Two possible takes on "natural": 1. They may mean "in nature", that is, not inside a building, and not even in a city (both of which would tend to give higher

Two possible takes on "natural": 1. They may mean "in nature", that is, not inside a building, and not even in a city (both of which would tend to give higher numbers). 2. They may mean "using something in nature as a measuring method" - in this case, Antarctic ice. I agree that it's a confusing/misleading term.