Answer:
The greenhouse effect in simple terms is the warming of Earth's surface.
One way it can be described as a world problem is the melting of ice in Antarctica, because of global warming, the ice is slowly melting, killing lots of animals.
Another way it can be described as a world problem is that the global sea level is rising. Because of these melting ice-caps, sea level is slowly rising and could potentially flood some cities or towns in the next few years.