Explain why scientists are concerned about human activities that cause global warming if global warming is caused by the greenhouse effect and the greenhouse effect is a natural part of Earth's climate

I read somewhere that scientist shouldn't be worried at all. They just kinda freaked out when they found out there was a hole in the ozone layer when there is some theories saying that There has always been a "hole" up there. They think its getting worse because of humans burning gasses and releasing it into the air, when actually the ozone layer is replenishing itself and getting better.