Science is what is published in peer reviewed journals and presented at conferences. What is not science is to make allegations of errors and not follow up on that with peer reviewed publications, conference presentations etc.
Now, I'm not an expert in climate scince, so I can't give a detailed response to problems raised by skeptics. But it is up to the skeptics to make their point in the scientific community. So far they haven't so their criticism of global warming theory is definitely not science.
I think this statement is complete rubbish. Any proponent of any scientific proposal (including AGW) must be completely open to challenge and analysis from any quarter. That is the only way to ensure credibility. If someone postulates something, it isn't up to the rest of us to prove the opposite and get published in a journal as part this. A very good example is the recent case of the 0.15 degree error that was found in the ground based US temperature reading post 2000. These were simply pointed out by a Steve McKintyre .... and I may be mistaken .... but I don't think his observations were peer reviewed or published in any journal. And that was science at its very best.
