So, let's get it started with our first mini lecture. What I would like to do is to share an example with you as to how exactly we're going to be making projections or forecasts into the future so that we can establish the most important global trends. I'd like you to take a very careful look at the following chart. On the charts, what you have is the evolution of the size of the human population on Earth since the year 1950. All the way up until the year 2015, we have actual record of the data because we know how many people inhabit the planet. As of the end of 2015, as you know, a little bit more than seven billion people were on Earth. And then what we have is four different projections or forecasts into the future to the year 2100. In a moment I'm going to ask you to think about which of these forecasts is the most likely, the one with the highest probability, and which one do you think has the lowest probability of being true. But before we get there, let me explain how these forecasts were calculated. First of all, the data come from the United Nations and the United Nations has a population division which is located in New York where they have nearly 4,000 people crunching numbers about population in the world. And every year they update four different kinds of forecasts. As you can see, they are labeled constant, high, medium, and low. The constant forecast is based on two very simple assumptions. The first assumption is that if today, on average, people in the world have this many kids, they continue to have the same number of babies until the end of the 21st century. And the second assumption is that if today people on average live this long, people will continue to live the same number of years on average until the end of the 21st century. If you use those two assumptions and you calculate through a mathematical model how many people you're going to have on the planet over the next few decades, you end up with nearly 29 billion people by the year 2100. That's what happens if nothing changes. In other words, if the number of babies born to the average person in the world remains the same and people continue to live the exact number of years that on average they live today. That's why this forecast is called constant. Now, you can also think about how you can relax or change those two assumptions that I just told you. And the United Nations basis those forecasts on three different assumptions that they label high, medium, and low. As you can see, the low projection into the future gives us a population by the year 2100, is actually smaller than the one that we have today in the world. And then the medium and the high gives you other numbers somewhere in between. So, think about it. My first question to you is: out of these four projections, which one do you think has the lowest probability of being true? In other words, if today you had to make an important decision and you needed to know the future size of the human population, which one would you rule out? Which one would you eliminate from consideration because you believe that it's way too unrealistic? Think about it for a moment. The answer to this question is that the constant projection is by far the most unrealistic and therefore the lowest probability. Why? Because you will see later on in this class that people are always changing their behavior. It's very difficult to assume that behavior remains constant. Now here's the other question that I wanted to ask you: if now we're left with the high, the medium, and the low projections, which of the three do you think has the highest probability of being true? So, if you look at the chart, the high gives you a world with about 16 and a half billion people by the year 2100. While the medium gives you a population of about 11 billion by the year 2100. And the low gives you a population that is slightly below today's level of seven billion people by the year 2100. Think about it for a moment. Now, in this case, the answer is by construction that the medium projection is the one with the highest probability. You see, whenever an international organization such as United Nations or a forecasting service or a consulting company or a professor, like myself, gives you three different forecasts about the future and they are labeled as high, medium, and low, what that organization or that person is trying to convey to you is that he or she believes that the medium projection is the most likely. And then they give you two other projections that they label as high and low so that you have a sense as to how, by changing the assumptions, you arrive at different forecasts for the future. Now, in this entire discussion about the size of the human population on earth, I have been using the word billion. Now, I fully realize that this class may be taken by students from all over the world. And I wanted to bring to your attention that I'm going to be using the word billion in the American context. Here in the United States, one billion is the number one followed by nine zeros and one trillion is the number one followed by 12 zeros. However, in your part of the world the convention may be different. So, for example in Europe, one billion is a one followed by 12 zeros, whereas one trillion is a one followed by 18 zeros. So, just as a reminder, in this entire class, I will be using the word one billion and the word one trillion following the American convention
