Mars has excessive cold periods. There is more than water to consider.

And giant dust storms. Which I kind of hope keeps us from sending humans and wrecking the place. The one good thing about us finally killing ourselves with pollution and war is that no matter what, the earth will still be here, just doing it's thing, until the next dominant thing arises, or becoming another mars because we truly did kill it.

@Hathacat thats what i used to think until I realised it’s very unlikely the human species will ever totally die out, even if the support capacity of the planet becomes very low. Have you read Olaf Stapleton’s Of Last and First Men? Interesting piece of sci fi about this, although now a bit outdated.