Guide: A Warm Response - Our Climate Change Challenge

Observed Climate Change

“How has climate and sea level changed in the recent past?” 

Observed Global Climate Change

The significant increases in concentrations of greenhouse gases since 1750 have made the
atmosphere more efficient at trapping heat generated from solar radiation and have
enhanced the natural greenhouse effect. In turn, the hydrological cycle has become more
dynamic because a warmer atmosphere can also hold more moisture. The observed ‘global
warming’ during the Instrumental Period (since 1861) has been 0.6°C± 0.2°C. Most of the
warming has occurred during two periods – 1910 to 1945 and since 1976 (see Figure 3). The 1990s was the warmest decade and 1998 remains the warmest year in the instrumental record followed closely by 2002, 2003 and 2004. New analyses of proxy data (e.g. tree rings) for the Northern Hemisphere indicate that the increase in temperature in the 20thcentury is likely to have been the largest of any century during the past 1,000 years. Moreover, it is very likely that rainfall has increased by up to 1% over most Northern Hemisphere mid-latitudes and that there has been up to 4% increase in the frequency of heavy precipitation events. In addition, global average sea level has risen by up to 0.2 metres during the 20th century mainly as a result of the thermal expansion of the oceans (IPCC, 2001c).

 

Global

Trends in the UK Climate

Temperature. The climate of the UK is warmer now than it has been at any time since
measurements began in the 17th century. Of the twenty warmest years in the 343-year
Central England Temperature Record no fewer than ten have occurred since 1989. The 1990s is the warmest decade in the entire series and is 0.5°C warmer than the average 1961-90 climate i.e. the World Meteorological Organisation’s current climate period which is used as the baseline throughout the global climate change community. The record also shows that winters of the last decade (1994 -2003) have warmed by 1.2°C since the beginning of the 19th century. The last 30 years have also witnessed an unusual sequence of warm summers – 1975/6, 1983, 1989, 1995, 1997 and 2003. The summer warming has led to more frequent hot (+25°C) and very hot (+30°C) days in the UK. The highest ever temperatures in the UK were recorded during August 2003 when the previous UK record of 37.1°C at Cheltenham on 3rd August 1990, was beaten by a number of stations on 10th August 2003. Brogdale near Faversham in Kent reported the highest at 38.5°C (101.3°F). These warm summer temperatures have been associated with high sunshine totals and 2003 was the sunniest summer (1720.8 hours) in the 90-year England and Wales Sunshine record closely followed by 1995, 1989 and 1990. The 20th century warming over Central England has resulted in the lengthening of the thermal growing season by about one month mainly as a result of the earlier onset of spring.

Precipitation. Unlike temperature there are no long-term trends evident in the amount of annual precipitation the UK receives and quantities vary greatly from year to year and from decade to decade. For example the year 2000, which was the wettest year (1232.5 mm) in the England and Wales Precipitation Series in the 20th century, had 62% more precipitation than 2003 (761.4 mm), a relatively dry year. There is, nevertheless, evidence of a changing seasonal distribution of precipitation over England and Wales, with winters getting wetter and summers drier. In this regard, 1995 was an exceptional year with a very wet winter and a very dry summer. The year 2000 also included the wettest autumn in the England and Wales series when very nearly twice the seasonal average rainfall of 257 mm was recorded.

Storms. Windiness is an important aspect of the UK climate. The 119-year record of gales
over the country suggests that since 1988 a record number of severe gales (Beaufort Force
9) have been witnessed, although the frequency of ordinary gale events (Beaufort Force 8)
has not changed. In 1990, for example, 20 severe gales occurred, two more than the
previous highest total in 1916. That said, the evidence for the recent increase in gale
frequencies being related to anthropogenic-induced warming remains unconvincing.

Sea Level. After adjustment for natural land movements, the average rate of sea-level rise during the last century around the UK coastline was approximately 1 mm per year. There is no evidence for a long-term change in UK storm surge statistics.

Changes in the Climate of Devon

Climate. The South West Peninsula is the mildest area in Britain being thrust well forward into the mild rain-bearing winds of the Atlantic. This oceanic location produces a warm temperate humid climate with only occasional frost that is tempered throughout the year by the influence of the sea. Such extreme Atlantic coastal areas are defined by Tansley as having “low summer and high winter temperatures, moderate precipitation and below average sunshine for their latitude” (Tansley, 1939).

Temperature. The sea of the western approaches has the highest average temperature of any sea area near to the UK i.e. 11 to 12°C. With winds blowing from the sea the annual average temperature of the coastal areas of Devon reflects the sea temperature. Woolacombe on the north coast is the warmest location at 11.2°C whilst the resorts of the English Riviera average around 10.6°C. Away from the coast altitude is the main factor affecting temperature. Princetown (414m) on Dartmoor has a mean annual temperature of 8.4°C whilst Hangingstone Hill (604m) records an average of 7.4°C. The long term average annual temperature for selected locations in Devon is shown at Figure 4.

Temperature shows both seasonal and diurnal variations but due to the modifying effect of
the sea the range of these variations is less than in most other parts of the UK. The sea
reaches its lowest temperature in February/early March thus making February Devon’s
coldest month. The strong maritime influence normally prevents extreme low temperatures but very low minima have been recorded when strong, cold easterly winds replace the dominant south-westerlies. Such an occasion was 13th January 1987 when a minimum temperature of -8.9°C was recorded at Exmouth. Away from the coast spells of frosty weather with clear and calm nights have produced records such as -15.0°C at Exeter Airport on 24th January 1958.

