Climate change is a subset of the larger issue, which is the economic system. Call it capitalism with a measure of corporate socialism. By its nature, capitalism must lead to its own destruction. Absent government checks, that destruction is of the ecosystem that supports it.

This isn't to downplay the very real threats of biological agents that will be easy to make with 3-D biotechnology.

And AI, which may be the biggest threat.

The human propensity for jumping from the superstitious religious frying pan into the superstitious ideological fire.

I think that the greatest challenge that civilization faces is the inherent failings of the human mind. We tend to tribalism. We tend to hide from reality when it isn't what we want. We trust personal experience more than evidence. In other words, we aren't very good at critical thinking. The greatest challenge to civilization are the same beings who created it to begin with.

Probably being civil!

I agree that climate change is the biggest problem. I think it is because it because there is not one single solution. Wind, solar, geothermal, hydroelectric, nuclear, and others all have their place. We are not going to completely eliminate fossil fuels. We need the higher octane for aviation uses. We will find our way through this.

Organized religion is the biggest challenge, it seems to me. Through scientific study and mutual cooperation, humankind has been able to solve most of its problems. But with organized religion, we get institutionalized superstition replacing science and righteous indignation sewing the seeds of dissension, leading to open hostility and eventually war.

Climate change. Hands down.

aliens.

Population growth

civilization itself

Fidelity to objective reality. I.e. Good mental health.

The reason the Reepublicans do not like to talk about climate change is because they are making a lot of money selling gas as in gasoline and natural gas. As long as profit talks they will no acknowledge climate change.

Man killing man kind. Climate change included (but not limited to).

Technology.

Not killing itself off