Undaunted by these spectacular own goals, both sets of protagonists have continued to churn out confident and confounding forecasts - and the mass of academics, civil servants, environmentalists and businessmen who make up the sensible centre of the debate have cautiously joined them in the prediction business. There is little choice, because many environmental issues are essentially about the future - and are bedevilled by long time- lags between cause and effect. A cancer caused by pollution today will take 20 to 40 years to develop, and delays built into the working of the planet can be even longer. Yet those same time-lags make it possible to sketch some of the outlines of the environment of Britain and the world in the 21st century.
For a start, it will be a much more crowded world. Short of a series of truly catastrophic visits by the Four Horsemen of the Apocalypse, there is no stopping a huge increase from the present population of 5.7 billion - because the mothers and fathers of the additional people have already been born. But timely action can limit the scale of the boom. UN projections for the year 2050 range from 7.8 billion people (if the world seriously and speedily tackles the problem) to 12.5 billion (if it does not). The difference is equal to the entire population of the world in 1984.
Britain and the world will also have to put up with an artificially thin ozone layer - and therefore increased levels of skin cancer and cataracts caused by the sun's harmful ultraviolet rays. The main man-made chemicals that are damaging the ozone stay in the atmosphere for more than 100 years, and there is no way of taking them back, so the problem will continue for the whole of the century. It will be particularly bad over temperate countries like ours, where the ozone thins more than in the tropics.
But here at least, early action has already taken the edge off the crisis and may have prevented catastrophe. Rapid steps to eliminate the harmful chemicals have ensured that levels high in the atmosphere will peak as the century opens and then slowly decline. We may not have acted in time to prevent the beginnings of an ozone "hole" from opening over the Arctic, but it is possible that the north will escape the disastrous depletion that has taken place over the Antarctic. This is just as well, because equivalent depletion over the Arctic would expose not penguins but vast areas of population, including Britain.
What we cannot so easily escape is the fact that we will, almost certainly, be living in a warmer world. Carbon dioxide, the main cause of global warming, also lasts for 100 years in the atmosphere. The fossil fuels that release it power the world and cannot quickly be abandoned; and poor countries justifiably insist that they must burn them in order to develop. So concentrations will continue to grow throughout the century.
The best realistic hope, says Sir John Houghton (chairman both of the Royal Commission on Environmental Pollution and of the world's official inter-governmental working group of scientists on the issue) is that carbon dioxide levels can be stabilised by 2100. This would mean, he says, that industrialised countries will have to start reducing their emissions from the very opening of the century. The world would still get warmer but, he hopes, slowly and slightly enough for us to be able to adapt without too much pain.
Such a gentle warming might make Britain more pleasant, bringing hot summers and balmy winters, spreading fields of sunflowers over the Home Counties and vines across Yorkshire's broad acres. But it could also bring such diseases as malaria and the disfiguring Leishmaniasis to Britain, and it would be almost certain to increase pests. Greenpeace, always adept at curdling the blood, warns of a huge rise in numbers of aphids, and of the arrival of Black Widow spiders and of cockroaches "literally boiling from the sewers".
On present performance, however, the world is unlikely to get off with such a gentle warming. Indeed, Britain and northern Europe may well actually get colder as the world heats up. A glance at the map shows why. Britain shares the same latitude as Labrador and much of Siberia. It is the Gulf Stream that makes the difference, but scientists believe that global warming is likely to weaken it. The future may be Nova Scotia in Stoke Newington rather than Tuscany in Todmorden.
Lurking in the back of the minds of many scientists - including Cambridge University's Peter Wadhams, the co-ordinator of the European Sub-Polar Ocean Programme - is increasing evidence from natural records frozen in the Arctic ice that climate change may happen surprisingly fast. As he points out, these show that climate has "flipped" between one state and another (such as an ice age and present conditions) over the space of a few decades, rather than evolving slowly over centuries or millennia. If that happens with global warming, the resulting disruption could severely reduce the chances of many of our children or grandchildren surviving the 21st century.
n The world's population is 5.7 billion. By 2021 it will probably be around 8 billion.
n Over 50,000 sq km of rainforest are felled each year; 8 million sq. km remain.
n The ozone layer over the UK is being depleted at a rate of 8 per cent a year. An ozone "hole" over the Arctic was detected for the first time this spring.
n Since records began in 1855, global average temperatures have increased by between 0.3 and 0.6 degrees Celsius. The seven hottest years on record were all in the 1980s.
n The world's marine catch has increased fourfold since 1950. Every major commercial fish species is now classified by the United Nations Food and Agriculture Organisation as "depleted" or "over-exploited".
n Between 1981 and 1993, Friends of the Earth's membership grew from 7,500 members to 204,000, Greenpeace's from 30,000 to 400,000.
n 20 per cent of UK beaches do not meet mandatory EC standards for bathing water.
n Be flexible. You can no longer assume (as previous generations have tend to do) that nature is unchanging.
n Become "green". The best hope for the planet's future is that millions of individuals will decide to behave with greater environmental responsibility. You too can make a difference.
n Conserve energy, both in the home and in your travelling habits. This will help the planet; and there may well be less of it around anyway.
n Buy sun-hats and sun-cream, and keep children out of the sun. Our climate may not grow warmer, but our exposure to ultra-violet light will almost certainly increase significantly.
n Plant trees.