Think before you enjoy it. The near-record temperatures expected today are a sign of things to come, and will become commoner and hotter in future years as man-made global warming takes hold, scientists predict.

With the mercury expected to approach 100F (37.8C) for only the second time in Britain’s history - but also the second time in under three years - researchers said the unusual heat was entirely consistent with predictions of climate change caused by rising emissions of greenhouses gases, such as carbon dioxide (CO2) from power stations, motor transport and, increasingly, aircraft.

Temperatures are expected to peak today. In some parts of southern England, in particular the London area, they may reach the high 30s, nearing or even exceeding 38C.

The UK’s air temperature record was set on 10 August 2003, when 100F was breached for the first time, with a reading of 101.3F (38.5C) at Brogdale, near Faversham in Kent.

The Met Office says there is a 30 per cent chance that this all-time record will be beaten today. Even if it is not, it certainly seems likely that the record for the hottest July day will be beaten, a record nearly a century old. It was set on 22 July 1911 in Epsom, Surrey, with a temperature of 36C (96.8F).

The immediate reason for the heatwave is a prolonged period of very settled weather, with cloudless skies and strong sunshine which has allowed temperatures to rise. Over recent days, hotter air has moved across from the Continent, causing the temperature to rise further.

But a look at the temperature records - and Britain has the longest-running set in the world - strongly suggests it is much more part of a pattern showing the advent of global warming. The Central England Temperature Record dates to 1659 and we show here a graph of it since 1772. The smooth line shows how far the annual mean temperature diverged from the mean of 1961-1990; and it will be seen that from about the 1980s, the line starts to rise steadily above all previous values. It is still rising.

For most of the 20th century, British temperatures did not exceed the record of 98F (36.7C) set at Raunds in Northamptonshire on 9 August 1911. But on 3 August 1990, this was smashed when 98.8 F (37.1C) was recorded in Cheltenham, then broken again at Brogdale in 2003. Now it may be broken yet again.

These new peaks do not in themselves prove global warming, say scientists - but global warming makes them much more likely. “As you get a warming trend in temperatures, which is what we are observing, the risk of exceeding extreme temperatures increases dramatically,” said Peter Stott of the Met Office’s Hadley Centre for Climate Prediction and Research. “This is what we saw with the European heatwave of 2003. When we analysed it, we found that the rise in average temperatures over the previous century of about one degree had doubled the risk of an extreme event like the heatwave of that year.

“And, as we go into the future, the risk of what was quite a rare event rises dramatically. We think by 2040, a summer like 2003 will be a regular event; the chances of it happening will increase from one in 250 all the way to one in two.” He added: “We have an increasing amount of confidence that we are observing rising temperatures caused by human-induced rising greenhouse-gas concentrations. Unless the world changes what it is doing, we are going to see these extreme temperatures very much more.”

People should remember the downside to rising temperatures, said Sir John Houghton, former head of the Met Office, and chairman of the scientific working group of the UN’s Intergovernmental Panel on Climate Change (IPCC) from 1988 to 2002. “Yes, we can enjoy the heatwave when it comes,” said Sir John. “There’s no point in saying let’s be miserable. But let’s remember, let’s recognise that the world is warming, and let’s remember that the downside is very severe. We are already going outside the range of what a lot of people can cope with and what a lot of ecosystems can cope with, and a lot of crops.”

Tony Juniper, the director of Friends of the Earth, said last night: “Our society needs to get real about climate change, and understand it isn’t a distant future event - it’s happening now and it’s causing major problems already. The longer we leave it, the worse it will get. Any person who claims to be representing the best interests of our society must demand urgent action, now, and not tomorrow.”

Feeling the heat

* Temperatures on buses in the hottest parts of Britain hit 52C yesterday while the London Underground reached 47C. EU guidelines state that cattle should not be transported at temperatures exceeding 27C.

* Speed restrictions were implemented on trains because of “hot rails” and sagging overhead wires.

* Council gritters were on stand-by to prevent roads melting.

* A Level 3 health warning has also been issued, requiring primary care trusts to provide additional care for those most at risk from dehydration and heat stroke.

* More than 170 people were forced to swelter on a grounded plane at Heathrow for five hours after its air conditioning broke down.

* Charities for the elderly expressed concern about the effects of the heatwave, advising those at risk to take precautions such as staying indoors or in the shade.

* The Government has issued smog warnings across Britain for the coming days.

* The House of Commons Speaker Michael Martin granted an “unprecedented” request to give 500 bottles of water to demonstrators outside Parliament suffering in the heat.

* With southern England facing the worst drought in 100 years, initial signs are that customers are being more careful with water - a total of 300 million fewer litres a day were used during June.

* Fish are threatened by reduced water levels as oxygen levels drop. Toxic algae bloom can form and fish struggle to breathe.

* Employers have been urged to relax office dress codes to enable workers to wear casual clothes in the heat.

By Michael McCarthy and Steve Connor Published: 19 July 2006http://news.independent.co.uk/environment/article1185369.ece