A common complaint we humans tend to have is that there are too many of us. Too many of us on the road, too large of a crowd to feel comfortable in class, too many students to fit in the dorms we have (true!), etc. In fact, if you ask an older person, perhaps a grandparent, if they notice there are more people today than when they were your age, they will almost invariably respond that there is -and that's 100% true. Since homo sapiens (humans) popped up on Earth between 4 and 2.5 million years ago we have been reproducing, and reproducing, and reproducing. In fact, doomsday Calvinist, the Rev. Thomas Malthus (remember the "people fornicate so much because of their sinful nature, and that leads to more people, and soon there won't be enough food" -- that's Thomas Malthus) was right about one thing, apparently; human populations most certainly grow exponentially (or, as Malthus put it, "geometrically"):
Notice, human population grows gradually until we hit around 1700AD. That's pretty much exactly when the Industrial Revolution hit and we started developing advanced technologies, medical care that was based in science instead of religion or superstition, and ideas like "safety" (I exagerrate a bit on that last one...). Since 1750 or so, population has grown at an incredible rate, which this next graph (which is really just a zoomed-in version of the one above) shows:
Yes, that's right, we doubled human populations in just less than 200 years from 1750 to around 1920, but then doubled it again from 1920 to 1970, and then added another 3 billion people from 1970 to 2000! This curve is pretty darn close to a perfect "exponential growth" curve, and is typical of unregulated populations (that is, populations that have no check to their growth).
The above graphs are why people get freaked out about population growth. It's happened fast! And it's not slowing down right away! But, it looks like it will slow down eventually, depending on your view of what we, as a global society of human beings, decide to do in terms of reproduction. Here are the projections:
Notice that the Y-axis is in billions, and that by the line at 2050, which is sort of the "pretty high level of confidence in these projections" date -- beyond 2050 the accuracy might be suspect, we will have between 7.4 billion and 10.6 billion people on Earth. Of course, the number depends on how we manage our fertility as a species, and if we go on the "business as usual" track, basically keeping levels of access to birth control, economic development, medical care, etc. at the same rates they are now we will probably stabilize our population at between 9 and 10 billion people. That stabilization trend is the "medium" projection on the above graph. However, if we encourage things like birth control, expand equal rights for women throughout parts of the world where those rights are withheld, and improve medical care, projections could be that we actually have a pretty serious decline in population starting around 2050 -- that's the "low" growth scenario, and could be caused by a conscious decision as a global society, or something more nefarious... like a lack of food or global disease (zombie apocolypse or whatever).
The alarming line, that sort of looks like population growth to date (i.e., population growth from 1750 to today), is if we "roll back" access to birth control and economic and political equality for women. Essentially, this "high" growth scenario would be like stepping back in time to the mid-1800s... and woudl result in unchecked population growth (unless that apocalypse kicks in, of course).
So which of these projections is more realistic? Most of the smart money is bet on the "medium growth" scenario, so population will likely stabilize around 9 billion people, with small fluctuations after 2050. However, there is a distinct possibility that the "low growth" scenario is possible, and in some countries (most of Western Europe, China, Japan) the low growth scenario is already a reality! Actually, the global trends are more toward things that lead to the low growth of future populations, chief among them are lower infant mortality (which means people have fewer children because more make it to adulthood) and increased rights and economic opportunity for women. In fact, globally, fertility has declined quite a bit:
The above graph shows a change in the "total fertility rate" (which is just the average number of children per woman of childbearing age) between 1950 and 2003. Basically, if a given country or world region has a total fertility rate of 2.1 then population will be completely stable. Why 2.1 children per woman of childbearing age? Well, unfortunately, even though medical care is excellent today in most places around the world, some babies do not survive to adulthood. This is true no matter where you live. Anything above 2.1 children per woman of childbearing age will indicate a region or country which has a growing population.
What is clear from the above bar graph is that every single region around the world has seen a significant drop in the average number of children a woman has. Some regions, like Africa, still have a lot of potential for that number to decline, but other regions, like Asia and Latin America have seen a HUGE decrease in children being born. Of course, context is everything. In Asia, if China had not enacted a one-child policy then fertility rates would be in the 4-5 children per woman range. In Latin America, economic change, enhanced medical care, and increases in education have really contributed to the total fertility rate decline.
Check out North America and Europe. Both of these regions have a problem, and it is due in large part to low total fertility rates! Here in the US we have a total fertility rate of 1.89 children per women, and in Europe the rate is really around 1.4 children per woman. Without immigration, which brings in new individuals, countries with fertility rates which are that low will shrink, and populations will become older (because there are fewer babies born, but still the same number of older adults... for a time). Europe is really feeling the effects of this, economically, and the United States is about to feel these effects.
Given that since 1950 (the darker bars in the above bar graph) total fertility rate has dropped quite a bit, as economic growth, increased opportunities for women, and other factors continue to improve in Africa and Asia (in particular), but elsewhere as well, it is quite likely that population growth will slow quite a bit.
Perhaps the neo-Malthusians (the folks who say "overpopulation is a problem") are missing something? Why do we get concerned about human population growth?