Nobody has a plan to sharply reduce human population in the short term, so the following comments are purely hypothetical — a thought experiment.
Question: If the choice is between a world of 10 billion happy humans and a world with 5 billion happy humans, is it fair to say the former has a humanity that's twice as happy? This is a philosophical question with, perhaps, no easy answer. I would answer No, but perhaps many would say Yes. The Yes-sayers may stop reading after the next paragraph; my comments aren't addressed at them.
A world with fewer happy humans will have more happy squirrels, happy dolphins, happy birds, happy insects and happy fish but some will find this suggestion silly. However, many would agree that humans would be happier in a world where other creatures thrive. Why do some promote a 10-gigahuman world? In what sense is it better than a 5-gigahuman world? Or is it just about accepting the inevitable: There will be a high population so let's hope it's for the best.
Some say that with ten billion instead of five billion, we'll have twice as many geniuses like Mozart and twice as many like Archimedes. This fails the sniff test! There were less than a billion humans alive in Mozart's time, and perhaps just 100 million in Archimedes' time. Yet both these great geniuses are still spoken of in superlative terms.
The problems of overpopulation are not hypothetical:
We can see them now.. Precious resources are being depleted: groundwater, phosphates and petroleum are among the most obvious examples. About fifty species of life already go extinct
every week during this Great Man-made Extinction and this number is increasing. Even without extinctions, there are profound changes to the ecology: I've already mentioned the widespread replacement of fish with jellyfish.
I'm sure that the supporters of overpopulation have glib answers to these concerns. Fusion power will provide the huge energy needed to replenish phosphates and groundwater. Extinctions are not a concern: only
H. sapiens matters. And clever chefs will find ways to prepare varieties of jellyfish as delicious as fish. I find these answers overly glib. Groundwater depletion is
already a serious concern in many parts of the world; a huge portion of arable land is already dedicated to humans and their food; pollution of various sorts is
already a big problem.
And extinctions are irreversible.
Overpopulation is a stupid idea. It was a reasonable fear in the mid to late twentieth century, but it's long since been resolved.
There are no resource issues we cannot solve that would prevent us sustaining the ~10 billion humans that represent our likely peak population. Of course, we might not be smart enough to actually implement those solutions - look at the reluctance we have to completely replace the burning of fossil fuels with nuclear fission - but the problems are political and ideological, they're not resource, technology, or population driven.
Population is just people. "Overpopulation" is a fundamentally anti-human concept, and belongs in the same ideological dustbin as other anti-humanitarian ideas such as apartheid, slavery, and fascism.
You acknowledge that 30 billion would be too much; that leaves me confused about your strong support for 10 billion. Especially since you admit that there may be obstacles to the dramatic changes needed to sustain such a population. Are you especially fond of jellyfish as a food?