Big companies are paying contrarians to undermine the work of climate scientists, according to a top UN official speaking before the release of a landmark review of climate science this weekby international researchers next Friday.

Halldór Thorgeirsson, a director who reports to the head of the UN body that governs the on-going high level international climate negotiations, said that scientists would need to be prepared for a counter-blast from sceptics.

"Vested interests are paying for the discrediting of scientists all the time. We need to be ready for that," he said.

His outspoken views will set the tone for a fractious meeting of the world's leading climate scientists, kicking off on Monday in Stockholm, that will set out the evidence that the world's governments use when formulating policies to deal with global warming for decades to come. More than 800 scientists have contributed to the report, the final details of which will be hammered out in a gruelling four-day session next week.

According to a draft of the "Summary for Policy Makers" dated June , seen by the Guardian– the most important part of the document – the scientists will argue that the evidence points to 95% certainty that climate change is occurring and is caused mainly by greenhouse gases released by humans – up from 90% certainty in the previous 2007 report. The 53 page document, seen by the Guardian which includes a note saying "do not cite, quote or distribute" says that levels of greenhouse gases in the atmosphere are now greater than at any time in the last 800,000 years, based on ice cores and other evidence, and the incidences of extreme rainfall are increasing, with rainfall likely to increase in the north but to decrease in the subtropics. The draft outlines evidence of "large-scale warming resulting primarily from anthropogenic increases in greenhouse gas concentrations". It says that if warming is to be limited to less than 2C in future, more than half of the carbon that can be emitted to hold to that goal has already been poured into the atmosphere.

The real impact of the report – the latest since 2007 and only the fifth such assessment since 1992 – will not be felt until governments meet this year in Poland to discuss a global response to warming, aiming to forge a treaty to replace the 1997 Kyoto protocol, which was rejected by the US and which placed no obligations on big developing countries such as China, now the world's biggest emitter of greenhouse gases.

The IPCC came in for severe criticism after a handful of flaws were found in its 2007 report, chiefly a mistake which suggested that most of the glaciers of the Himalayas could disappear by 2035. he error was seized by detractors, and led to the discovery of several other claims that were insufficiently backed by research. The IPCC said it was unsurprising that a few mistakes had crept into what was than a thousand pages of dense scientific research.

One of the crucial issues in the latest IPCC report is how sensitive the climate is to carbon concentrations in the atmosphere. Early leaked drafts of the summary suggest that the earth's systems are extremely sensitive to greenhouse gases, and are likely to continue to rise as fossil fuels are burned at ever faster rates, but they also suggest that the lower end of estimates of future temperature rise will be reduced. Formerly, the scientific consensus was that temperatures would rise by at least 2C, but in the new report this is likely to be reduced slightly to a 1.5C projected increase at the lowest end of the range. This slight reduction, which scientists stress does not reduce the known dangers of warming, such as more droughts and floods and fiercer storms, has been seized on by some climate sceptics, who see it as evidence that global warming will be less severe than thought.However, there are no certainties in the report as yet – though drafts have been seen by the Guardian. The final assessment will be subject to the wranglings of climate scientists and government-appointed experts next week.

Prof Nilay Shah, of Imperial College London, who compiled a recent report on how to reduce carbon emissions, compared the world's lack of action on climate change to the complacency on safety procedures before the devastating fire at King's Cross underground station in 1987, in which 31 people lost their lives. Before the fire, he pointed out, smoking was allowed on Underground stations, piles of flammable rubbish were allowed to accumulate, and many escalators were made of wood. After the dangers of these became apparent, the design of stations was improved to remove these hazards, but it took the disaster to stimulate change.Sceptics have been lining up to put forward their views that the IPCC's fifth assessment report is flawed. Many of their arguments focus on the recent slowdown in the upward march of global temperatures, attributed by climate experts to the effects of the oceans in absorbing heat and the natural variability of the world's climate systems. However, scientists point out that ten of the warmest years in the temperature record have occurred in the past decade and a half. There have also been other strong indicators of climate change, including the shrinking of Arctic sea ice - which reached its lowest recorded extent last year and is also diminishing in volume - and the retreat of glaciers around the world.

To those who are in disagreement with climate science, however – even though recent research has found that more than 90% of scientific studies support the finding that climate change is happening as a result of human actions – the remaining areas of uncertainty, such as the role of the oceans in absorbing heat and the role of clouds and human-made aerosols in deflecting the sun's rays from the earth's surface, are a cause to doubt more than a century of climate science. Myron Ebell, director of the centre for energy and environment at the right-leaning US thinktank Competitive Enterprise Institute, and one of the US's most prominent climate sceptics, told the Guardian: "The science contradicts the modellers' dire predictions. The divergence between reality and model projections in the last two decades provides strong evidence that global warming, although it may become a problem some decades in the future, is not a crisis and is highly unlikely to become a crisis. We should be worried that the alarmist establishment continues using junk science to promote disastrous policies that will make the world much poorer and will consign poor people in poor countries to perpetual poverty."

The CEI has in the past received funding from Exxon Mobil, the oil company, and the American Petroleum Institute, Texaco, General Motors and the Koch Family Foundations, controlled by the Koch brothers who made their fortune from fossil fuels.

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What is the Intergovernmental Panel on Climate Change?

Set up in 1988, the IPCC is a UN body that evaluates the state of climate science. It produces major assessments every five to seven years – the last was published in Paris in 2007 and said scientists were 90% certain humans were responsible for the warming the planet was experiencing. The panel was awarded the Nobel peace prize in the same year, to be shared jointly with former US vice-president Al Gore.

What is being published next week?

The summary of the first part of the so-called fifth assessment report (AR5), which focuses on the scientific evidence behind climate change and man's role in it. The IPCC will meet in Stockholm to discuss the final draft which will be presented to governments and the public on Friday.

Later parts of AR5, due in 2014, will report on the impacts of climate change, such as more extreme weather, how we can adapt to a warmer world, such as building flood defences and adapting farming practices, and "mitigation" – how greenhouse gas emissions can be cut.

Do these reports still matter?

Yes. The IPCC's major assessments are extremely influential and widely read. But many people, including some of the scientists who put together the reports without pay, say that more targeted and more frequent reports would be more useful.

Adam Vaughan