How did we get to 8 billion people?

To predict the future we must first understand the past. The road to 1 billion people took us nearly all of human history to reach, in around 1800. In the 2 centuries since, we’ve increased our numbers 8 fold, and the rate has been accelerating over most of that time. Indeed, when I made one of my first radio programmes 20 years ago, the front cover of one science magazine greeted the world’s 6 billionth person. This - as the Cambridge-educated economist Thomas Malthus pointed out would happen back in 1798 - is exponential growth. He realised that our ability to reproduce would go faster the more of us there are. Our ability to produce food, on the other hand, doesn’t. He was one of the first to sound the alarm concerning the ability of populations to outstrip the resources that sustain them. Here to explain more about how we arrived at today’s 8 billion, and the trajectory we’re still on, is Alice Reid, director of CAMPOP, the Cambridge Group for the History of Population and Social Structure.

Alice - Well, the answer to that is we don't know. It is an estimate. We do have censuses and we monitor births and deaths in most, but not all of the world. But we never measure everyone at the same time. So the UN just chose this date to mark 8 billion.

Chris - But we think that's vaguely how many of us there are.

Alice - Yes. Yeah, it's fairly accurate, I should say.

Chris - How fast is the population growing? I mentioned the history of exponential growth and the fact that it has accelerated, it has gone ferociously fast. But what do we think the current rate is as an average across the planet at the moment?

Alice - It's less than 1% a year. So you are right that population growth really picked up after about 1750, 1800. It was quickest in the 1960s when it was about 2.2% per year and it's actually slowed since then. And population growth is going to get slower and slower up until about 2100.

Chris - It's patchy though, isn't it? Because if one asks, what is the population of our country doing? We'll get a very different answer than if I ask, for instance, what's the population of Nigeria doing?

Alice - It's very variable. And many places have populations which are already falling. For example, Italy, many places in Eastern Europe, many places in East Asia, Japan, China and so on. But many parts of Africa still have quite rapidly growing populations.

Chris - Now, if we look back in history, what do you think drove the changes that we saw? Those rates change, very fast, and now in some cases, as you're saying, some countries have diminution in growth or even a shrinkage where other places are still growing quite quickly.

Alice - So most countries have either been through or are going through a process called demographic transition. And this is the change from quite high and variable mortality and fertility rates. So lots of births and lots of deaths, and then a change to low and variable, less variable mortality and fertility. And different countries have gone through this process at different times. And for example, Britain and most of Europe, North America went through this process in the 19th and early 20th century. But other places are still going through this process. Of course, individual countries are also affected by migration, which feeds into that balance of people too.

Chris - I did read a statistic somewhere, I must admit, I haven't fact checked it, but it seems plausible. And it said because of ferocious rates of growth, which you were pointing to in the 1960s and that sort of ilk, that 15% of the people who've ever lived are alive right now. Does that sound plausible to you?

Alice - No <laugh>, well those sorts of statistics have been analyzed and debunked and I can't remember exactly where 15% fits, but there's a great episode of more or less on that. So you can go and listen to that.

Chris - <laugh>. When do you think then, based on what we know and what facts, figures and stats we can rely on, when do you think we are going to reach peak person? When will the population top out at whatever number it's going to?

Alice - Current estimates suggest that it will probably peak at about 10 and a half billion, somewhere around that. Before the year 2100.

Chris - Right. And is that assuming just everywhere continues, continues to grow at the rate it is now? Are people factoring into these predictions climate change, migrations, the fact that some parts of the world will become much nicer to live in, but they're far fewer and outstripped by the number that will become worse places to live because of things like climate change?

Alice - Each country's estimates are done on a country level basis and take into account what's happening to mortality. And most of the assessments reckon that mortality will continue to decline slowly and that fertility will, depending on what's happening to fertility in that country, will either sort of decline if it's not already below replacement, which is below about two children per woman. So I don't think there are massive issues with mortality built into those to do with climate change. I think most of the assessments of population are that mortality will continue to decrease slowly. Of course that may not happen if there are problems with mortality events connected to climate change,

Chris - Where do we think the hot spots are for the next few decades?

Alice - Well, most of the population growth will occur in Africa. Population is declining already in China, although fertility has been low, below replacement levels in China, since about 1990. Population overall is finally starting to decline, but population growth is likely to carry on in Africa where fertility is still three or four, five children per woman.

Chris - Is there any reason why the population isn't higher than it is already?

Alice - Well, that's because of the demographic transition, because fertility does tend to start to fall at some point after mortality has fallen. So as populations' living standards get better with falling mortality, then women start to want to have fewer children, partly because more of their children are surviving into adulthood. Partly because some of those changes, um, which occur, give women more opportunities. Partly because they begin to see that they have control over their fertility and can reduce it. So if fertility hadn't declined and wasn't declining, then the population would be bigger than it is.

Chris - And we'll probably be hearing a bit more about that in a little while from Kathleen Mogelgaard. Alice Reid, thank you very much indeed for outlining the current population situation for us.