File photo: A Diana monkey at London Zoo eats fruit frozen in ice to keep cool.

London - The extraordinary heatwave that is about to slam into southern England on Wednesday will probably see temperatures in London of 35C or even 36C (which is 95 to 96.8 degrees in old-fashioned Fahrenheit), according to the Met Office - may well turn people's thoughts once more to a phrase we have almost stopped using: global warming.

Nowadays we almost always call the phenomenon “climate change”. When, 25 years or so ago, we first realised that we were altering the atmosphere to devastating effect by our emissions of industrial gases, it was the dramatic warming expected to follow which focused everyone's minds; but after a period of clearly rising global air temperatures in the 1980s and 1990s, in the Noughties, for reasons not entirely clear, the rise hit a plateau, where it has more or less remained.

So climate change, which incorporates other phenomena such as more violent weather events and sea-level rise, rather than just evident heat, has become the preferred term; but “global warming” may be about to come back into vogue.

I suggested in this column a month ago that there were signs that 2015 might be the year when the warming took off again, as it is bound to do if we keep adding vast amounts of CO2 to the atmosphere. These signs are continuing. The United States' National Oceanic and Atmospheric Administration reports that May 2015 was the warmest May, globally, since records began in 1880, and the January-May globallyaveraged temperature was also arecord.

Now these signs are coming our way. This week's heat, caused by a plume of very hot air moving north from Spain, will be quite exceptional, and if on Wednesday, which is 1 July, it does hit 36C, it will be close to the hottest July day ever seen in Britain - a record set on 19 July, 2006, when the mercury at Charlwood in Surrey hit 36.3C (97.3F). (And it will certainly be the hottest day ever seen at Wimbledon.)

Before that, the old July record had lasted since 1911, when 36C was reached in Epsom, Surrey. So if we do hit it again on Wednesday, if will be as a hot a July day, barring one, as we have seen for more than a century - and there is always the possibility that the 2006 record itself maybe broken.

If so, minds will no doubt turn to Britain's absolute temperature record, set on 10 August 2003, when 38.5C, or 101.3F, was reached at Brogdale near Faversham in Kent. This not only broke the 100 degrees Fahrenheit mark for the first time in Britain's history, but it surpassed the previous record of 37.1C or 98.8F, set in Cheltenham on August 2 1990, by a clear two-and-a-half degrees, which is a staggering leap. Might this be broken again this summer? Well, the bookies are already taking bets on it.

Even our biggest supercomputers cannot predict the weather accurately more than a few days in advance, and making long-term forecasts is a mug's game. But the Met Office says more plumes of very hot air from the south are likely, and the signs are that this summer may see temperatures wholly out of the ordinary - at least, as far as what we have always accepted as ordinary.

The idea of the warming globe may be about to become evident to us, in a dramaticway; and this in the year when the world community tries to do a deal to fix it, in Paris in December. So watch the mercury. And don't forget to take that bottle of water with you, if you go out in the sun.

The Independent