Global warming is caused by mankind, is here to stay and is getting worse, leading climate scientists have concluded.

In a bleak report to world leaders aimed at assessing the impact and extent of climate change, 2,500 scientists from 113 countries said that the evidence of worsening global warming was overwhelming.

So strong is the evidence linking the warming climate to human actions that only the most irresponsible world leaders can ignore or deny global warming and its causes, a senior UN official said at the launch of the report.

Achim Steiner, the executive director of the UN Environmental Programme (UNEP), said that the authoritative report should remove any doubt over whether manmade climate change was taking place. “February 2 will perhaps one day be remembered as the day the question mark was removed,” he said. “Anyone who would continue to risk inaction will one day in the history books be considered irresponsible.”

David Miliband, the Environment Secretary, was equally certain that the “debate over the science of climate change is well and truly over” and called for immediate international action to address the problem.

He said that there was only a short time left — as little as ten years, according to some scientists — to act to stop runaway

temperature rises that will bring a host of natural disasters. Presidents, prime ministers and heads of government treasuries must, he urged, ensure that there is “international political commitment to avoid dangerous climate change”.

The Summary for Policymakers report was one of a series to be issued this year as part of the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC). It showed that since the third assessment, carried out in 2001, there has been a deterioration.

The IPCC is regarded as the authoritative voice on the science of global warming, chiefly because its procedures lend themselves to thoroughness and conservatism. Its reports are produced by researchers with acknowledged expertise who have to convince thousands of their peers that their opinions are backed by rigorous evidence. It was established by the UN in 1988, and produces reports every five to six years.

Rajendra Pachauri, the panel chairman, introduced the report, which he said revealed that mankind was having a dramatic impact on the world’s climate: “We are in a sense doing things that haven’t happened in 650,000 years.”

If governments continue to allow unfettered use of fossil fuels, temperatures are estimated to rise by at least 2.4C (4.3F) and perhaps as high as 6.4C. A 4C average rise is the most likely, the report concluded. Six years ago the predictions were for a 1.4C to 5.8C rise, with 3C the most likely, and the report pointed to rising carbon dioxide emissions since 2001 as the cause of the increase.

Even in the most optimistic scenario considered, in which the global economy reduces reliance on heavy industry and rejects fossil fuels in favour of renewable energy, current carbon emissions mean that the world is likely to undergo a 1.8C temperature rise.

The prediction makes the European Union’s target of limiting the rise to 2C even more challenging, and suggests that the 2-3C limit proposed in the Stern report last year will be harder to achieve than was thought.

Scientists agreed that they could say with “very high confidence” that human activities since the Industrial Revolution had warmed the world. Most 20th-century temperature rises were, they said, “very likely” to have been caused by mankind, a strengthening of confidence since 2001 when they said that it was likely.

They concluded: “Global atmospheric concentrations of carbon dioxide, methane and nitrous oxide have increased markedly as a result of human activities since 1750 and now far exceed preindustrial values.

“The global increases in carbon dioxide concentration are due primarily to fossil-fuel use and land-use change, while those of methane and nitrous oxide are primarily due to agriculture. Continued green-house-gas emissions at or above current rates would cause further warming and induce many changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th.”

Susan Soloman, who jointly led the scientists, said of greenhouse gases: “We have a strong confidence that these things are driving climate change to a substantial degree. There can be no question that greenhouse gases are dominated by human activity.” The report identified effects that have already been observed, including reduction of ice cover, ocean temperature rises and increased sea levels, and more extreme weather such as droughts and heat waves. For the first time the IPCC identified hurricane intensity as having worsened.

Global warming will cause the effects to increase in frequency and strength. By the end of the century, the scientists said, Arctic ice is likely to have disappeared and summer heatwaves in Europe will be common. The report addressed the issue of solar radiation, which has been cited by sceptics about manmade climate change as the likely cause of global warming. The scientists dismissed it as anything but a minor possible contributor.

Despite the White House’s reluctance to take action against climate change, the US sent a delegation to the IPCC and approved the report.

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