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It was that summer, scientists now realise, when global warming at last made itself unmistakably felt. People in the Northern Hemisphere knew that summer 2003 was remarkable. Britain had record high temperatures; Europe had out-of-control forest fires, great rivers drying to a trickle and thousands of heat-related deaths. But how remarkable that summer was is only now becoming clear.

June, July and August were the warmest three months recorded in western and central Europe. And they were the warmest by a very long way. Like Britain, Portugal, Germany and Switzerland had record national highs. Over a great rectangular block stretching from west of Paris to northern Italy, taking in Switzerland and southern Germany, the average temperature for the summer months was 3.78 °C higher than the long-term norm, says the University of East Anglia’s Tyndall Centre for Climate Change Research, one of the world’s leading institutions for monitoring and analysing temperature records. That might not seem a lot until you are aware of the usual pattern. But then you realise it is enormous. There is nothing like this in previous data. It is considered so exceptional that Professor Phil Jones, the unit’s director, says openly – in a way that few scientists have done – that 2003’s extreme temperatures may be directly attributed to global warming caused by human actions, rather than natural climate variations.

Meteorologists have hitherto contented themselves with the formula that recent high temperatures are ‘consistent with predictions’ of climate change. For the great block of the map in question, the unit has reliable temperature records dating back to 1781. Using as a baseline the average summer temperature between 1961 and 1990, departures from the temperature norm, ‘anomalies’, can easily be plotted.

Over the past 200 years, there have been at least half a dozen excess temperature anomalies approaching, or even exceeding, 2 °C. But there has been nothing remotely like that year, when the anomaly was nearly 4 °C. ‘That is quite remarkable,’ Professor Jones says. ‘It’s very unusual in a statistical sense. If this series had a normal statistical distribution you wouldn’t get this number. The return period (i.e. how often it could be expected to recur) would be something like one in 1,000 years. If we look at an excess above the average of nearly 4 °C, then perhaps nearly 3 °C of that is natural variability, because we’ve seen that in past summers. But the final degree of it is likely to be due to global warming, caused by human action.’

That year’s summer had in a sense been one that climate scientists had long been anticipating. Until then, the warming had been manifesting itself mainly in winters that were less cold rather than in summers that were much hotter. Last week, the UN predicted that winters were warming so quickly that some of Europe’s lower-level ski resorts will die out.

But sooner or later the unprecedented hot summer was bound to come – and that year it did. Over a large swathe of the western part of the European continent, records were broken in all three months. It wasn’t only monthly averages, but daily extremes and the lengths of spells above thresholds. National records were set in at least four countries.

One of the most dramatic features of the summer was the hot nights, especially in the first half of August. The high night-time temperatures were related to the 15,000 extra deaths in France during August, compared with previous years. They gradually increased during the first 12 days of the month, peaking at about 2,000 a day on August 12 and 13 and severely overloading the medical services. Then they dropped dramatically after August 14 when minimum temperatures fell by about 5°C. The elderly were most affected – their death rate rose 70 per cent.

For Britain, the year as a whole is likely to be the warmest recorded. But despite the temperature record on August 10, the summer itself – defined as the June, July and August period – comes behind 1976 and 1995, when there were longer periods of intense heat. At the moment, the year is likely to be the third-hottest in the global temperature record (which goes back to 1856), behind 1998 and 2002. But when the records for October, November and December are collated, it might move into second place. The ten hottest years in the record have occurred since 1990.

Professor Jones is in no doubt about the astonishing nature of that year’s European summer. ‘The temperatures recorded that year were out of all proportion to the previous record,’ he says. ‘It was the warmest summer in the last 500 years and probably way beyond that. It was enormously exceptional.’

His colleagues at the Tyndall Centre are planning a study of it. ‘It was a summer that had not been experienced before, either in terms of the temperature extremes that were reached, or the range and diversity of the effects of the extreme heat,’ says the centre’s executive director, Professor Mike Hulme. ‘It will certainly have left its mark on a number of countries as to how they think and plan for climate change, much as the 2000 floods revolutionised the way the Government is thinking about flooding in the UK. The 2003 heatwave will have similar repercussions across Europe.’