Here are a few tips for combining your keywords into powerful searches:
"Kyoto Protocol"
Without the quotation marks you could get an article that mentions protocol but not the Kyoto Protocol specifically.
sustainab*
This returns articles that include the words: sustainab-le, sustainab-ly, sustainab-ility, etc.
"global warming" OR "climate change"
Also look for the advanced search feature. If you want both topics included in your search, make sure select the AND operator to connect them.
("global warming" OR "climate change") AND "Kyoto Protocol"