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Across two decades and thousands of pages of reports, 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 on the topic.
This conservative bias, say some scientists, could have significant political implications, as reports from the group – the U.N. Intergovernmental Panel on Climate Change – influence policy and planning decisions worldwide, from national governments down to local town councils.
As the latest round of United Nations climate talks in Doha wrap up this week, climate experts warn that the IPCC's failure to adequately project the threats that rising global carbon emissions represent has serious consequences: The IPCC’s overly conservative reading of the science, they say, means governments and the public could be blindsided by the rapid onset of the flooding, extreme storms, drought, and other impacts associated with catastrophic global warming.
"We're underestimating the fact that climate change is rearing its head," said Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research and a lead author of key sections of the 2001 and 2007 IPCC reports. "And we're underestimating the role of humans, and this means we're underestimating what it means for the future and what we should be planning for."
Underplaying the intensity
A comparison of past IPCC predictions against 22 years of weather data and the latest climate science find that the IPCC has consistently underplayed the intensity of global warming in each of its four major reports released since 1990.
The drastic decline of summer Arctic sea ice is one recent example: In the 2007 report, the IPCC concluded the Arctic would not lose its summer ice before 2070 at the earliest. But the ice pack has shrunk far faster than any scenario scientists felt policymakers should consider; now researchers say the region could see ice-free summers within 20 years.
Sea-level rise is another. In its 2001 report, the IPCC predicted an annual sea-level rise of less than 2 millimeters per year. But from 1993 through 2006, the oceans actually rose 3.3 millimeters per year, more than 50 percent above that projection.
Some climate researchers also worry that recent institutional changes could accentuate the organization's conservative bias in the fifth IPCC assessment, to be released in parts starting in September 2013.
The tendency to underplay climate impacts needs to be recognized, conclude the authors of a recent paper exploring this bias. Failure to do so, they wrote in their study published last month in the journal Global Environmental Change, "could prevent the full recognition, articulation and acknowledgement of dramatic natural phenomena that may in fact be occurring."
The conservative bias stems from several sources, scientists say. Part can be attributed to science's aversion to drama and dramatic conclusions: So-called outlier events – results at far ends of the spectrum – are often pruned. Such controversial findings require years of painstaking, independent verification.
Yet some events in nature are dramatic, conclude University of California, San Diego, history and science professor Naomi Oreskes and Princeton University geosciences professor Michael Oppenheimer, co-authors of the study looking at the IPCC's bias. "If the drama arises primarily from social, political or economic impacts," they wrote, "then it is crucial that the associated risk be understood fully, and not discounted.”