It is clearly possible to operate at the level of zero carbon emissions. People did it for thousands of years before the industrial revolution and modern technology runs just fine on renewable or nuclear energy.
Since the emissions per person can be zero, any level of emissions reduction can be achieved with any number of people, you just need more emissions reductions per person when you have more people. "We should have both more emissions reductions and more people" is thereby an entirely consistent position.
More than that, emissions reductions generally require R&D and have some economic cost, but an aging population is economically weaker and has fewer resources to spare. Meanwhile older people who aren't going to live much longer and have no descendants have less incentive to care about what happens in the future and spend those scarce resources on things like emissions reductions. People having more kids avoids that death spiral.
Also, I mean, who are we trying to save the world for?
Since the emissions per person can be zero, any level of emissions reduction can be achieved with any number of people, you just need more emissions reductions per person when you have more people. "We should have both more emissions reductions and more people" is thereby an entirely consistent position.
More than that, emissions reductions generally require R&D and have some economic cost, but an aging population is economically weaker and has fewer resources to spare. Meanwhile older people who aren't going to live much longer and have no descendants have less incentive to care about what happens in the future and spend those scarce resources on things like emissions reductions. People having more kids avoids that death spiral.
Also, I mean, who are we trying to save the world for?