I guess you slept during your science classes.
There are natural forces that force climate change. The strongest natural forces provide the climate changes that cause Ice Ages. We emerged from the last Ice Age some 12,000 years ago, and reached the warmest interglacial point some 8,000 years ago. Since then, natural forces have been cooling the planet, with occasional dips.
But recent human pollution is forcing warming, and these artificial forces are overwhelming natural forces, particularly since the 1970's.