The four hottest months on record have occurred in the past 23 years. In fact, hardly a week goes by without some new record of extreme weather being set somewhere. But do all these records really add up to a huge lurch in Britain’s climate, or are they statistical freaks of little long-term significance?
The Met Office, the guardian of the nation’s weather archives, protects new weather records like crown jewels. Its information base is built on a network of 500 weather stations dotted across the country.
Their sample of daily readings are scaled up into a national picture, rather like the way an opinion poll takes a representative sample of people and scales up their answers to the whole population. But exactly how representative are these stations of the weather situation across Britain? The question continues to provoke great debate. Just as a bad opinion poll can be biased towards one part of the country, so weather stations may be in the wrong places.
The Met Office is sensitive about such criticism, particularly when applied to urban areas. Some stations that were built in rural areas have been swallowed up by urban sprawl, with tarmac, concrete and bricks sending temperatures higher.
The weather station at Heathrow airport is one such location. It is increasingly affected by the encroaching airport expansion, offering one explanation why it often records Britain’s highest daily temperature of the day. The highest single temperature for this month — 30C — was recorded at the airport. This phenomenon means that a reading taken at Heathrow years ago may not be comparable with today, despite meteorologists’ best efforts to protect weather instruments from surrounding interference.
However, with so much of Britain now built upon, the experts also accept that if they are to get an accurate national weather picture some recordings have to be conducted in urban settings. All the weather stations record temperature inside a box with ventilation slats, known as the Stevenson screen. The box, which looks a little like a beehive, lets air waft around the thermometer inside without interference from direct sunlight, wind, rain or heat on a stand 1.25m (4ft 1in) off the ground. The stations send in reports of their daily average temperature, based on the day’s maximum and minimum temperatures. The results are plotted across the whole country on a grid of 5-kilometre (3-mile) square boxes, taking into account the effects of mountains, hills and other topography, and also distance from the sea. Over the months and years those averages give a picture of Britain’s climate. The resulting data is the most accurate picture of the weather yet compiled, although it presents its own problems.
Weather measurements in the past were not as extensive or accurate as those of today, which risks exaggerating or underplaying current trends. Furthermore, the historical span of the records is limited: the thermometer was invented only in about 1654 and the first standard temperature scale, Fahrenheit, was devised around 1709. The Central England Temperature (CET) records, set up by Gordon Manley and continued by the Met Office, have been central to calculating how the weather has changed.
While the reliability of this data is questioned — particularly when weather stations are moved to new sites — there is plenty of other evidence that Britain is growing significantly warmer. As well as monitoring air temperature just off the ground, the Met Office also uses soil temperatures. The ground behaves more like a storage radiator, slowly soaking up heat, and so is far less susceptible to the daily fluctuations in air temperature.
Underground temperature readings down to 100cm (39in) below the surface show a consistently rising trend in temperatures. Similarly, the surface of the sea changes far more gradually than the air temperature. Again Britain is blessed with long records, stretching back to 1850, and these too reflect the slow warming trend that has accelerated in recent decades.
Of course, a hundred or so years of rising temperatures could all be quite natural. Throughout history temperatures have fluctuated, sometimes so violently they brought down civilisations, such as the Maya of Central America a thousand years ago.
In medieval times a plunge in temperatures in Britain led to widespread famine and the abandonment of upland settlements such as Dartmoor.
But what really concerns the climatologists now is the size of the CET rise since the mid-1970s, when average temperatures began to accelerate.
The jump, only a fraction of a degree Celsius, is dismissed as of little significance by people less persuaded of climate change. But the average temperature in Britain in the depths of the last Ice Age was only about 5C lower than today.
Most people accept that the temperatures are rising, but climate change sceptics claim they are caused by natural changes in the Sun’s activity.
The best known solar changes are the roughly 11-year cycle of sunspots; at present we are at solar minimum when the number of sunspots reaches its lowest.
At this stage the Sun’s heat output drops by around 0.07 per cent, and you would expect global temperatures to drop as a result, but so far the warming continues unabated and this year is on course to be one of the hottest on record across the globe. There are other changes in solar activity, but even these do not explain the changes.
The brightness of the Sun also waxes and wanes over longer periods of time.
According to a paper published last week in the journal Nature, solar brightness has increased over the past 400 years, based on measurements of a radioactive isotope, beryllium-10, left in polar icecaps by the Sun’s activity — this form of beryllium is produced in the atmosphere in nuclear reactions that depend on solar activity. The Met Office and many other experts acknowledge that changes in the Sun are important, and take them into account in their calculations of climate change, but the amount is too small to explain the 20th-century warming.
The sceptics can argue that other factors in the Sun’s activity also change, such as variability in solar ultraviolet or magnetised plasma output, but these are poorly understood and it is uncertain how these could affect the climate.
In fact, the only way to explain the recent upsurge in temperatures is from man-made pollution — the rise in greenhouse gases is the only factor that matches the sharp rise in temperatures. If you take out the greenhouse effect, the take-off in warming since the 1970s largely disappears.
No matter how you try to find another scapegoat, the finger of blame for the plethora of new temperature records in the United Kingdom and worldwide points to carbon dioxide and other greenhouse gases. If so, then we can expect many more record-breaking temperatures in the years to come.
Summertime temperatures
2006
Maximum average: 20.4
Minimum average: 11.1
Mean average: 15.7
2005
Max average: 19.1
Min average: 10.5
Mean average: 14.8
2004
Max average: 18.9
Min average: 10.9
Mean average: 14.9
2003
Max average: 20.3
Min average: 11.3
Mean average: 15.8
2002
Max average: 18.4
Min average: 10.5
Mean average: 14.4
These temperatures use data taken from about 180 observing sites across Britain between June and August
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