What do you feel is the number 1 threat to mankind's survival?

Human overpopulation will eventually correct itself, if we don't do it ourselves (starvation, disease, war, natural disasters etc), Such things will not make us go extinct, but eventually would force some kind of population balance. I do not consider it our main threat.

However, global climate change is happening at an unprecedented rate (taking decades to hundreds of years, instead of thousands of years).

If the climate changes too dramatically and too quickly, there will not be enough time for the Earth to change back for our severe endangerment or extinction.

That is our number one threat. My hope is we will have the time, wisdom, and ability to adapt in time.

There used to be many more human species. Some of them survived long enough to live during some of the same time periods as our species. It is certain with at least a few of them regional climate change (not global, but regional), was enough to severely endanger them. It's also possible we wiped out the remainder either through direct genocide or through intermarriage. Neanderthals are one example. Some of us modern humans definitely have some Neanderthal DNA, but there is also ample evidence to suggest that Neanderthals were less equipped to handle the significant warming of their habitats and the subsequent changes in food sources. Our species was better able to adapt (but we had thousands of years to adapt to such changes. Even our seemingly superior adaptability may be insufficient for the current rate of climate change, even if it is hundreds of years (which I doubt).

Of course, there are many other threats as well, that are less likely but equally or even more devastating.