How many rats are there in the world?

Here are some examples of the estimated number of rats worldwide and in select cities:

It has been estimated that there are up to seven billion rats across the planet.

http://www.npr.org/sections/thetwo-way/2011/11/03/141946751/along-with-humans-who-else-is-in-the-7-billion-club

Here is a recent story about 2 billion rats in the country of China alone:

http://www.freerepublic.com/focus/f-news/1865142/posts

There are between 1.0 and 1.5 billion rats in Britain - at least one rat per person. Officials estimate the total number of rats in the French capital at eight million - or four rats for every Parisian.

http://www.bbc.com/news/magazine-20716625

This, of course, is all speculative as the true number of rats worldwide is difficult to determine.

[Links updated and figures changed to 2015 realities as well by shuehorn on August 22, 2016]