Let's be honest, renewables are already geoengineering (changing water flow, air flow, albedo, etc.), just done in an uncontrolled fashion. Nuclear energy or renewables do not solve the long term problem unless coupled with large scale geoengineering. Granted all of the above are vast improvements over fossil fuels.
Thermodynamics is a bitch. If you make a nuclear reactor, you make heat. You add additional heat to the system, either at the source (energy production isn't 100% efficient), or at the point of consumption (the waste product of using energy is always heat). So, if you switch everything to nuclear, you're still adding heat to the system that wasn't there before (in addition to whatever the sun is blasting us with). If energy use goes up, and it always does, it just means we add more heat faster.
Literally the only way we can have our cake and eat it too is geoengineering. Solar shields in the earth-sun Lagrange point are my preference and least disruptive to other natural processes.
If we reach a point where such enormous space installations are possible with multi national budgets and technological progeess, we still have to live with the largest mass extinction, Destroyed soils, disequilibrated ecosystems.
Then what?
Life will be possible. But not as worthwhile as it was.
Whether life is "worthwhile" is a subjective and personal decision. Different people will have different considerations of what makes life "worthwhile."
I think not having meteorological anomalies on a yearly basis, growing crops in a climate where humanity evolved, and having no dead zones on the planet is on a little bit different step of the hierarchy of needs than what people have different consideration on.
There are people who live in places where there are yearly meteorological "anomalies" on a yearly basis (hurricanes, heat waves, etc), and most people live in places that don't have the same climate as where we evolved. People wouldn't live in "dead zones", by definition. There's already areas of Earth where people don't live because their climates are too extreme and that doesn't make me sad.
I'm not saying climate change is fine and not to be worried about, I just don't see how future generations would consider life less worthwhile because of it.
Use the renewable and nuclear energy to remove the IR shield in the atmosphere (store atmospheric carbon in the ground), rather than put a shield in space. A space shield doesn't address CO2 levels in the atmosphere or oceans.
It's just a question of scale and thermodynamics. Using the renewables to do carbon capture is probably a good idea, because anything is better than the giant greenhouse gas. But that really is geoengineering too. And it'll only work for a period. As energy use increases, you will modify the planet more and more simply due to collecting and distributing the energy. Energy must flow from concentrated forms to dispersed forms. That dispersed form is usually heat.
Reduce emissions. That's cheaper, more effective and safer than any other method.
Geo engineering commonly only tries to fix temperature. While that would be a big achievement, it does not change the CO2 ppm. And that translates to ocean acidification. Which translates to mass extinctions. Which is still an existential threat also for land living species, and us.
There is only one solution to fix both (and many other, related / caused problems): Fix the source.
We cannot engineer our way out of all the individual symptoms. Just leave fossil fuels in the ground.
There are ways to capture carbon in seawater, kelp is the fastest growing I know of.
Okay, that type of geo engineering is a good counter argument against my previous comment. It comes with other issues and does not solve all issues, but still, good point.
IQ tests are designed relative to the population so the median is always (or should always be) 100. The point is that is measures people against one another in the present. If we all got 10% smarter, our individual IQ scores would stay the same.