[Question] If Starship works, how much would it cost to create a system of rotable space mirrors that reduces temperatures on earth by 1° C?

There are many pro­posed geo­eng­ineer­ing solu­tions that could re­duce tem­per­a­tures on earth. Un­for­tu­nately, a lot of them have a mix of side effects and lock-in effects where the changes in tem­per­a­ture come years af­ter de­ploy­ment which cre­ates risk of un­in­tended con­se­quences.

If we would have a con­stel­la­tion of space mir­rors that can be ro­tated as we de­sire to let in less or more sun­light, how much would it cost to bring up enough to re­duce tem­per­a­tures on earth by an av­er­age of 1° C? Let’s say that Elon’s promise of be­ing able to lunch a Star­ship that brings up 100,000kg for 1,000,000\$ works out, what would it cost to pro­duce and de­ploy those mir­rors?

• Fermi estimate

Cross Sec area of earth= 1.3e15

Pro­por­tion needed to cover for 1C temp, 1.3%

Area needed=1.7e13

As­sume alu­minium foil is used.

As­sume that it needs to have 500nm thick­ness to block light.

As­sume most of the mass is ul­tra­thin foil.

So 8.5e6 cu­bic me­ters of foil

At 2700 kg/​m^3 thats 2.3e10 kg

Mak­ing 2.3e11\$ at that price tag.

Ie 230 Billion \$.

Plus an­other 41 billion \$ for alu­minium at 1.8\$/​kg cur­rent prices.

• There would also al­most cer­tainly be some on-go­ing costs. The La­grange points aren’t fully sta­ble, so even­tu­ally we’d need to ship up pro­pel­lant for sta­tion keep­ing. [EDIT]: We could likely use so­lar ra­di­a­tion for sta­tion keep­ing… Prob­a­bly on much longer timescales mir­rors would need to be re­placed. Th­ese costs would prob­a­bly be less than 1% of the up­front in­vest­ment, but if the sys­tem grad­u­ally falls apart, you then find your­self in a bad situ­a­tion.

• For con­text, the US congress passed the CARES act quite quickly (rel­a­tive to nor­mal timelines for pass­ing leg­is­la­ture) ear­lier this year and that dropped \$2.2 trillion into the econ­omy.

• Seems like this ques­tion re­lies on a huge num­ber of tech­ni­cal ques­tions and as­sump­tions such that a back of the en­velope es­ti­mate would be mean­ingless and a rigor­ous ex­am­i­na­tion would be highly difficult, nigh im­pos­si­ble. Nat­u­ral albedo fluc­tu­ates wildly on a global scale from year to year and there are so many con­found­ing fac­tors and feed­back sys­tems in global cli­mate that it seems in­sane to even es­ti­mate how much ar­tifi­cial mir­ror sur­face is needed, let alone how much it would cost not just to launch all that ma­te­rial, but to co­or­di­nate or­bital pat­terns and con­trol sys­tems for it.

• I think it’s use­ful to dis­t­in­guish knowl­edge of truth from gears-level un­der­stand­ing, these two differ­ent things can oc­cur in any com­bi­na­tion. Your point is that at­tain­ing spe­cific un­der­stand­ing of a plan that’s good enough to make the es­ti­mate in ques­tion is a hope­less en­deavor, and you list par­tic­u­lar is­sues with get­ting such a plan fleshed out.

But it’s also pos­si­ble to know truths about the world with­out un­der­stand­ing why they are true or how they came to be known (origi­nally). The main ex­am­ple of this is seek­ing ex­pert con­sen­sus in an area you don’t un­der­stand: by find­ing out what the con­sen­sus is, you get a rea­son­able cre­dence in what the truth of the mat­ter is, with­out nec­es­sar­ily un­der­stand­ing why it’s this way, or how speci­fi­cally any­one came to know it’s this way.

This post asks for a Fermi es­ti­mate, which is an­other way in which a very vague model can yield truths about the world. Even if a de­tailed model is unattain­able, such truths might be in reach.

(It’s of­ten a lost pur­pose to seek truths about the world in­stead of seek­ing un­der­stand­ing, so it’s nat­u­ral to scorn some forms of pur­suit of truths. I have a lot of sym­pa­thy for this po­si­tion. That doesn’t make such forms of pur­suit of truths un­work­able, just not rele­vant to im­prov­ing un­der­stand­ing of what’s go­ing on.)