There seems to be little doubt that global warming is a real phenomenon. However, as (1) ... as it may seem, scientists disagree about the causes of this (2) ... change. One explanation is the greenhouse effect. This is where pollution in the atmosphere traps the sun's rays, like a giant greenhouse. While nobody claims that releasing chemicals into the atmosphere can be (3) ..., not everyone is convinced that all the temperature change is due to industrial activity. It's an important question, because a (4) ... can only be found once we understand exactly what is causing the problem.