Thanks for the pointers Nathan: I'll do some reading up on those. And your point about building a climate model being the best way to get at good conclusions is well taken.
This is basically me just thinking outloud about what a researcher would do if he doesn't think he has a meaningful grasp of the underlying physics (which is not to imply thinking that no-one has a grasp on the physics). Obviously temperature records are a spatial and a temporal patchwork which need joint analysis for any reasonable estimates. Intuitively one would like to marginalise over variables like weather, but to take into account that weather must be similar in nearby areas.
Looking at the wikipedia entry for [Berkeley Earth](http://en.wikipedia.org/wiki/Berkeley_Earth) has a link to one of their papers which discusses their procedure for estimating the average. This seems a reasonable way of doing things, but it's not clearly the "one true" way to do things, so I'm just idly thinking about stuff an amateur might toy with.