It's clearly the current lifestyle. Africans are destroying the world much less than the industrialized world because they're too poor to live the climate-wrecking lifestyle of the West.

A key issue though is that it takes a while for lifestyles to change. The higher the population, the quicker the switch needs to be done to avoid catastrophic consequences.

If the Earth's population were 100 million, it might be fine to take a century to switch away from fossil fuels. But at nearly 10 billion, if it takes a century, the results will be catastrophic.

In the end, that's more or less the same thing. But the question is, do we need more people? It's also easier to be sustainable if we require less.

I mean, yes but also no. There's just way too many people, period. Merely 60 years ago the human population was sitting around 3 billion people. Now it's 8. Earth's resources are finite, and at this rate of growth I would not be surprised if we ran out of non-renewables (with no renewable alternatives that scale as well as non-renewables) in our lifetime or our children's.