If humans aren't the primary cause of the current global warming, what is?

Climate scientists still have not figured out for sure what causes ice ages. Most of North America was covered by glaciers 20,000 years ago and those melted about 10,000 years go. Nobody knows why. So I would question anyone who claims to know for sure why the climate is warming now. It is plausible that the added CO2 is the cause, or part of the cause, but until they can prove for sure what causes the ice age cycles, I am skeptical of any claims they they know what is going on now and what will happen in the future.