Scientists are troubled by the increasing incidence of extreme weatheraround the world, writes Brendan McWilliams.
The highest air temperature in the shade ever recorded on this planet occurred on September 18th 1922 in a place called al'Aziziyah in Libya. The official temperature that day reached 58°C, a clear world record both before and since.
But temperatures in southern Europe in the past few weeks have not been far behind; values in excess of 50 degrees, by all accounts, have not been too uncommon.
Of course, extreme heat in those parts of the world is more the norm than the exception at this time of year.
Indeed, the ancient Romans had an explanation for it. They noticed that for a 40-day period during July and early August, Sirius, the brightest star in the sky and popularly called the "Dog Star", rises and sets with the sun; they assumed, wrongly of course, that it was the radiant power of this star, added to that of the sun, which provided the extra heat to make the midsummer days so sultry and unbearable.
They called the heatwave caniculares dies or "the dog days".
But the current heatwave has broken many records. Over the past few months, for example, temperatures in parts of southern France have been a full 5 -7 degrees above the long-term average. Likewise in Switzerland, not a country generally given to extremes, it has been warmer recently that at any time in the past 250 years.
At a superficial level, the cause of the heat is clear from any weather map. A huge anticyclone covers most of Scandinavia and continental Europe. Moreover, the circulation is such that air affecting many southern regions has already passed over the hot-plate of the scorching sands of the Sahara, through Libya and Tunisia; the African heat already in the air is augmented by the clear anticyclonic skies, which allow the noonday sun to raise the local temperature still further.
And since much of this heat is retained overnight by the soil, behaving like a giant storage heater, the effect is almost cumulative, resulting in higher and higher temperatures, day by day, week by week, for as long as the anticyclone stays in place.
Heatwaves in themselves, however, are not all that uncommon. Meteorological records of all kinds are broken every year somewhere in the world.
But weather people view the current scene in conjunction with other extreme events which have occurred recently: in the central United States, for example, there were 20 per cent more tornados during May this year than had ever occurred in a single month before; and in India the regular pre-monsoon heatwave was much more severe than usual, bringing soaring temperatures, which contributed to the deaths of an estimated 1,400 people. Scientists are troubled that in recent years the number of such extreme events appears to be increasing. And while there is no conclusive scientific link, it is hard to resist the temptation to conclude that there must be some connection between this increasing frequency of extremes and the gradual rise in average global temperature.
During the 20th century, this increase was around 0.6 degrees, the largest change in average global temperature in any 100-year period for at least 1,000 years.
Moreover, during the last two decades, the rate of increase in global temperature has increased dramatically.
This has resulted in a sequence of exceptionally warm years in the recent past, 1998 being the warmest on record, and last year, 2002, the second warmest, with average global temperature about half a degree above the long-term norm.
The current high temperatures over much of the northern hemisphere, combined with the recent mild winter, when fed into the figures which make up the averages, may well result in the current year, 2003, being the warmest ever experienced on this planet.
The very least we can expect in the future, the experts tell us, is an increase in average global temperature of about 1.5 degrees between now and 2100; in the worst-case scenario, with no corrective action taken to prevent it, the rise this century could be a catastrophic 6 degrees.
Recent experience provides hints that even the lower of these figures might bring with it extreme weather events of a kind and severity quite outside our experience at present.