Anomalies
Just something that piqued my curiosity - can anyone explain to me, at length appropriate to a blog comments box, what the "global temperature anomaly" beloved of global warming charts actually is? It's clearly some kind of modelling residual, but looking at the plots, I don't think it can simply be a difference from the sample period mean, can it? Thanks awfully.
"Anomalies and Absolute Temperatures
ReplyDeleteOur analysis concerns only temperature anomalies, not absolute temperature. Temperature anomalies are computed relative to the base period 1951-1980. The reason to work with anomalies, rather than absolute temperature is that absolute temperature varies markedly in short distances, while monthly or annual temperature anomalies are representative of a much larger region. Indeed, we have shown (Hansen and Lebedeff, 1987) that temperature anomalies are strongly correlated out to distances of the order of 1000 km. For a more detailed discussion, see The Elusive Absolute Surface Air Temperature."
http://data.giss.nasa.gov/gistemp/
A temperature anomaly is the difference between a short-term average temperature (e.g. monthly) at a particular place, and the long-term average temperature for that place (and the same time of year) during a baseline period.
ReplyDeleteThe choice of baseline period is arbitrary, but 1951–1980 is a common choice, following Hansen and Lebedeff 1987.
The point of working with the anomaly rather than directly with temperatures, is that you can combine the anomalies from places with different climates to get regional or global anomalies. On a local scale, you may have one weather station in a valley and the other on top of a nearby hill: the hilltop may be consistently a couple of degrees colder than the valley, but if both stations show a similar warming trend relative to a baseline, then this is evidence for local climate change. On a global scale, a weather station in the tropics may be 50 degrees warmer than one in Antarctica, but by averaging their anomalies, you can find evidence for global climate change.
Hansen and Lebedeff 1987 is the paper that introduced the idea, though of course there have been many refinements since.
(Declaration of interest: I contributed a small amount of programming to Clear Climate Code.)
Awesome, thanks - I was wondering why the residuals weren't randomly distributed, but presumably this is because of the warming trend itself.
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