Real Climate has been speaking with two voices on how to compare observations of climate with models. Last August they asserted that one-year’s sea ice extent could be compared with models:

A few people have already remarked on some pretty surprising numbers in Arctic sea ice extent this year (the New York Times has also noticed). The minimum extent is usually in early to mid September, but this year, conditions by Aug 9 had already beaten all previous record minima. Given that there is at least a few more weeks of melting to go, it looks like the record set in 2005 will be unequivocally surpassed. It could be interesting to follow especially in light of model predictions discussed previously.

Today, they say that looking at 8 years of temperature records is misguided:

John Tierney and Roger Pielke Jr. have recently discussed attempts to validate (or falsify) IPCC projections of global temperature change over the period 2000-2007. Others have attempted to show that last year’s numbers imply that ‘Global Warming has stopped’ or that it is ‘taking a break’ (Uli Kulke, Die Welt)). However, as most of our readers will realise, these comparisons are flawed since they basically compare long term climate change to short term weather variability.

So according to Real Climate one-year’s ice extent data can be compared to climate models, but 8 years of temperature data cannot.

Right. This is why I believe that whatever one’s position of climate change is, everyone should agree that rigorous forecast verification is needed.

Post Script. I see at Real Climate commenters are already calling me a “skeptic” for even discussing forecast verification. For the record I accept the consensus of the IPCC WGI. If asking questions about forecast verification is to be tabooo, then climate science is in worse shape than I thought.