Human population dynamics| Human populations| AP Environmental Science| Khan Academy

What we have here is a really interesting visual that shows world population growth from 1750 all the way to 2100. Obviously, this isn't 2100 yet, so it's doing some projecting for roughly the next 80 years. It also shows the absolute world population over that time period.

So just to make sure we're understanding this graphic, our horizontal axis here, we can see the years going by. You can view that as our time axis, starting with 1750 right over here, going all the way to 2100. Now we are here in, when I'm making this video, is 2021, but this graphic was made in 2015. So everything beyond 2015 is considered to be a projection.

Now, on this left-hand vertical axis, this is really the rate of world population growth—the growth rate that it has here. I could write that down just to make it more clear: this is the growth rate axis. And we have a right vertical axis here because we're really showing two different things. We're showing the annual growth rate of world population in red and then we're showing the absolute world population in this blue curve right over here.

So we could view this axis as world population. Now there's a bunch of really interesting things here. One, you might just wonder what was the world population around 1750. If you look at this, and these are obviously estimates because you did not have a global census back in 1750, it was around 700 or 800 million people. Now, what's interesting is that's not a lot larger than the number of people that we had at, say, the time of Jesus, which is also the time of the Roman Empire and Han China, where it's estimated that around that time period, around 2000 years ago, the world population was around 300 million people.

But then we see something interesting happening over the next several centuries. From 1750, it seems to start growing pretty dramatically, and we could see why that is. We can see the growth rate of population for a while; if we look at this red curve, the growth rate of world population was around 0.6 percent. But it looks like in the early 20th century, right around here, the rate of growth of world population starts to really increase, where at least based on this, it looks like around the 1970s, the world population growth rate peaked out at 2.1 percent.

Now, the question might be: why was this happening? Why did the rate of growth increase so much? Well, the main argument is as we became a more industrial society, health care would have improved, so child mortality would have gone down. People would have died of fewer diseases. At the same time, we would have gotten more efficient with agriculture. We would have had modern farming methods, so more people, so food would get cheaper; it would be more abundant, and more people would have access to nutrition. All of those would drive the growth rate up.

Now, an interesting question that folks have been thinking about for many hundreds of years is: is there a limit to how much human population the Earth can sustain? So they've introduced this idea known as carrying capacity. Why do we use the letter K for capacity when capacity starts with a C? Because a version of that word in German starts with a K.

There's always been this notion that for a given species, there's got to be some maximum capacity that an ecosystem can sustain. There are folks like Thomas Malthus who theorized that there must be some limit to how many people there could be on the planet, just based on how much land there is, how much nutrition, or how much resources there are.

But the carrying capacity for humanity is really an open question because our technology is constantly on the move. We constantly are getting better at using our resources more efficiently, going into new ecosystems. Now, it's a huge debate: how sustainable is that? As we become more efficient, are we also trashing certain parts of our ecosystem? But the carrying capacity of humans—we really don't know how many people the planet Earth could sustain.

But we could see that the rate of population growth starts to decline after 1970. The main argument for this—and we've seen this, and we covered this in other videos—when we study countries that are still developing and countries that would be considered industrial or even post-industrial, is that as countries develop and become wealthier, you might have more women entering into the workforce. You might have more family planning; women have more control over their destiny, get education longer, and people just have fewer children.

As people have fewer children, you could see that especially in industrial and post-industrial countries, the growth rate starts to come down. The aggregate growth rate is coming down, arguably because more and more countries are becoming wealthier, have better health care, and better rights for women. We even see today that when you look at countries that are developing, their growth rate is around 1.2 percent, while more economically developed nations are around 0.2.

So there's definitely that correlation between the two. Of course, this growth rate that you see in red is going to affect the absolute population. So it's no coincidence that at the same time that the growth rate started to go up like that, we see that the population in absolute terms starts to accelerate.

Just over, let's say, the last hundred years, we've gone from 1.65 billion, or I guess if we go back 100 years to 1920, it looks like we're approaching 2 billion folks. Now we're sitting at roughly seven and a half billion folks. So the world population roughly doubled over the, let's say, 2000 years prior to this chart, but then over this chart, over just the last 270 years, our world population has grown pretty much by a factor of 10.

Now, we don't know how sustainable that would be if our growth rate were to continue, but it does look like that growth rate is moderating. At least the projections in this chart have starting to approach 11 billion, maybe over time 12 billion, and maybe stabilize someplace around there. But it's an interesting thing to think about.