly not a UK dataset, the North
Atlantic Oscillation (NAO) is also relevant as it represents the incidence
of Atlantic depressions and the associated westerly winds affecting Britain. It
commences in 1823 and is a measure of the difference of atmospheric pressure
between Iceland and the Azores/Gibraltar region.
FOCUSSING on the UK, there is no doubt that UK temperatures have generally
been higher
than long term averages since the late 1980s as shown by the CET
(CET graphs:
annual;
seasonal). However,
Valentia Observatory on the south west tip of Ireland, Tiree in the western isles of Scotland,
Plymouth in southwest England and
Aberdeen/Dyce on the east coast of Scotland also show the 1990s as being warm but unlike the CET, the warmest year in these records did not occur in the 1990s but in 1949, 1959, 1989 and 1933 respectively and all four stations show the 1990s warming to be less exceptional than the CET.
As Valentia and Tiree are classified as rural, these two temperature records are more likely to be accurate representations of surface temperature for their area than the CET is for the relatively urban Central England which is almost certainly influenced by the "urban heat island" effect. The long temperature records of these four stations (Valentia; Tiree; Plymouth & Aberdeen/Dyce) show that the warmth of recent years in Britain is not unprecedented and that there has not been a clear upward trend in temperature with time as portrayed by the surface temperature record which is used to imply that increasing atmospheric CO2 concentrations are causing global temperatures to rise.
A LIKELY
explanation for much of this warming is the increased incidence of westerly winds
off the North Atlantic in winter
& spring as shown by the LDWTs
and the NAO.
CET seasonal graph shows that the winter, spring and autumn seasons all contributed to the notable warming of the 1990s while apart from the exceptional summer of 1995, the summer season warmed relatively little. Other
north European countries have also shared in this winter warmth as shown by the
long temperature records of
De
Bilt in the Netherlands,
Zurich and
Geneva in Switzerland. Some might argue that the current higher
incidence of westerly winds in winter is connected to increasing levels of
greenhouse gases in the atmosphere. However, this graph
clearly shows that the NAO was equally high for a number of years in the early part of the last
century when greenhouse gas concentrations were much lower.
UK WEATHER hit the headlines in October 2000 when widespread flooding during
the
wettest autumn on record prompted claims that the climate had changed.
Heavy rains continued beyond October 2000 resulting in the year to 31 March 2001
being the wettest
12 month period on record. In October 2001, it was high temperatures that
hit the headlines owing to the warmest
October on record. High rainfall records and high temperature records
are likely to be broken in wet and warm decades. The
CET &
EWR graphs clearly show that temperature and rainfall in Britain are constantly changing from year to year and from one decade to the next and that in the context of these long weather records, recent years are unusual but not unprecedented. These records are not proof of
a changed climate but are the expected result of a variable deviating around its
mean according to rules described in standard statistical theories. All of the long term datasets mentioned above show
large variations between years and between decades. Natural variability is a
very reasonable albeit less sensational explanation than climate change/global
warming for the weather events of recent years.
CONSIDER the following brief synopsis of weather variations in the UK over
the last 20 years. The early to mid 1980s were notable for some very
cold winter months: January 1980; December 1981; January 1982; January 1985; February
1986 & January 1987. Indeed, the coldest day of the 20th century was recorded in many parts of
England & Wales on 12 January 1987. As recently as December 1995, a new UK record
night low temperature was recorded at Altnaharra in northern Scotland. In contrast, the winters of 1988/89 and
1989/90 were exceptionally mild with the average temperature for the whole winter
some 2-3 degC above average in many places. Since then, all but three winters
(1990/91; 1995/96 & 1996/97) have been milder than average although with
smaller anomalies.
WHAT of the summers? Recent summers have mostly been a little warmer than average and fairly unsettled. However, the the early to mid 1990s were
notable for some very warm summer months: August 1990; July 1994; July
1995; August 1995 & August 1997. Indeed, many places in England & Wales
experienced their hottest day on record in the first few days of August 1990.
Moreover, there was a long trend of dry months in many east and south east parts of
England & Wales from August 1988 to June 1992 and another long trend of dry
months from April 1995 to May 1997 which necessitated
Yorkshire Water using tankers to ferry water across the Pennines to prevent a
reservoir running dry at one point. Contrast this with November 2000 when an
area the size of Lancashire was said to be under water owing to floods.
THESE variations over two decades may be consistent with predictions that the UK climate
will be subject to more extremes as well as having generally higher temperatures
in the 21st Century owing to global warming. Or are they simply proof that UK weather varies as it
has always done and is likely to do so in the future? Notable Weather Events provides a long historical list of "extreme
weather" events in Britain.
