They agree also that warming will have dramatic effects - more droughts and floods, for example - and could accelerate out of control. Even if governments were to act now to stop any further rise in the man-made emissions that cause global warming, the climate would go on heating up and the seas would continue to rise for centuries to come.
Until now, most scientists have been cautious about global warming: although agreeing that temperatures have been rising this century, they have been reluctant to say this is anything more than natural climatic variation. But a new report, from the official Intergovernmental Panel on Climatic Change (IPCC), marks the end of serious argument about the threat of global warming. The report is summarised in two confidential draft documents obtained by the Independent on Sunday.
The consensus of 2,000 top meteorologists and other experts consulted by the IPCC is that tropical diseases will increasingly spread into temperate areas, that droughts and floods will increase, that forests will die out, and that harvests in poor countries could fall dramatically. And they warn that Britain and Northern Europe, despite the recent run of hot summers and mild winters, could rapidly become colder because of changes in the Gulf Stream.
In its last principal report, in 1990, the IPCC said that although the world had grown warmer over the last century, it was not possible to attribute any or all of this to human activities. The energy industries and their supporters, and the oil-producing countries, have used this report, as well as the work of a small number of dissident scientists, to deride the risks.
Now the experts have reached a consensus rare in science. Until recently, they thought that we would have to wait until the end of the century to be sure that pollution is warming the climate. But with the help of greatly improved scientific techniques, they have decided that there are enough signs already that this is happening.
Over the past year, it has been established that flowers and grasses have been spreading rapidly in the north of Antarctica; that several of the frozen continent's ice sheets have disintegrated; that the seas are rising faster than had been thought; and that there are ominous signs of changes in the North Atlantic that may affect the Gulf Stream.
All these developments were first reported in the Independent on Sunday, and last November we forecast that this would be a very warm year. It is now clear that it is one of the hottest ever. Nine of the ten warmest years on record have occurred since the early 1980s.
The documents summarising the IPCC report - which is likely to be approved by world governments at a conference in Rome in December - say that this century has been the warmest ever recorded. Global average temperatures have increased by between 0.3 degrees and Centigrade and 0.6C since the late 1890s; and most of this increase has taken place in the past 40 years.
As a result, the seas have risen between 10cm and 25cm so far this century, partly from the melting of glaciers (which have shrunk further in Europe, for example, than at any time in the last 5,000 years), partly from the break-up of ice sheets, and partly because sea-water expands as it becomes warmer.
Winter rainfall has increased over Europe, Russia and North America - a sign of warming - while snow cover in the Northern Hemisphere has fallen by 10 per cent over the past 20 years.
The warming has resumed during the past two years after a brief lull. The lull was caused by the eruption in 1991 of Mount Pinatubo, a volcano in the Philippines, which threw 20 million tons of sulphur into the upper atmosphere. This screened out some of the Sun's radiation, causing temperatures to drop until 1993. But as the dust has settled back to earth, temperatures have risen again.
The episode has helped strengthen the scientists' belief in global warming, partly because it shows that the climate responds to changes in the atmosphere, and partly because computer models that experts use to calculate global warming predicted the after-effects of the explosion.
The IPCC documents say that the new evidence indicates "a detectable human influence on global climate". They also say that the warming of the past century is likely to have been at least partly caused by increasing emissions of carbon dioxide and other greenhouse gases, such as methane and nitrous oxide. And the documents warn that rising pollution will inevitably increase the warming. The result will be the end of a 10,000-year period in which relatively stable climate has allowed human civilisation to begin and flourish.
The documents say that tropical diseases such as malaria, yellow fever and dengue fever are already beginning to move out of their traditional areas as the climate heats up. They forecast that this could leadto another 50 million to 80 million cases of malaria and another 3.5 million of river blindness.
Floods will increase as sea levels rise - by between a foot and a yard over the next century - and as rainfall increases in some areas. Arid parts of the world however, are likely to get even drier, as the increased heat evaporates what little moisture they have.
Global food production may remain much the same, the documents say, but some of the poorest and most populous parts of the world, including much of sub-Saharan Africa and South and East Asia, are likely to see harvests drop by up to a fifth as the heat takes its toll of crops.
The warming of the climate is likely to happen many times faster than natural vegetation can adapt. The documents say that one-third of the world's forests will be at risk, that alpine areas will disappear, and that coral reefs will die.
But they suggest that Britain and Western Europe could quickly become colder as global warming takes hold. This is because the changing climate could affect the Gulf Stream, which keeps us much warmer than we would otherwise be - Britain is on the same latitude as Labrador. Research suggests that these changes can take place very rapidly in climate zones - over a few decades - and the documents indicate that there is evidence that these changes may be under way.
At the 1992 Rio Earth Summit, the world's rich countries agreed to stabilise their emissions of carbon dioxide by the end of the century. But the documents show that this will not be sufficient, because many of the greenhouse gases persist in the atmosphere for more than 100 years.
The documents show that emissions could be greatly reduced by new technologies for saving energy and introducing renewable, non-polluting, sources of energy. But even if these did succeed in causing concentrations of the gases in the atmosphere to level off, there is so much inertia in the world's natural systems that the climate will go on warming up and the seas will go on rising for centuries. The documents conclude that rapid action is needed to stop global warming accelerating out of control.Reuse content