I'd be surprised if you ever found a single answer for this, which, if you think about it isn't surprising. Taking the temperature of a defined place, like Bristol, is straightforward. But how would you collect temperatures for everywhere on the globe? Temperatures on land would be difficult enough, what about large uninhabited places like deserts. You'd have to place heat sensing equipment at close regular intervals to build up readings for the whole desert. But what about the oceans, what would you do for them? Somehow place heat sensors every few metres? Even every few kilometres for all the worlds oceans would be impossible. If you think about the logistics involved it would be too monumental a task to undertake. I think the nearest you'll get are average temperatures for large landmasses, like America.