July and August are the warmest months in Devon with an average daily maximum temperature of around 18°C. Instances of extreme high temperatures are rare being associated with hot air brought from mainland Europe on south-easterly winds and accompanied by strong sunshine. The hottest day in Devon was 3rd August 1990 when 35.4°C (96°F) was recorded at Saunton Sands. Temperatures in excess of 31°C (88°F) were recorded at a number of locations in the county on 29th June 1976.

Figure 4 - long-term average annual temperature in Devo

Using data supplied by the Met Office it is now possible to assess the temperature rise
associated with climate change over the 20th century at most locations across the county.
The details for Ilfracombe (north coast), Dartmoor (central moorland), Exeter (central
lowland) and Slapton (south coast) are shown at Figures 5a – d. Each of the four locations
has recorded a rise of between 0.8 and 0.9°C since 1900. Whilst some warming occurred
during the first half of the century most of the warming has been since the mid 1960s. The
warmest decade has been the 10 year period since 1995 during which 7 of the 10 warmest
years recorded in Exeter have occurred. Only 1921, 1989 and 1990 were warmer.

Ilfracombe and Dartmoor

 

Figures 5a-d

 

Precipitation. Rainfall is caused by the condensation of water vapour in air that is being
lifted. This can occur within a frontal system associated with an Atlantic depression (i.e
cyclonic rainfall), by convection during the warm summer months and where moist air is forced to ascend hills (i.e. orographic rainfall). As Atlantic depressions are more vigorous in autumn and winter, most of the rain that falls in these seasons is cyclonic. Such rainfall totals are often increased by the impact of altitude on the passage of fronts across the south-west peninsula. In summer, rainfall in the form of showers and thunderstorms results from convection currents caused by solar heating.

Annual rainfall totals of 900 to 1000mm are normal around the coasts of Devon. This amount doubles over the upland areas of Dartmoor (2278mm) and Exmoor but may be somewhat less in the rain shadow to the lee of high ground (807mm in Exeter). The driest place in Devon is Exmouth with 749 mm. The long term average annual precipitation for selected locations in Devon is at Figure 7.

 

Figure 6

The south-west peninsula is prone to rare but very heavy rainfall events of relatively short duration. On 15th August 1952 228.6mm of rain fell on Long Barrow, Exmoor in 12 hours. The swollen East and West Lyn rivers destroyed the centre of Lynmouth in north Devon and claimed 34 lives.

Unlike temperature, which can be averaged on an annual or seasonal basis over a wide area, rainfall varies considerably between location and on a daily, monthly, seasonal, annual and climatic (i.e. over 30 years) scale. In Devon there are many rainfall gauges reporting daily precipitation quantities and some stations like Teignmouth have continuous records
stretching back to 1900 (see Burgess, 2001). Such data has been collated by the Met Office
and it is now possible to assess the variation in precipitation associated with climate change
over the 20th century at most locations across the county. The details for Ilfracombe,
Dartmoor, Exeter and Slapton are shown at Figures 7a – d.
Analysis of these records shows some statistically significant trends in the annual amount of precipitation. On Dartmoor and at Ilfracombe rainfall has increased since the late 1970s.
Conversely, there has been significant drying in Exeter but no measurable change on the
south coast at Slapton. In all records 1921 was a very dry year being over 40% drier than
the average. 1960 and 2000 were extremely wet when over 50% more than the average
annual precipitation fell.

In addition to these regional changes in annual rainfall, there are some important changes
taking place in the seasonality of rainfall as measured by the changing ratio of winter to
summer rainfall. This index, known as the ‘seasonality ratio’, shows a significant difference in all 4 records between the periods before and after 1976 denoting that summers are getting drier and winters wetter. This is largely due to the unusual sequence of hot summers experienced across the UK as mentioned above.

Annual Precipitation

Figures 7a-d

 

Sea Level. In addition to climate-induced changes in sea level, regional land movements
also need to be considered to provide an estimate of relative sea level rise. One of the main
reasons for regional land movement in the UK is the on-going isostatic adjustment caused by deglaciation at the end of the last Ice Age about 18,000 years ago. Whilst much of northern Britain is rebounding now that the weight of ice sheets up to 1.5 km thick has been removed, much of southern Britain is sinking forming features such as the distinctive ria coastline of southern Devon. For the South West the present rate of subsidence is estimated at between 0.1 and 1.4 mm/year. Therefore, it is likely that relative sea level rise along the coasts of Devon has been greater than the average sea level rise around the coast of Britain over the 20th century i.e. +0.1 metre.

 Summary

The mean annual temperature in Devon has increased since the cooler period of the 1950s
and 1960s in line with both global and UK warming trends. Most of the warmest years on
record have occurred over the last fifteen years. There has also been an unusual sequence of warm summers during this post 1960s period with summer 2003 now being acknowledged as a climate change event. From an annual precipitation perspective there have been some significant changes in rainfall totals in both directions which contradict a projected marginal drying caused by climate change. That said, annual rainfall totals remains inherently variable due to entirely natural causes. However, there is a significant change in the seasonality of both UK and Devon rainfall with winters becoming wetter and summers drier. This is in line with modelling predictions. Relative sea level continues to rise.