Monitor changing value in web page

this is a very specific question which has proven beyond me.

I'd like to continuously monitor some value in a given website. Let's say I want to make a graph about human population and that I happen to want to use http://www.worldometers.info as my source.

As you can see there, current world population is the ever changing value which I'd like to monitor. This value is not present in the source web, but it is updated via a javascript.

Considerations/Doubts

1. Is web-scrapping a nice approach to this problem? If yes, what would be a nice start to look into? I've already seen some ways, but they get only the current value and must be updated every 'frame' capture these values.

2. Is it possible to it in a manner that do not saturate the target web? The intent of this is double. One is to remain as invisible as possible. It would actually be great if this value could be monitored inside the explorer (preferably Firefox). The other would be to cause no harm to the target web.

Thank you all very much for your time.