Final answer:
The greenhouse effect is a natural phenomenon that keeps the world warm by trapping and re-radiating heat. Human activities have caused an increase in greenhouse gas concentrations, resulting in global warming.
Step-by-step explanation:
The greenhouse effect is a natural phenomenon that keeps the world warm by allowing certain gases in the atmosphere to trap and re-radiate heat back to Earth's surface. Without the greenhouse effect, the average surface temperature of the Earth would be much colder and life as we know it would not be possible. However, the current problem is that human activities have intensified the greenhouse effect by increasing greenhouse gas concentrations in the atmosphere, leading to global warming.