If you try a terraforming experiment on Mars and it fails, nobody's life/house/country/etc is ruined, which isn't true of geoengineering on Earth. I agree with the sentiment that there really is no Plan B to fixing climate change but that doesn't mean the goals of space colonization and fixing climate change are contrary.
Terraforming on Mars would take millennia, and over that long amount of time, settlement would presumably continue and the amount of colonists would grow. Thus, eventually flaws in the terraforming effort would eventually impact on local people.
Precious? The primary elements "wasted" in space exploration are aluminum, silicon, hydrogen, and oxygen. All of those things Earth is just lousy with. Even launching a rocket a day wouldn't put any sort of dent in the availability of any of those elements. A rocket a day also wouldn't meaningfully add to levels of harmful pollution. The Earth is really big. It has lots of pretty much everything.
Doing stuff in space is expensive in an economic sense but it's not really all that resource intensive. Most of the cost is paying people to design, test, fabricate, and operate the hardware.
Ok, but to put a rocket in space you need a surrounding economy on a specific technological level which is costly resourcewise. Could SpaceX or NASA happen in e.g. Congo, Nepal or Papua New Guinea alone?