Which catastrophic risks does a mars colony mitigate? ... Climate change : yes
If a Mars colony mitigates catastrophic risk (extinction risk?) from climate change,
then climate change is not an existential risk to human civilization on earth.
If humans can thrive on Mars, Earth based humanity will be able to cope with any climate change less drastic than transforming the climate of Earth to something as hostile as the current climate of Mars.
Is climate change seriously considered to be an existential risk? It seems to 1st order climate change would just move population densities, to 2nd order there might be net less or net more land and ag resources after the climate change, and either 2nd or 3rd order, the rate of hurricanes and other weather storms is changed.
It doesn't seem to me that something which reduces human population from 6 billion to 2 billion should be considered an existential threat. A threat, yes, an expense we would prefer not to tolerate, perhaps. But a game ender? Not the way I play.
Article on space.com