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.
Not all blame IPCC for failing to produce sufficient alarm with policymakers, however.
Andrew Dessler, professor of atmospheric sciences at Texas A&M University, sees no need for the IPCC to do anything differently. "The burden of communication falls on policymakers, not scientists," he said. Scientists are responsible for providing the hard data. It is up to policymakers to lead, connect the dots, and explain to the public the necessity of responding to global warming.
But the consequences of a conservative bias by climate scientists can be significant, others like Oreskes note. A society blind to the full range of potential outcomes, particularly the most disruptive, can remain apathetic to the need for change, pushing hard decisions off into the future.
The melting Arctic ice pack may offer such an example.
Scientists suspect that a diminished Arctic ice pack has the power to shift weather patterns across the Northern Hemisphere. Less ice, the hypothesis goes, would weaken and shift the jet stream, causing it to block normal weather patterns and hold storms, dry spells and heat waves in place so that they pound a single location for days, weeks or months.
But with the ice supposed to stay intact until 2070 or later, this was largely a theoretical problem for the future.
No longer: Summer ice in the Arctic hit a new low in 2012, and now some scientists say there is likely a link between that meltdown and the record-breaking drought that caused an estimated $28 billion in damage across the United States, as well as the soggy summer that left Britain drenched.
Even Hurricane Sandy has a potential Arctic tie-in, with researchers suggesting that the anomalous strong high pressure weather system over Greenland, forcing Sandy ashore in October, was influenced by the ice cap's decline.
These events – and especially the rapidity with which they are occurring – were not foreseen by IPCC models.
Dismissed as outliers
Likewise, weather forecasters not associated with the IPCC, using short-term models, almost uniformly failed to predict the drought that gripped most of the United States this summer. The reason? The few computer models that did forecast a major drought were dismissed as outliers, according to a report by Climate Central, a science research and communication organization.
"It's like going to a doctor," said Princeton's Oppenheimer. "When data is weak you ask your doctor for his or her best judgment.
"That is what IPCC is supposed to do."
IPCC's Fifth Assessment will be released in four parts from September 2013 through September 2014. Reforms within the organization have resulted in a more demanding consensus process – one that may produce even greater caution in its conclusions, say several former senior IPCC authors.
"The next report shows every sign of being even more conservative than the previous ones," said Trenberth. Instead of 10 lead authors per chapter, 14 or 15 scientists will have a say, making consensus-building harder.
"That builds in more conservatism, caveats, and wiggle room," Trenberth said.
IPCC's internal rules and deadlines have also been tightened, preventing the inclusion of some of the most up-to-date studies, he added.