One of the people I interviewed for a job at my library mentioned this as an approach taken in library instruction - to help students focus their research. The example is candy.
a. Level 1 = If you really only want to find any candy store, to stop at the first one you find, and order something - then search: "candy." But whatever resource you use (yellow page website; RefUSA), you might come up with a lot of sites.
b. Level 2 = If, though, you want a certain kind of candy (I don't know anyone that truly likes all candies), the you would want to narrow it down to words such as "chocolates" or "hard candies." In other words, you would choose a category or class of candies. Use the quotes to search a phrase, i.e. "milk chocolate."
c. Level 3 = But really, most of us want something specific, either for ourselves or others. For example, we should choose which kind of hard candies we want. This is the search that is most rewarding and most difficult. You might have to "shop around" a bit to find exactly what you want to search for.
If you would up with a desire for butterscotch hard candies, then you would search using several words, such as "butterscotch hard candies smooth 'old fashioned'" - note that it is string of four words and 1 phrase. You might want to include words such as "retail" or the phrase, "free shipping."
So the student given the topic of global warming, would choose Level 3 and create a search strategy that would help him/her save time and get great results. The key point is to focus your attention and include all that you want to search. So instead of starting with "global warming" as the search phrase, you might enter:
["greenhouse gases" "polar bears" arctic melting ice flows protect wildlife] - yes, that is a lot for a first search, but if I do this in Google, then the first site I find is a wonderful one from Nature Canada (note: more on what online resource to choose later - very important).
With our aim of writing a five to ten page paper, this topic would excite me and be of interest to the professor or others in the class.