My concern is not that scientists are biased. Quite the contrary, most scientists I know are painstakingly objective. Despite that, once a scientist publicly advocates a position--even if that position is founded in fact based on the current state of knowledge--that scientist is likely to succumb to the natural tendency to turn that position into desire and defend that position selectively and vehemently.
That is where we are in the climate change debate. Two sides, with ardently staked positions, selectively choosing 'facts' to support that predetermined position and raging against the 'facts' in disagreement.
Science is better than that.Haab's right: science is better than that. But scientists are not, never have been, nor, arguably, should they be expected exist beyond politics.
Why should natural scientists, any more than social scientists (including economists), be barred by their work from advocating for positions they believe in? In fact, there is a long history of political advocacy by scientists. It is worth recalling that Linus Pauling won two Nobel Prizes, one for chemistry and the other for peace. Albert Einstein famously wrote to President Roosevelt, first, to advocate the building of an atomic bomb out of concern that Germany was already trying to do so, and later, to advocate against using the atomic bomb. J. Robert Oppenheimer - scientific director of the Manhattan Project - outraged many politicians by his political statements during the Red Scare, and eventually had his security clearance revoked.
Scientists should not be expected to fly above the political fray. They should, however, be expected to keep their science as free as possible from their own political biases. This, I take it, is the most we might legitimately and realistically expect. But even when scientists - like other fallible humans - fail to keep their science separate from their politics, the scientific method itself provides something of a check. Because science is a competitive enterprise, if one scientist achieves dubious results based on a flawed methodology, data mining, etc., whether by accident or political motivation, other scientists will sooner or later reveal the flaws through their analyses of methods, efforts to replicate findings, etc. Simply put, politicized science will ultimately be repaired by more and better science. And scientists who falsify findings or purposefully bias their analyses sooner or later are publicly shamed. The risk of shame alone, I believe, provides a reasonably strong incentive for most scientists to be careful in their work and parsimonious in the conclusions they draw from it.