ok Dave...3 scenarios.

scenario 1- we build a huge pair of sunglasses for the planet to block the normal uv and solar rays...or "air scrubbers that clean the air"....
at GREAT cost of materials ...time (we don't have) ...labour and monumental environmental damage and waste...
and then what?
Naturally we continue to turn this into an inhospitable place by exceeding a new higher and even more devastating threshold.
We aren't about to stop doing what we do in this scenario...
It just buys us a bit more time to do yet worse damage.

Scenario 2-
Instead of the Macro project.
Micro projects, nano tech could reverse a great deal of the damage done here...
We would have to release GI and AI and SI created nano tech that has been designed not only quickly but has been foreseen and designed to be error proof . The nano tech needs to be designed by intelligence greater than ours to take in all the needed factors for stabilizing the planet properly ...we can't do it alone...it would be too full of potential disaster and the time would take to create it would be too long...
also the speed of the nano tech to do their job and their numbers must be very high to do it effectively to reverse the damage current and reverse said damage...
All of that relies on having enough time...enough time to come up with a (preferably SI ) intelligence that can perform the design and implementation and maintenance ...time we may not have.
But it is the cheaper alternative and more realistic to alter and reverse the dangerous course our planet is on.
Of course as long as we keep thinking that an invention, innovation, or some other "powers that be" will save us...
It will still be business as usual and we continue of dangerous ways ...
So the nanotech would be a buffer that would be perpetually strained due to our behaviour ...and as such there would be risks of collapses in the ability to do its jobs.

Scenario 3 - We get our collective crap together. Stop the massive waste of warring, and military budgets, stop bickering and fighting, stop starving and impoverishing one another, stop killing one another, stop polluting and living wasteful lives, stop the wholesale plunder of limited natural resources we will need for the future, stop the rampant cancerous consumerism that everyone is addicted to.
Get along, use our combined knowledge and sciences and cure this place, fix it up, change the concept of what a normal lifestyle is and what one should be aspiring to, and we could do it, we still have time.
We can do it, we have enough of the important infrastructure built on this planet, and enough people to do it.
Conversely we have enough people, plenty, but they don't get along, and that is the first thing that has to stop.
Those that don't want to fix this place , have to go, for the sake of the future of our species, and very very likely, for the sake of almost all species here as well.
So that minority or majority ..(however the numbers work out)..that impedes, must go.
It is a harsh scenario, but if such folks as don't wish to live in peace with one another and get along and work together, then they are impeding.
and that could result in no one here at all to bicker and fight and argue and kill and blow up and exterminate the other...

And the longer we postpone effective solutions, the cost of the bill mounts higher.
The price to be paid escalates.
it isn't a static bill.
If we tighten our belts, help one another, cease hostilities, suck up the price now, it means we avoid a more terrible price in the future, or the final price if we do nothing.