James and William have more info on UBS-GWI, a composite index of 15 American weather futures markets. It will certainly have to expand geographically to be "global", but it seems like a good start.
I do have two concerns: first, it's unclear to me whether the index will be weighted by the level of trading in the particular submarkets. While that could well make sense to an energy trader, it biases the information from a straight-science perspective. Second, it will necessarily be an Urban Heat Island index, I think. That doesn't validate the worn-out skeptic argument of cool rural temperature stations transforming into warmer urban heat islands, but it does mean, for once, that land use will be an important factor for purposes of the index. For example, the highly laudable movement towards green rooftops and higher-albedo building and paving materials will affect the index without affecting global temps.
Regardless, a very promising start. I've sent some questions off to UBS, and we'll see if they respond.