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Climate Risks Have Been Underestimated for the Last 20 Years

The world's most authoritative voice on climate science has consistently understated the rate and intensity of climate change and the danger those impacts represent, say a growing number of studies.

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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.

Current 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. 

Yet since that 2007 assessment, numerous observations and studies have shown that the speed and ferocity of climate change are at the extreme edge or outpacing IPCC projections on many fronts, including carbon emissions, temperature rise, continental ice-sheet melt, Arctic sea ice decline, and sea level rise (see  sidebar).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.

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."

IPCC scientist and Pennsylvania State University professor of meteorology Michael Mann, who was not involved in the March IPCC report, said the IPCC missed an opportunity to provide politicians with a clear picture of the extent of the climate crisis. "Many scientists felt that report erred by underplaying the degree of confidence in the linkage between climate change and certain types of severe weather, including heat wave severity, heavy precipitation and drought, and hurricane intensity,” he said.

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