Population Growth and the Future of Our Planet

As someone who teaches environmental science and who is supposed to be committed to all things sustainable, I know that I should be certain about not having any more children. Our planet is already over populated, resources are running low, and threats of global warming leave an uncertain future for our planet and its inhabitants. If we want the world population to stabilize (or hopefully even shrink a bit), people need to have fewer children. The "replacement level fertility rate" in the developed countries is 2.1. This means that if each woman has 2.1 children (1 to replace herself, 1 to replace her partner, and 0.1 to account for infant and childhood mortality), then our population will stay stable. If women have more than that, population will continue to grow.
So, I know that should not want any more children, but right now, as I hold my tiny 2 month old in my arms, I am just not ready to commit to that. I know I should be, but I'm not. I am leaning towards not have any more children (and my husband is really, really, really leaning towards no more children), but I just can't say that I am 100% sure that I never want to do this again. Never want to feel a baby kick me from the inside. Never want to see a teeny tiny new life that I created in the first moment that it enters the world. Never want to snuggle a nursing baby at my breast.
After I had Finn I knew that we were planning on having a second child. In some ways I didn't treasure his baby moments as much as I should have, but I was simply living in the moment and eager to watch him change and grow. I always knew that I would be going through this again with a second child. Since I have given birth to Phoebe, however, there have been many discussions about whether or not there will be any more children, and the answer seems to point to "not likely". This has made me treasure her newborn moments, but it has also made me very emotional. Its like I have a cloud hanging over my head and every moment with her is my last with a baby in this stage. It is certainly making me cherish this time, but I also feel like I can't miss a moment. Every time she reaches a new milestone, I am actually sad that she is growing up too fast and that these moments are passing much too quickly.
So, I have decided to try and stop thinking about this as being my final child. I want to leave it open ended, and that way, even if Phoebe is my last, at least I won't spend her baby-hood dwelling on. Or at least I will dwell on it less.
This week in class I am teaching about population growth and I will be the one explaining to my students that if world population continues to grow, it will likely be a bleak future for all of us on this planet. And I know that while I am in front of the class lecturing, I will secretly be feeling guilty that I already have 2 children and can't definitely commit to not having a third. Shouldn't I be an environmental crusader and be setting an example by only having one child? Or at least being done after 2? However, the number of people on the planet isn't exactly the whole story. How we live and the footprint that we leave is also a big part of the picture. I recently read an article in National Geographic entitled Population 7 Billion (definitely worth reading) that discusses this point exactly and inside I felt a teeny tiny sigh of relief. Hopefully we can teach our children how to live in closer harmony with our earth. How to use fewer resources, create less waste, live more simply and locally. Hopefully we will reach a point when we don't need to worry about population growth because we are no longer putting a strain on our planet. Maybe the numbers won't matter quite as much as they do today.
In the meantime, I am going to cherish my 2 children while secretly not completely ruling out an unlikely third. I am gong to teach my children to be good and gentle citizens of this planet who hopefully won't have as large of an impact as my generation has had. And I am going to continue to make lifestyle choices that will decrease my personal environmental impact (hopefully making up for some of the added impact of having children).
I hope I have not offended anyone with this post. The decision on how many children to have is a very individual one and I understand equally well why a woman would have zero children or why she would have 5. (That family on tv who has 20-some kids however, I completely don't get that.....). I think I just feel hypocritical being someone who lectures about sustainable populations while not necessarily being able to commit to not increasing the population... and these thoughts run through my mind a lot. It's sort of like if you were a marriage counselor who had been divorced a few times... I don't want my students to lose their faith in me and what I teach!