Scientists generally agree that Earth is getting warmer due to what's called the greenhouse effect. A greenhouse used for plants stays warm because sunlight and heat coming through the glass roof are mostly trapped. The heat doesn't radiate back into the atmosphere because it's partly trapped by the glass. Something like that happens on Earth. The atmosphere acts like a blanket that keeps the Sun's heat from leaking away into space.
 

1. In the first sentence, the phrase what's called the greenhouse effect presents a/an