The definite article the signals to the reader that the noun is specific, not arbitrary, and not new to the reader.
The Greenhouse Effect is a theory in environmental science which warns of the warming of the globe due to an increase in carbon dioxide in the atmosphere.[Only one Greenhouse Effect, one globe, and one atmosphere are assumed to exist; the context of the globe serves to make the warming unique.]
Use a definite article if you intend a singular noun to refer to an entire class of items. Using a definite article with a singular noun in this way is particularly common with species of animals, inventions, or musical instruments.
The farm alligator is more dangerous than the
wild alligator because the farm alligator does not
--"Take It or Leave It," Valley Comic News (modified)
See Usage Rules for Articles for further information about how definite articles are used in sentences.