Scientists and academics who have taken part in major international and domestic climate assessments say that politics and alarmism need to be taken out of the study on global warming.

A panel of climate experts are telling the House Science Committee that politics often gets in the way of good science at the Intergovernmental Panel on Climate Change (IPCC), as well as in the U.S. government’s own climate research.

Climate scientists and researchers who dissent even slightly from the talking points of politicians and environmental groups are intimidated and ostracized, said one congressional witness. Politics, the witness said, takes a lead role over science in the study of global warming.

“Academics who research climate change out of curiosity but find less than alarming things are ignored, unless they rise to prominence in which case they are harassed and smeared,” said Richard Tol, an economics professor at the University of Sussex.

Tol gained notoriety when reports came out that he removed his name from the IPCC’s fifth climate assessment because it had become too “alarmist”. Since then Tol says he has been attacked by environmentalists and left-leaning media for not agreeing that global warming would bring catastrophe.

“Governments nominate academics to the IPCC – but we should be clear that it is often the environment agencies that do the nominating,” Tol said in his written testimony, adding that it’s “rare that a government agency with a purely scientific agenda takes the lead on IPCC matters.”

“As a result, certain researchers are promoted at the expense of more qualified colleagues,” Tol said. “Other competent people are excluded because their views do not match those of their government. Some authors do not have the right skills or expertise, and are nominated on the strength of their connections only.”

Tol’s concerns were echoed by fellow panelist Dr. Michael Oppenheimer of Princeton University. Oppenheimer is far from a climate skeptic and has participated in IPCC assessments for decades.

While Oppenheimer says the IPCC has done many great things, he agrees that there needs to be reforms to increase transparency and openness of the organization’s process. Oppenheimer did, however, say that IPCC reports’ “Summary for Policymakers” (SPM) sections are effectively approved by governments that can distort the scientific findings.

“In the end, the SPM is approved by governments,” Oppenheimer said in his testimony before Congress. “Admittedly, the SPM approval process is imperfect.”

Oppenheimer added that “there have been occasions where government interventions, by causing omissions, have diluted IPCC findings.”

House Republicans have become increasingly interested in the scientific process at the IPCC and in the U.S. government after numerous reports have come out that governments have been lobbying the IPCC to omit some details or emphasize others in their climate reports.

“Not all IPCC authors are equal,” Tol told Congress. “Some hold positions of power in key chapters, others subordinate positions in irrelevant chapters. The IPCC leadership has in the past been very adept at putting troublesome authors in positions where they cannot harm the cause.”

Numerous reports have also come out about skeptical climate scientists being bullied or intimidated because of their affiliations and dissenting views.

Recently, Swedish meteorologist Lennart Bengtsson was forced to leave the U.K.’s Global Warming Policy Foundation, a science nonprofit that is skeptical of the notion that global warming will catastrophic.

“I received emails from colleagues all over the world telling me it was a ‘questionable’ group,” he told the U.K.’s Daily Mail. “But what made me the most upset was when a colleague from the U.S. resigned as co-author of a paper, simply because I was involved.”

Bengtsson claimed he also had an upcoming study rejected by a prominent scientific journal for political reasons.

“The problem we have now in the scientific community is that some scientists are mixing up their scientific role with that of climate activist,” Bengtsson told the U.K. Times.

According to Tol, this sort of treatment of dissenting scientists is not uncommon, as those who don’t toe the line on global warming are smeared by activist groups and the media.

“Other eminent meteorologists have been treated like Bengtsson was… merely for sticking to the academic literature, as reflected by the IPCC, that there is no statistical evidence that the impact of natural disaster has increased because of climate change,” Tol said.

“If similar-minded people come together, they often reinforce each others’ prejudices,” Tol said. “The IPCC should deploy the methods developed in business management and social psychology to guard against group think.”

Update: Texas Republican Rep. Lamar Smith said the Obama administration “should stop trying to scare Americans and then impose costly, unnecessary regulations on them. The President says there is no debate. Actually the debate has only just begun. When assessing climate change, we need to make sure that findings are driven by science, not an alarmist, partisan agenda.”

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