My own theory, based on the timing of things, is that we are coming out of a natural cold period around a couple hundred years ago, and the average temperature is indeed getting higher over time. Beyond that, there are a lot of feedback issues with ocean currents and such, and, like the difference in sea ice between the north and south hemispheres, there may be things happening that are counterintuitive.

When it comes down to it, figuring out the Earth's temperature is a guess, based on averages from many places under many conditions.

All weather is basically a guess. The peak of the hurricane season was September 10th. How will last year's predictions work out? There's an El Nina forming in the Pacific that will probably play merry hob with my own personal weather.