The current issue of National Geographic includes a cover story about the human population reaching 7 billion this year. There is a neat video illustration too. (And a fun video for a World Party.) Population is a topic they are going to explore throughout the year.
While I haven't used my degree in a long time, I have a Bachelor of Science in Environmental Science. Even when I graduated from college in 1996, population was a huge issue. Many environmental issues are tied to population: use of resources is dependent on both the number of people using them and the amount those people use.
There are a lot of reasons we have only 2 kids. Some are practical (I only have 2 arms, managing the 2 we have is enough, we'd need a bigger car) but one big reason is that I just can't justify (for us) having more than replacement value. (Replacement value is 2.1 in most developed countries.) It goes against everything I learned. In the US, we use a much bigger percentage of resources than our share.
Even if the birthrate in every country could miraculously be reduced to 2.1 (some are actually lower), the population would still grow as today's children grew to adulthood. Really, the explanatory graphs in the article help, but the basic idea is that there is a larger group of kids than the current reproductive population, so it takes time for population growth to even out.
Why is population a big deal? Well, 7 billion people are using a finite number of resources. Some of us are using more than our share leaving many with much less than their share. If we all used resources at the low end of the scale, 7 billion might not be such a big number. If we all used resources at the top end of the scale, 7 billion (or even 6 or 5 or 4 billion) is too many to sustain. See, it isn't so much how many people are using the resources as how they use them. More people just stresses the situation.
So welcome to a world in which we will soon be 1 in 7 billion. Think about reading the National Geographic article. It's pretty interesting.