I was thinking about Doug Keenan's WSJ article about statistical significance in the global temperature records - for those unfamiliar with it, we don't know whether the recent warming is significant or not because we don't know what statistical model to adopt to describe the climate's normal behaviour. Doug has published a "director's cut" of the article at his website.
I found myself wondering how the Royal Society had explained the recent warming to the public in their new paper on climate change and, more particularly, how they had addressed the question of statistical significance. Here's the relevant excerpt:
Measurements show that averaged over the globe, the surface has warmed by about 0.8°C (with an uncertainty of about ±0.2°C) since 1850. This warming has not been gradual, but has been largely concentrated in two periods, from around 1910 to around 1940 and from around 1975 to around 2000. The warming periods are found in three independent temperature records over land, over sea and in ocean surface water. Even within these warming periods there has been considerable year-to-year variability. The warming has also not been geographically uniform – some regions, most markedly the high-latitude northern continents, have experienced greater warming; a few regions have experienced little warming, or even a slight cooling.
When these surface temperatures are averaged over periods of a decade, to remove some of the year-to-year variability, each decade since the 1970s has been clearly warmer (given known uncertainties) than the one immediately preceding it. The decade 2000-2009 was, globally, around 0.15°C warmer than the decade 1990-1999.
So, no mention of statistical significance. This is a bit disappointing really - this is our national science academy. I'm not sure that saying that recent decades are warmer than earlier ones is saying anything very much at all.
The other thing that interests me is the reference to known uncertainties. What is the magnitude of the known uncertainties in the temperature records? I'm not sure I've seen these before (or perhaps I've forgotten).