IPCC Vice-Chair Jean-Pascal van Ypersele countered that "the mandate of IPCC is to assess where there is consensus, and to reflect the full diversity of views that are scientifically valid where there is not."* He conceded that by requiring teams of authors to agree upon a report’s text, the IPCC process is inherently conservative. Getting the balance right, he said in an e-mail, is "not always easy."
Oreskes, Oppenheimer and their co-authors argue the conservative bias pervades all of climate science.
But the underestimation by the IPCC is particularly worrisome, scientists say, because the organization is charged specifically with advising policy makers on the most relevant, accurate climate science.
Established in 1988 by the United Nations, the IPCC does no original climactic research. Its role is to review current science from around the world, then synthesize and summarize that data within comprehensive reports meant for policymakers.
Such assessments typically take five to seven years to complete in a slow, bureaucratic process: Thousands of scientists from around the globe, working as unpaid volunteers, first sift through the scientific literature, identifying trends and writing a draft report. That draft is reviewed and thoroughly revised by other scientists. Then a summary for policymakers, condensing the science even further, is written and subjected to a painstaking, line-by-line revision by representatives from more than 100 world governments – all of whom must approve the final summary document.
IPCC's four assessments – massive, multi-tome volumes released in 1990, 1995, 2001 and 2007 – are considered the gold standard in climate science. The fourth report earned both intense criticism from climate skeptics and the honor of the 2007 Nobel Peace Prize, shared with former Vice President Al Gore.
Yet since that 2007 assessment, numerous observations and studies have shown that the speed and ferocity of climate change are outpacing IPCC projections on many fronts, including CO2 emissions, temperature rise, continental ice-sheet melt, Arctic sea ice decline, and sea level rise (see sidebar).
Pattern of under-projection
The pattern, said Oreskes in an interview, is under- rather than over-projection. "These data simply do not support the allegations by skeptics that scientists have been alarmists," she said.
One example: In November, scientists at the National Center for Atmospheric Research in Boulder, Colo., took a closer look at the computer models underpinning most climate predictions and concluded future warming is likely to be on the high side of climate projections.
Another example: This summer, NASA climatologist James Hansen co-authored an analysis of recent extreme weather across the globe. Hansen's team arrived at a strikingly different conclusion from an IPCC special assessment on the topic released just months earlier.
The Hansen study, published in August in the journal Proceedings of the National Academy of Sciences, concluded that rapid climate change over the past 30 years has loaded the dice in favor of extreme weather. The chance of extreme summer heat is now 13 percent higher than in 1980, the report found. Record heat waves seen by Europe in 2003, Russia in 2010, and Texas in 2011 would not have happened without human-caused global warming, it concluded.
Hansen's conclusion contrasted sharply with the hedging in the IPCC special assessment on extreme weather, published in March, 2012: "Confidence in projecting changes in the direction and magnitude of climate extremes depends on many factors," the report's summary for policymakers began. "Even the sign of projected changes in some climate extremes over this time frame is uncertain."