Leading climate scientists now agree that human pollution, mainly from fossil fuels, has added substantially to global warming in the past 50 years and that the Earth is likely to get far hotter than previously predicted, with immense consequences for people and wildlife.
A summary of the 1,000-page final draft of new research by the Intergovernmental Panel on Climate Change -- a UN-sponsored group made up of the world's leading atmosphere scientists -- has been sent to governments this week. It is expected to add urgency to talks on global climate change in the Hague next month, at which Britain intends to take a lead in urging states to set targets and timetables to cut emissions.
The report suggests that the upper range of warming over the next 100 years could be far higher than estimated in 1995. Its worst-case scenario raises the average global temperature by 6C (11F) above the 1990 level. Average temperatures today are 5C (9F) higher than they were at the end of the last ice age. Five years ago the panel predicted that average temperatures would, at worst, rise by 3C (6.3F)
The leaked document is the first major update of climate prediction since 1995, when the panel concluded that there was "a discernible human influence" on the Earth's climate because of the greenhouse effect -- which is caused by the buildup of heat-trapping chemicals in the atmosphere.
The panel has now concluded that the burning of fossil fuels and emission of man-made chemicals has "contributed substantially to the observed warming over the last 50 years". The scientists believe, too, that temperatures could rise far higher and faster than previously predicted if emissions are not curtailed.
The human influence on the Earth's climate has long been debated, but this is the first time such an authoritative group has gone so far.
While there are still uncertainties, scientists say there is an "increasing body of observations that provide a collective picture of a warming world that cannot be solely explained by natural forces. Emissions of greenhouse gases and aerosols due to human activities continue to alter the atmosphere in ways that affect the climate system," the report says. Many panel members in the US say that the summary represents the closest thing to a consensus possible in science.
They point to the additional data gathered in the past three years, which shows that the world is much warmer than any similar string of years in many centuries; improvements in computer models designed to project future trends; and better understanding of the influence of other emissions, such as particles of sulphates which can cool the Earth by reflecting sunlight back into space.
Speaking in a personal capacity yesterday, one of the British authors of the report, Mike Hume of the University of East Anglia, described the research as "incremental back-up" to what had been suspected for years.
"Our current best insights lead us to see a continuation of global warming and an acceleration of it," he said.
Evidence of increased warming has been found in retreating glaciers, thinning polar sea ice, retreating snow packs, increased precipitation, and the big rise in weather-related natural disasters.
Global warming will deeply affect poor countries, leading to huge numbers of refugees, crop failures, and extreme weather. Most emissions of carbon, the main greenhouse-warming gas, are from rich countries. The US is responsible for 23% of carbon emissions; Britain's 3% is the same as that of Africa.
Kevin Trenberth, head of climate analysis at the US National Centre for Atmospheric Research (NCAR) in Boulder, Colorado, said the report showed that global warming was a real problem. "More and more people working in atmospheric science or on climate or ecology have had to come to grips with the fact that climate change is affecting what they're looking at," he said.
"It reinforces what we were able to say in 1995," said Tom Wigley, another NCAR climatologist. "It shows the previous projections in 1990 and 1995 were conservative."
Some governments may insist on changing the text before it is published in May.
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