For several years, the strongest argument climate contrarians had was a paper by Spencer and Christy that showed a discrepancy between changes in surface and lower-stratosphere temperatures that was at variance with all numerical climate models. I think the lower-stratosphere temperatures came from satellite measurements. Climate scientists had several other lines of argument that suggested the result was wrong, but they couldn't pinpoint a systematic error anywhere in Spencer and Christy's paper. It was genuinely puzzling, and I remember thinking that if anything actually blew open the anthropogenic global warming hypothesis, it would be that.

Then, some years ago, Spencer and Christy realized that when correcting for longitude drift of the observing satellites off their sun-synchronous orbits, they'd used the wrong sign. Putting in the right correction brought the results well within the error bars of the models. I think that was the moment Ron Bailey of Reason magazine decided global warming wasn't a hoax.

Right now, we're in an interesting situation: global temperature has been flat for several years and is scraping the lower edge of model error bars. An open possibility is that the climate sensitivity to CO2 has been calculated a litle high based on some transient effect that is now over. Or we could be in the middle of some transient effect that is holding temperatures down. Of course the contrarians insist once again that it means it's all a hoax and the sensitivity is zero, or effectively zero. I haven't heard any believeable mechanism as to how this could be, and I think I've heard this tune before.