It means that they estimate the population in 2150 will be 12 billion.
Is this sentence correct:
1-They estimate global population out to 2150 with 12 billion people on the planet.
I suppose 'on the planet' is redundant, but what does 'out to' mean here? Is 'out to 2150 with 12 billion people' correct?
I think they have used "to estimate out" as a verb. It is strangely worded and, for me, "out" is redundant and verging on incorrect. The verb is simply "to estimate". I find "They have estimated ... out to ..." very unnatural.
They estimate that the global population in 2150 will be 12 billion.
I don't believe it is "estimate out" as a phrasal verb. They have estimated the population for years in the future. They have done this (from the present, presumably) out to the year 2150.
If they had said "projected" I think the "out" would make more sense.
In any event, simply re-stating it as Mike suggests is the best.