I'm an only child and would much rather have had siblings so will (hopefully) have at least two. I do at the same time care about the environment and climate change. (I also think many people with siblings would have preferred being only children: but you can't really predict these things ahead of time)
DH and I have arranged our lives in a way that will somewhat reduce our footprint - smaller old house in a central location so we can cycle/walk everywhere, looking to incorporate solar power and massive insulation into a renovation project next year, vegetarian household and likely to raise future children vegetarian, use cloth nappies, etc.
But to be honest, I actually think reducing global inequality and helping people in other countries out of poverty would have more of an impact at population level than having one fewer child and carrying on as normal. When countries reach a certain level of development, the number of children they have plummets. It's happened all over the world, regardless of religion or previous culture. If a family of nine has their situation improved to such an extent those children each decide to have no more than two children, then realistically, an improvement in the wellbeing of that family would have more impact than a family here having one fewer child- especially given large families are still the norm in lots of countries that are catching us up in terms of carbon.
It irks me when people who worry about overpopulation think the answer is wealthy people having one fewer child. Really, it's all wealthy people radically redesigning the way we love our lives, and focusing our attention on reducing global poverty. Continuing as we are, even with slightly fewer people, isn't enough.