The IPCC is the watchdog established in 1998 by the World Meteorological Organization (WMO) and the United Nations Environment Program (UNEP), whose six-yearly climate assessments are usually taken for granted by environmentalists, politicians and experts.
The report argues the computer predictions for global warming and the effects of carbon emissions have been proved to be inaccurate.
According to the report, the world is warming at a rate of 0.12C per decade since 1951, compared to a prediction of 0.13C per decade, which means the world has been warming at only just over half the rate claimed by the IPCC in its last assessment published in 2007.
The current 31-page “summary for policymakers” is based on a more technical 2,000-page analysis, which will be issued at the same time.
The prediction flaw could be explained by forecast computers failing to take into account natural variability in the climate, therefore exaggerating the effect of increased carbon emissions on world temperatures.
The summary also shows that scientists have now discovered that between 950 and 1250 AD, before the Industrial Revolution, parts of the world were as warm as they are now.
So despite a 2012 draft stating that the world is at it’s warmest for 1,300 years, the latest document states: “Surface temperature reconstructions show multi-decadal intervals during the Medieval Climate Anomaly (950-1250) that were in some regions as warm as in the late 20th Century.”
The 2007 report also included predictions of a decline in Antarctic sea ice, but the latest document does not explain why this year it is at a record high.
The 2013 report states: “'Most models simulate a small decreasing trend in Antarctic sea ice extent, in contrast to the small increasing trend in observations ...There is low confidence in the scientific understanding of the small observed increase in Antarctic sea ice extent.”
The 2007 forecast for more intense hurricanes has also been dropped without mention in the new document after this year was one of the quietest hurricane seasons in history and the US is currently enjoying its longest-ever period – almost eight years – without a single hurricane of Category 3 or above.
One of the report’s authors, Professor Myles Allen, the director of Oxford University’s Climate Research Network, urged people not to look at the IPCC as a “bible” on climate change.
“It is not a bible, it is a scientific review, an assessment of the literature and scientists are constantly revising their research to account for new data,” he said.
Next week, 40 of the 250 authors who contributed to the report and representatives of most of the 195 governments that fund the IPCC will hold a four-day meeting in Stockholm to discuss the findings.
The governments have tabled 1,800 questions in relation to the report.
One of the central issues is why the IPCC failed to account for the “pause” in global warming, which they did not predict in their computer models.
Professor Judith Curry, head of climate science at Georgia Institute of Technology in Atlanta, said the leaked summary showed that “the science is clearly not settled, and is in a state of flux”.
She added it therefore made no sense that the IPCC was claiming that its confidence in its forecasts and conclusions has increased.
Thus, for example, the IPCC keeps insisting that it is 95 percent certain that global warming is mainly human’s fault. In the new report, the IPCC says it is “extremely likely” that human influence caused more than half the temperature rises from 1951 to 2010.
It says the world will continue to warm catastrophically unless people undertake measures to curb greenhouse gases.
“This is incomprehensible to me,” Prof Curry said voicing hopes that the “inconsistencies will be pointed out” at the meeting.
“The consensus-seeking process used by the IPCC creates and amplifies biases in the science. It should be abandoned in favor of a more traditional review that presents arguments for and against – which would better support scientific progress, and be more useful for policy makers,” she said.
Some experts claim the greenhouse gas conclusions could hardly be different, as all UN climate change activities must adhere to the 1992 UN Framework Convention on Climate Change (FCCC) and its target to “stabilize greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.”
Dr Benny Peiser of the Global Warming Policy Foundation described the leaked report as a “staggering concoction of confusion, speculation and sheer ignorance”.
IPCC spokesman Jonathan Lynn refused to comment, saying the leaked report was “still a work in progress”.
Meanwhile independent climate researcher Nic Lewis accused The British Met Office of issuing “erroneous statements and misrepresentations” about the pause in global warming, saying its climate computer model is fundamentally flawed.
This has serious implications, because the Met Office’s HadCM3 model is used to determine the UK Government’s climate projections, which influence policy.
The Met Office said it would examine the paper and respond in due course.
Tom Harris, executive director of the Ottawa-based International Climate Science Coalition, says the key question to be answered in the climate change debate is not “whether climate change is real, or whether human activities have some impact, but whether reputable science indicates that it is worth spending hundreds of billions of dollars to restructure our energy infrastructure to avoid a man-made climate catastrophe.”