It's tempting to infer from the reports of University of California physicist Richard Muller's conversion that climate sceptics really can change their spots. Analyses by Muller's Berkeley Earth Surface Temperature project, which have been made publicly available, reveal that the Earth's land surface is on average 1.5C warmer than it was when Mozart was born, and that, as Muller puts it "humans are almost entirely the cause". He says that his findings are even stronger than those of the Intergovernmental Panel on Climate Change, which presents the consensus of the climate-science community that most of the warming in the past half century is almost certainly due to human activities. "Call me a converted sceptic", says Muller in the New York Times.

Full marks for the professor's scientific integrity, then. But those of us who agree with the conclusions of nearly every serious climate scientist on the planet shouldn't be too triumphant. Muller was never your usual sceptic, picking and choosing his data to shore up an ideological position. He was sceptical only in the proper scientific sense of withholding judgment until he felt persuaded by the evidence.

Besides, Muller already stated four years ago that he accepted the consensus view – not because everyone else said so, but because he'd conducted his own research. That didn't stop him from pointing out the (real) flaws with the infamous "hockey stick" graph of temperature change over the past millennium, nor from accusing Al Gore of cherry-picking facts in An Inconvenient Truth.

In one sense, Muller is here acting as a model scientist: demanding strong evidence, damning distortions in any direction, and most of all, exemplifying the Royal Society's motto Nullius in verba, "take no one's word for it". But that's not necessarily as virtuous as it seems. For one thing, as the Royal Society's founders discovered, you have to take someone's word for some things, since you lack the time and knowledge to verify everything yourself. And as one climatologist said, Muller's findings only "demonstrate once again what scientists have known with some degree of certainty for nearly two decades". Wasn't it verging on arrogant to have so doubted his peers' abilities? There's a fine line between trusting your own judgment and assuming everyone else is a blinkered incompetent.

All the same, Muller's self-confessed volte-face is commendably frank. It's also unusual. In another rare instance, James Lovelock was refreshingly insouciant when he recently admitted that climate change, while serious, might not be quite as apocalyptic as he had previously forecast – precisely the kind of doom-mongering view that fuelled Muller's scepticism. There's surely something in Lovelock's suggestion that being an independent scientist makes it easier to change your mind – the academic system still struggles to accept that getting things wrong occasionally is part of being a scientist.

But the problem is as much constitutional as institutional. Despite their claim that evidence is the arbiter, scientists rarely alter their views in major ways. Sure, they are often surprised by their discoveries, but on fundamental questions they are typically trenchant. The great astronomer Tycho Brahe never accepted the Copernican cosmos, Joseph Priestley never renounced phlogiston, Einstein never fully accepted quantum theory. Most great scientists have carried some obsolete convictions to the grave, which is why Max Planck claimed that science advances one funeral at a time.

This sounds scandalous, but actually it's useful. Big questions in science are rarely resolved at a stroke by transparent experimental results. So they require vigorous debate, and the opposing views need resolute champions. Richard Dawkins and EO Wilson are currently locking horns about the existence of group selection in Darwinian evolution precisely because the answer is so far from obvious. I'd place money on neither ever rescinding.

The fact is that most scientists seek not to convert themselves but to convert others. That's fair enough, for it's those others who can most objectively judge who has the best case.

Could this mean we actually need climate sceptics? Better to say that we need to subject both sides of the debate to rigorous scientific testing. Just as Muller has done.