Read the whole article at The Telegraph and follow the links.
3.5 degrees C in two decades? That would indeed be a remarkable temperature rise in anybody's money. (Remember, since 1850 global mean temperatures have risen by about 0.8 degrees C – and we're supposed to find that worrying and significant).
To his credit, this troubled Leo Hickman, too.
So began his wild goose chase to track down the source of the BBC's factoid.
As you'll see from his superb piece he never got a terribly satisfactory answer.