I am trying to find the title and author of an extremely pulpy science fiction yarn. Unfortunately I do not know how the cover was - the version I had was secondhand, and in the vicissitudes of its life the cover was lost by the time the book came to me. It was definitely old - I suspect from either the 1950s, or possibly the 1960s.
The story is set in the future, when the Earth is under attack from another planet. The protagonist is an airman, I think from the Second World War (or shortly afterward). His plane crashed and we was frozen in a glacier (rather like Buck Rogers) until he was found and thawed out. In the process he was also “upgraded”; in particular I remember that his muscle strength was enhanced, and his blood was replaced with amniotic fluid with a higher oxygen carrying factor, that gave him the capacity to hold his breath for long times. This would be a plot point later.
Because of these enhancements he is placed with a commando team who will travel in a spaceship to the planet of the attackers, carrying an atom bomb on board of sufficient force to destroy the planet. When they get there they find that the planet supports two intelligent races. One dwelling on the land is the one attacking the Earth. Rapidly increasing desertification means that the land area will soon not support life, and so they are planning to move to the Earth after it has been conquered. The other race lives in the oceans. As an interim measure, the land dwellers are moving into the oceans, which are less affected by the global warming, by building underwater cities. This encroachment is opposed by the marine race, leading to a vigorous war between them.
I don’t recall too much of the protagonist’s adventures. They are captured on arrival and taken to one of the underwater cities. The protagonist escapes, and due to his ability to hold his breath, is able to make contact with the marine race. Thank to his assistance, I believe (but I’m not sure) that the marine race win a comprehensive victory against the other, eliminating the threat to Earth.
I remember the ending better. The Earthmen work out that if the atom bomb is placed in a suitable canyon, then instead of blowing the planet apart, its detonation will shift the planet into a new orbit, further from the sun, and so cure the global warming problem. I remember feeling rather skeptical about this, even at the time. But I don’t know if the scheme worked or not, because the last few pages were missing.
Extra detail: After thinking hard about this, I remember that after his capture the protagonist was interrogated by a sexy scantily-clad alien princess. To find out what planet they came from, she used a telescope, whose primary mirror was formed by a rotating bath of mercury (and so had a perfect parabolic shape). This is an inconsequential but slightly unusual touch, so maybe it can jog someone's memory.