Three years ago, the United Nations issued what many considered the bible of climate change.

The 3,000-page report famously said the evidence for long-term global warming was "unequivocal." That's science-speak for: The argument's over.

But these days that thunderous 2007 verdict is sounding, well, a lot like tomorrow's weather forecast: It's very likely to be right. But there's some doubt.

Why the cloudy outlook? For starters, last month the U.N.-sponsored Intergovernmental Panel on Climate Change was forced to apologize for a gaudy — and false — claim that the Himalaya glaciers could disappear by 2035. Turns out that warning might be off by about 300 years. The bad info wasn't science but conjecture from an activist group's report. Big oops.

Then came the reports that the head of the U.N. panel, Rajendra Pachauri, was battling accusations of financial conflicts of interest linked to his consulting business. He has denied doing anything improper, and so far no one has proved otherwise.

And then there's the scandal known as Climategate. That's the furor over purloined e-mails showing a few top climate scientists in England and the U.S. fuzzing over some contradictory evidence and conspiring to muzzle skeptics and bury research that didn't agree with their own findings. Investigators still are trying to get to the bottom of that.

So now the U.N. panel's credibility is heavily damaged — and so is the science of global warming. Doubts about the science — and scientists — are creeping in. Many people can't help but wonder: Are some of these climate scientists trying to find the facts or hide them?

You could see that uncertainty in the recent global warming summit in Copenhagen, where the world's powers agreed to nothing of consequence.

You can see it in the U.S. Senate, where an expensive and complicated cap-and-trade carbon bill is dead.

You can sense that public opinion is turning against the idea of massively expensive solutions. In a recent Rasmussen poll, slightly more than half the people surveyed said that warming is a serious problem. But a rising number of people — half in the latest poll — blame long-term planetary trends, not human activity.

Let's take a deep breath here. The climate skeptics have poked some holes in the science and exposed the apparently unethical behavior of a few top scientists. They've found some disturbing mistakes in the panel's report.

None of this disproves the essential conclusion that the planet is warming, and there's still strong evidence that it is driven by human activity. Even if you throw out the tainted research, the trends — rising sea levels, temperature changes and retreating polar ice — are convincing and have been documented over many decades by different groups of scientists around the world.

Yet, the U.N.'s credibility on climate change is in tatters, and that's going to affect the debate. In a recent article in the journal Nature, five climate scientists called for a drastic overhaul of the panel. They want to make it smaller, more independent and nimble. They want to make sure that the scientists chosen to work on the reports aren't selected because they already agree with the global warming orthodoxy. That kind of change is essential to restore the panel's credibility.

Meanwhile, the critical question of what can and should be done to slow global warming — how fast and at what expense — remains open to debate, as it should.

One climate expert, John Christy of the University of Alabama, wrote in Nature: "The truth, and this is frustrating for policymakers, is that scientists' ignorance of the climate system is enormous. There is still much messy, contentious, snail-paced and now, hopefully, transparent work to do."

Hmm. Humility. How refreshing. And scientific.