That sounds like an interview estimation question.

Presume total extinction due to climate emergency.

GWP = annual gross world product.

r = estimate of risk that we kill ourselves through other means per year e.g. 1 in 100 due to nuclear war.

Presume zero growth (e.g. world growth is offset by population growth, environmental damage, discount rate).

So:

Cost = GWP / 2r

”Fossil fuels doesn’t change the climate, humans change the climate, we merely provide the means and are not liable for cleaning up this mess.”