Solar shading is on the order of $1 trillion per decade, assuming SuperHeavy+Starship and full reusability as I recall ($10,000 per ton). As a bonus, the capability to launch 10s of millions of tons to orbit fundamentally changes the capability and reach of humanity.
Sucking gigatons of CO2 from the air at the current efficiency is basically infinite dollars, but it could conceivably be cost competitive with the solar shade in a few decades. It basically just requires massive amounts of fully renewable energy, so the benefit is it’s a massive renewable energy generation subsidy.
But renewables are already more than cost competitive, the problem we have now is energy storage (batteries).
I mean, maybe a solar shade is possible, but it still requires a massive engineering investment and maybe has some accidental risks like “ends photosynthesis” (which is hyperbole, but still, it’s a big one shot project with global consequences).
DAC has limitations that likely keep it from being the sole solution to CO2 levels, but it can be developed incrementally without much risk and then deployed on the margins where it makes sense (e.g., on top of geothermal, in the Sahara next to a PV array).
The L1 Lagrange point is not perfectly stable. Anything we put there would drift after about a decade. So it requires constant upkeep but also means nothing you do there can be catastrophic.
Both technologies are not “one-shot” but rather are things which are deployed along a spectrum measured in “gigatons of CO2 extracted” or “tons of diffractive material deployed”.
You don’t put one big mirror in place as a binary thing. You deploy thousands to millions of tons of “mirrors” overall, ~100 tons at a time. Imagine a payload of carbon fiber snowflakes which get dropped off, coated in such a way that they self orient broad-side toward the sun.
I wonder what the effective carbon offset of 100 tons of diffractive material at L1 buys you. Given that you could draw a direct cost/benefit comparison.
Sucking gigatons of CO2 from the air at the current efficiency is basically infinite dollars, but it could conceivably be cost competitive with the solar shade in a few decades. It basically just requires massive amounts of fully renewable energy, so the benefit is it’s a massive renewable energy generation subsidy.
But renewables are already more than cost competitive, the problem we have now is energy storage (batteries).