The report, last released in 2007, is an international manual to help governments assess their climate strategies.
Leaked drafts of the 2013 report, to be released in Stockholm, Sweden, have already sparked a predictable war of words between the broad scientific community and climate change skeptics.
That's because a decade-long pause, or variability, in rising global temperatures has introduced some uncertainty into models that measure what's known as climate sensitivity to greenhouse gas emissions.
Sensitivity relates to the impact of doubling CO2 in the atmosphere — something humans are well on their way to doing — and what temperature impact such a threshold would have globally.
The intergovernmental panel is expected to lower the bottom end of the temperature range by half a degree Celsius while leaving the top end unchanged, and at the same time confirming with 95 per cent certainty that humans are behind much of the global warming since 1950.
Christian Holz of the Climate Action Network Canada says this year's report may not have the same public impact of the previous 2007 version, which came as governments were gearing up for the Copenhagen summit on reducing global emissions.
Holz says another five years of scientific discovery and observation by thousands of climate researchers have only reinforced and fine-tuned earlier findings.
"The science is very clear and the debate is over," said Holz — although the debate clearly rages on in some quarters.
Even among governments that loudly agree man-made climate change must be addressed, the intergovernmental panel report offers a guide on how aggressively to tackle the problem — a major political preoccupation.
In June, a leaked draft showed the climate sensitivity range to doubled CO2 concentrations is between 1.5 C and 4.5 C, down from the 2 C to 4.5 C temperature range cited in the 2007 report.
Leaked comments on the June draft also suggested this sensitivity range is "a key issue of concern" — in the words of a British government official — because it serves as a measure of urgency for public policy action.
Any improvement of the best-case sensitivity scenario may give reluctant governments more wiggle room to delay aggressive emissions cuts
However the broad thrust of the report will be that global temperatures continue trending upward and that CO2 emissions must be dramatically curbed, likely to net zero by mid century, in order to forestall a global temperature increase above 2 C, the agreed-upon limit at the United Nations.
"You don't need to have the last decimal to see that the overall number isn't looking good," Connie Hedegaard, the European Union climate action commissioner, told the Associated Press.Suggest a correction