The untold story of the world’s biggest nuclear bomb
  However, a study from 1963 suggested that, if detonated 28 miles
  (45 kilometers) above the surface of the Earth, a 10,000-megaton
  weapon could set fires over an area 500 miles (800 kilometers)
  in diameter. Which is to say, an area about the size of France.

If thousands of such gigaton bombs were developed then MAD (Mutually Assured Destruction) would be more taxpayer efficient because we could simply stop funding expensive rocket and submarine weapons delivery systems, as they would only add an unnecessarily complicated step to the process. Just detonate your gigaton arsenal on your own nation's soil. When an animal is in great pain and cannot be saved, we usually consider the most compassionate course of action to euthanize. So in the event of MAD, just instantly vaporize your own population to spare them a few months of clinging to life with radiation sickness, starvation, cannibalism, and the lawless raping and murdering of the earth's final warlords. The multiple gigaton bombs detonated on your own soil will ensure the agonizing death of all your enemies that don't similarly self-euthanize.