global warming before the ice age in 2010, page-76

"I have read that the estimated effect of the urban heat bubble was something like a hundredth of a degree."

I have no figures to back it up, but London is a lot warmer than nearby small towns a similar distance from the ocean, typically 1-3 degrees - even warmer than towns to the south - you can actually feel the significant difference when you travel in and out of London.