(The author is a Reuters columnist. The opinions expressed are his / her own)
By Gerard Wynn
LONDON, Sept 26 (Reuters) - The latest United Nations blockbuster may lay the foundations for a more considered response to climate change, as the basic science becomes more accepted, and as opposition falls in a recovering world economy.
The last report, published in 2007, shared the Nobel peace price before running into a firestorm of criticism over errors,- including the altitude of the Netherlands and what appeared to be a typo on the year when Himalayan glaciers might thaw.
The report coincided with a major climate summit, which fell short of a U.N. climate treaty in 2009, and a proposed U.S. climate bill, defeated the following year - battles both lost against the backdrop of bigger economic priorities.
The report by the Intergovernmental Panel on Climate Change (IPCC) demonstrated a human cause for climate change. But that leaves a host of unanswered questions, including how much climate change there will be, where and when, and with what impact on human systems including fisheries and agriculture.
The IPCC may do well to publish more targeted reports in future, giving pointed, relevant advice to policymakers, leaving perhaps space for a slimmer version of the present reference tomes.
Such targeted reports may find a more receptive ear, now that both hyped expectations, for grand treaties and global carbon markets, and the fear of economic meltdown have gone.
The IPCC publishes its mammoth reports in instalments once every six years or so.
The first release from the latest round, to be published on Friday, updates the science including a review of the causes for observed higher temperatures and other climate changes.
It is now widely accepted that rising greenhouse gas emissions are largely responsible for a warming of the earth, by creating an energy imbalance where the planet is absorbing more heat than it radiates back into space.
The IPCC assigns to each influence a radiative forcing, defined as their cumulative influence since 1750 (before the Industrial Revolution), measured in watts per square metre averaged over the planet.
The radiative forcing from a change in solar output or higher greenhouse gas emissions is well known.
The surface temperature change that results is less certain.
That depends on feedbacks in the climate system: for example, a positive radiative forcing may cause sea ice to melt, creating additional warming because open sea reflects less heat back into outer space than ice.
That temperature responsiveness is called the planet's climate sensitivity.
The two biggest uncertainties in climate science are the radiative forcing of pollution which changes cloud formation and can cool the earth (a negative forcing); and the planet's overall climate sensitivity, given uncertainty about the amount of warming in the oceans where most of the earth's extra energy is stored.
NOW AND THEN
Many messages in the final draft of the latest report (which could be tweaked slightly before publication on Friday) are broadly unchanged from six years ago.
Both reports found accelerating sea level rise and exceptional recent warming on a timescale of about 1,000 years, and both projected these effects would continue this century.
They recorded very similar estimates for the radiative forcing of the main greenhouse gases, and projected continuing climate change even if there were very substantial cuts in greenhouse gas emissions.
There were some changes, however, reinforcing the usefulness of these periodic updates.
For the first time, the draft of the latest summary report described a slowing in the rate of warming on the earth's surface, to 0.05 degrees Celsius in the last decade, from a trend since the middle of the last century of 0.12 degrees per decade.
There were various explanations, from less heat reaching the surface of the earth, to more heat hidden in the deep oceans as a result of natural cycles, or a lower climate sensitivity.
The new report also cut the best estimate for the cooling effect of pollution. And it said human greenhouse gas emissions were "extremely likely" to be responsible for most warming since the middle of the last century, from "very likely."
OUT OF DATE?
As with many bulky reference documents, however, the IPCC assessment reports are already out of date when they are published.
That could be another argument for an overhaul of the panel's reporting system, to slimmer, more targeted studies.
The cut-off date for published journal articles to appear in the latest report was March 2013.
Reflecting the lag in journal publication, the cut-off date was July 2012 for articles submitted to an academic journal.
Taking the single, highly charged issue of the recent slowdown in warming, that means that the new IPCC report missed a detailed study of the subject published online in the journal Nature last month.
The authors attributed the recent pause to a natural decadal cooling effect in the eastern Pacific Ocean.
Scientists at the U.S. National Center for Atmospheric Research presented similar findings in an unpublished paper written last month.
They attributed the pause to the so-called Pacific Decadal Oscillation, which changes winds and ocean currents in such a way as to affect how much heat gets deposited into the deeper ocean, below 300 metres depth.
Given the slow turnover of IPCC reports, readers would have to wait until around 2019 to find such conclusions.
A dedicated report on such a topical issue would make more sense, and a more useful communication of the science where governments may be becoming more receptive. (Editing by James Jukwey)