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I’m Jonathan Tomkin from the University of Illinois. This is the last lecture for this week, and we’re going to be summing up our overall vision of some of the important concepts behind sustainability by looking at how earlier thinkers worried about population growth, and we’ll see that their worries are very similar to the worries of today. This is a diagram of the world’s population. This does not appear to be a sustainable growth pattern; it looks near exponential. We’ll discuss why this is unsustainable, uh, in this lecture, but next week we’ll find out if it really is. Global population is 7 billion now. Most recent growth rates for the world population is about 1%, and if that were to continue, that would imply that the global population will double in about 70 years. Imagine 14 billion people by 2080. What would happen if this came to pass? It’s a tough question, but luckily for us, somebody has already been doing the hard thinking on this 200 years ago. Thomas Malthus thought that this kind of growth was not sustainable, and he wrote down his reasoning in 1798. Many of these concerns are very relevant for today. Let’s explore his reasoning. Firstly, he noted that any population growth that was exponential, a-and this means that it doubles in any given period of time, and so here on this slide I have a-a picture of the world population growth and an example of how an exponential growth might happen. For every additional unit of time, the population doubles, so population goes from one to two, it doubles to four, it doubles to eight, and it doubles to sixteen. That is an exponential pattern of growth. If we have a 1% growth rate, as we see in the world today, this implies that there is an annual doubling every 70 years. So you could imagine for this, uh, time amount that every 70 year step would double the amount of population on the planet. Secondly, he assumed that agricultural production was arithmetic in its growth, or geometric is another, that is it increases with time in a straight line, and so, as you can see from this data set, perhaps it starts out at two, and then we add two for every time step, so it goes to two, four, six, eight, ten. That’s how much food we can produce, um, per population, if you like. Now, take a minute to think about what would actually happen if we tried to compare these two curves. You can either imagine, um, by filling in the spaces on this graph, or you could actually pull out a-a notepad and sketch it for yourself. On the x-axis put the time — we’re doing steps one, two, three, four and so on — and on the y-axis we have the units of either population or the amount of food produced. So take a minute to either think about this or sketch it out before you continue.

Your two curves should look something like this. Population increases at a greater and greater rate — it’s exponential — while food production increases at a constant rate. If one unit of food is needed for one unit of population, what happens after the point where the two curves intersect? Again, I want you to take a minute and either note down, or just think, what would happen to the world population, or the population of the example, after this point of intersection. The system breaks down. Famine, war, or disease have to prevent the population from exceeding the food supply. We can’t have more people than there is food to supply the needs for.

Your two curves should look something like this. Population increases at a greater and greater rate — it’s exponential — while food production increases at a constant rate. If one unit of food is needed for one unit of population, what happens after the point where the two curves intersect? Again, I want you to take a minute and either note down, or just think, what would happen to the world population, or the population of the example, after this point of intersection. The system breaks down. Famine, war, or disease have to prevent the population from exceeding the food supply. We can’t have more people than there is food to supply the needs for.

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