The theoretical has become the actual. Hazards that just a few years ago seemed entirely within the realms of scientific speculation are affecting us, here and now. The consequences for us and our children over the next 10, 20, 50 years could be catastrophic and will certainly be far-reaching.
A little over 20 years ago, we received the first warnings that the build- up of a group of non-toxic, ubiquitous industrial chemicals, CFCs and others, in the atmosphere could theoretically deplete the ozone layerthat shields life from much of the incoming ultraviolet light in sunshine.
Ten years ago, the first ozone hole was detected over the Antarctic by the British Antarctic Survey. It was a startling demonstration of the power of pollution; very low concentrations of a fairly innocuous contaminant could drastically alter the physics of the atmosphere over an entire continent.
But it was earlier this month that ozone destruction really came home. Instruments at either end of the UK - at Lerwick in Shetland and Cambourne in Cornwall - measured the lowest levels of stratospheric ozone recorded over Britain in 20 years of watching the skies. An ozone hole - thankfully, a short-lived one - was opening over Britain.
At the same time, other instruments detected an unprecedented surge in UVB radiation shining down on to these islands. The levels of this potentially dangerous radiation, which can cause skin cancers, were of the same strength as those normally found in May when the sun is much higher in the sky.
Fortunately, we are attempting to abort the dangerous and unwitting experiment with the ozone layer. A series of international agreements is cracking down on the chemicals that destroy it. But this does not mean the threat is over. There is a fierce argument about whether the pace of the phase-out is fast enough. Holes will continue to appear for decades to come.
But our bigger and still more dangerous experiment with the climate is only just beginning. Global warming has moved out of the realms of theory; it's with us and with a vengeance.
Scientists have been warning for more than a century that burning fossil fuels and forests would raise temperatures, alter climate and raise sea levels. Last November, under the auspices of the United Nations' Intergovernmental Panel on Climate Change, they reached a consensus that this temperature rise can now be detected. The Eighties and Nineties have seen nine of the 10 hottest years on record.
But unlike the ozone layer, there are as yet no agreements in place that will slow down and reverse this process. Even if, overnight, the world miraculously halved its use of fossil fuels, global warming would continue - because of the thermal momentum already built up and because 50 per cent would not be a sufficiently large cutback.
So it is going to be a hotter world in the next century. If we continue to remain hooked on fossil fuel (there is still at least a century worth of reserves underground) the century after that will be hotter still. The question that climatologists and their models cannot yet answer is exactly how much warmer. And precisely where it will be warmer, wetter, drier, or stormier.
Every aspect of society, of our children and grandchildren's lives, will be affected. Today's thirtysomethings will probably have to cope with climate shifts in their retirement years. But their children and grandchildren will have to learn to take much more drastic measures.
Our economies, health, agriculture, leisure activities and water supplies will all have to adjust. Nations may come into conflict because of water and food shortages exacerbated by global warming, so diplomacy and defence strategies will be affected, too.
Because the scientists are still several years from being able to make good regional predictions, it is impossible to know exactly what global warming has in store for Britain through the 21st century. Today's supercomputer forecasts suggest a slightly warmer and even rainier group of islands. But the stronger heat in summer could easily counterbalance the extra rainfall through increased evaporation, making Britain a drier place.
Several recent very mild winters in the UK and the record-breaking heat of last summer could be harbingers of the global warming. So could the English drought between 1989 and 1992, and the much more severe drought of last year that is now certain to resume this summer. The consequences for how we collect, distribute and use water will be far-reaching.
The rainfall shortage has made the water companies question all their assumptions about resources and demand, and caused a frantic investment in extra supplies in some regions.
Worldwide, there are other signals that global warming is already here. Most mountain glaciers for which we have good records are shortening, retreating higher and higher as they melt away.
One of the strongest warming trends has been in the Antarctic Peninsula - 2.5C warmer in 50 years. In the last Antarctic summer (our winter of 94/95), huge, thick floating ice shelves covering about 2,000 square miles of sea rapidly broke up - the first time this has been observed.
The most advanced predictions, such as those being produced by the Meteorological Office's Hadley Centre in Bracknell, Berkshire, suggest that globally average surface temperatures will rise 1C between now and 2040.
While 1C in 50 years may sound trivial, the 0.2C a decade rise this implies is faster than any since the last Ice Age ended 10,000 years ago. This alone is enough to disrupt natural ecosystems and agriculture.
But on top of this temperature rise will come changes in rainfall, wind patterns and soil moisture content, adding to the impact.
They are only forecasts based on an incomplete understanding of how the earth's atmosphere, oceans, icecaps and plant life will respond to the warming - dampening or accelerating it. But if the huge on-land ice caps of Greenland and Antarctica were to become detached, slide into the sea and melt, sea levels would rise not by the two feet forecast for 2100 using computer models - but by hundreds of feet. A land area the size of a large continent would disappear.
If global warming were to halt or divert the Gulf Stream, Britain and western Europe would become much colder. The risks of widespread, climate- caused death, disease and famine are ever higher in an increasingly crowded world, with dense populations packed into areas prone to drought, floods and crop failures.
A few decades beyond the millennium, we may see the Nineties as a wasted decade in which we chose to continue adding to these risks despite strong early warnings.Reuse content