I agree with Rich that population is the world's biggest problem.

I disagree, because as long as we have energy, we can sustain high P on the
Earth, and benefit from the social diversity, cheap labor, and the profusion of
human minds. OTOH, if we run out of energy, life will be **** no matter how
low the P becomes (and it would fall like a rock in such a situation).

Ultimately, only the SPS will ensure sufficient energy for a modern society.
It is the only energy source without a Hubbert peak.

But even if P were the major problem in our world, space would not solve it.
It will *always* be easier to add people than to fly 'em to the moon.

But
terraforming the Sahara or the seabed is almost within the grasp of current
technology,

But in the Sahara, you have to live with some rather crappy national
governments (i.e. Qaddafi). And who would want to live in the cold, darkness,
overpressure and drowning hazard of the seabeds?