As part of our discussion of transition science, we will talk about five different transitions that human societies have witnessed over the last 200 years. Both the best-known transitions that we are undergoing in many countries and that we have undergone in other countries already, is the demographic transition. The origins of the idea of demographic transition can be traced to the works of Robert Malthus. Malthus was a political economist who lived in the late 18th, early 19th century, and he's famous for having suggested that human population grows in a geometric progression, whereas food production grows in an arithmetic progression. What that means is that food production grows much more slowly compared to human population, according to Malthus. As a result of this difference between the slow growth of food production and the rapid growth of population, Malthus believed that human beings, human societies would need to control their population, without which there would be massive hunger and famine that would occur in many different places. Both technological and institutional changes over the last 50-60 years have shown that Malthus's predictions were not right. Instead of having population grow in a geometric fashion and improvements in food production happen much more slowly, food production has kept pace with changes in population and has exceeded it. So what is it that accounts for the slow growth of population in contrast to what Malthus predicted and what many people came to believe? This change in population growth rate is the result of, here's what we called the demographic transition. Very simply put, the demographic transition is the result of relationship between birth rates and death rates of human beings. According to the theory of demographic transition, when the numbers of people are small, when food availability is low per person, and when children provide substantial economic benefits to their parents, families tend to be large. Households tend to have many children. Birth rates are high. When you don't have sufficient health coverage, death rates are also high. Birth rates and death rates both being high, maintain the population in equilibrium, and this is the stage one in the graphic that you see. In stage two, with improved availability of healthcare, with improved sanitation and hygiene, death rates begin to fall, and they can fall very rapidly even as birthrates remain high. This is when you see a rapid increase in population happening. This is the stage two in the graphic that you see. In the third stage, death rates begin to fall more slowly. They still continue to fall, but they fall more slowly and birth rates also begin to fall. This can happen because of a number of reasons. As healthcare continues to improve, death rates decline and families have to bring up their children can often find that the cost of bringing up children is relatively high and they may begin to reduce the number of children they have. Also in the first two stages, families have large numbers of children because some children die and they don't know how many children will survive. With the goal of maintaining at least some offspring, they may have more children than they want. But in the third stage, as healthcare improves, death rates become low and fewer children die. Families also begin reducing the number of children they have. In the fourth stage of the demographic transition, both birth rates and death rates have declined substantially. This is what you see in many western European countries or in Japan, where the numbers of children is small per family, death rates are very low. But birth rates are so low that they're below replacement of the people who are dying. In this stage of the demographic transition, population can even decline. The only way, if you're in a country in this stage is to increase your population through immigration, or by supporting or by encouraging families and women to have more children during this stage. The final stage of the demographic transition, birth rates and death rates are more or less in equilibrium. You're beginning to see this in some countries in the world. At this point, demographic transition is happening in a lot of different countries, and nearly every country birth rates are far lower that what they were in the 1950s. It has already occurred in most of the Western European countries. As a result of this change, and this changing the equilibrium between birth and death rates, the structure of the population, the age structure of the population also changes. In the first stage, the largest number of people in the population belongs to the young age cohort. It may be less than 10-15 years old. That's the largest proportion of the population. As the demographic transitions proceeds a pace, the number of people in different age cohorts becomes much more similar, and instead of having a triangular or pyramidal shape to the age structure of the population, you get a much more evenly spread out number of people, proportion of people in different age groups. The occurrence of demographic transition of this related to a whole range of different forces. It is related to improvements in healthcare, reduction in death rate because of better sanitation and better hygiene, improved availability of food and choices, behavioral choices made by people to have fewer children. So the occurrence of demographic transition as a result of technological changes related to health and care of people when they're sick. It is related to institutional changes as governments may seek to support reduced birth rates or to support improved sanitation additives related to the choices that individuals make, behavioral changes. The second transition that is important to talk about is the energy transition. The energy transition can take place both at the national level and at the household level. What does the energy transition signify? It signifies a switch by countries or by households from one source of energy to another. So the first energy transition was one where we move from using different kinds of biomass, whether it is plants, or wood, or field waste from fields for energy to using coal for energy. The shift from biomass to coal was the first energy transition. But since then, we've had a number of different energy transitions as well. We have changed from using coal to using another fossil fuel, oil and gas. Now we're in the midst of the next energy transition, where sources of renewable energy are increasing rapidly their share of the total energy used by human beings. We're still in the very early stages of renewable energy transition, but it is happening, and on this transition being completed, hinge or hopes for reducing emissions that are leading to climate change. What accounts for the changes that we see today around the renewable energy transition? Perhaps the most important is the availability of renewable energy at a competitive cost. This has happened as a result of technological shifts that allow solar energy and wind energy to compete with fossil fuels in terms of cost. The second set of changes that are also happening is the willingness of users to adopt new forms of energy, to use energy from new sources. The third important element that supports a renewable energy transition, is government policies that favor improvements in production of renewable energy. Different countries have adopted these policies to [inaudible] extents. But for the energy transition to be complete, for it to happen faster than it is happening now, changes in government policies are essential. Once again, we see that transition when it comes to energy are possible because of changes in what users do, what governments do in terms of policies, and what ecological change enables us to do.