-To determine the necessity of reducing greenhouse gas emissions, we must study what would happen if we did not take action. We cannot simply forecast, or see what will happen in the 21st century. It is hard enough to forecast the weather. It would not make sense to forecast the climate system for the next century. So we need to have a futurological mindset, to explore possible futures because the decisive elements for the future are entirely or mostly out of our control. So we will be passive in the face of the future or at least it will not be our choice. Mostly, the strategies we implement to reduce greenhouse gas emissions need to be able to stand in any of these possible futures. To determine which major forces will define our emission levels in the 21st century, we use the Kaya identity which breaks down our emissions and states that the main influences on our emissions will be first, the demography, the number of people on the planet during the century, the economic growth, the speed at which these people will gain wealth, the energy content of said growth, if this growth will focus on immaterial goods and have low energy content or still on the production of material goods which use industrial processes and thus energy, and the energy system which will be used to produce this wealth. First, regarding demography, in the UN median scenario, the population continues growing throughout the 21st century or at least the first half until it reaches 9.5 billion or 10 billion people. Then, there are various scenarios. In the first one, the population starts decreasing after the South achieves its demographic transition. Other scenarios still find a trend of demographic growth, with some expecting up to 15 billion people. One important thing to note is that, depending on the scenario, the number of people can be multiplied by 2 between a scenario where we go down to 7 or 7.5 billion at the end of the century and others which go up to 15 billion, which is the supremum of these demographic scenarios. Then, the next issue is whether we will gain wealth fast or not so fast during the 21st century. A slight differential of about 1% in the growth rate, depending on whether wealth grows by 1 or 2% per year during the 21st century, could lead, at the end of the century to a factor of 3 between the wealth level produced with a 1% growth rate and the level produced with a 2% growth rate. The different scenarios created also lead to a factor of 5 for wealth creation. The more wealth we create, the more greenhouse gas we will probably produce. And this wealth creation is obviously tied to demography. With more people, it will be harder to create wealth for everyone even though we will at least need to produce agricultural wealth for the population to survive. Usually, in the scenarios where population is lower, economic growth is higher. For the 21st century, all the scenarios predict the continuation of the trend of a less energy-intensive production. In other words, how many kilowatt-hours, how much oil is necessary to produce one dollar of wealth? Since the 1960s, the energy intensity of the production decreases, if only because we have evolved from industrial to service economies, with more immaterial and less energy-intensive goods. Again, in all scenarios, the trend of less energy intensity continues but with a factor of 2 between the most optimistic scenarios and the least optimistic. However, the decrease in energy intensity does not go with a decrease in energy consumption because the increase in economic growth largely counterbalances it. So energy consumption will continue to grow throughout the century in every scenario. Lastly, to produce this energy, there are many different possible energy sources. Without any specific climate policies, renewable energy sources can emerge spontaneously and become competitive and available worldwide. However, in a world with weaker growth, coal could be massively used as it remains the most widespread energy on the surface of the Earth and the cheapest. Because of that, carbon intensity can either strongly decrease, not decrease or increase again if coal is being reimplemented instead of oil and gas. The conclusion is that, if nothing is done, the temperature will increase, as you can see here, to the right of the graph. And under the B1 scenario, in which the population becomes stable during the 21st century and is environmentally conscious independently of climate issues, it is possible to remain naturally under the two-degree mark. But even with this scenario, there is a confidence band regarding the results so we may go over this mark. And for all other scenarios, we will not remain naturally under the two-degree mark and we will have to take action. To put things in terms of greenhouse gas concentrations, we started with CO2 concentrations of 280 parts per million at the start of the industrial era. In 1992, for the Rio Declaration, the concentration was 370 parts per million. Today, we are around 400 ppm. To remain under the two-degree mark, we must not go over between 450 and 550 parts per million depending on the climate models. This will not happen spontaneously. It is probable that we will go well over and reach concentrations of 800 or 900 parts per million at the end of the century, which is not consistent at all with the goals set during the COP 21 summit in Paris. The use of futurology is necessary to try and understand that the climate strategies we need to implement must stand in a world in which things go quite well spontaneously and we should not overinvest from the start, as well as in a world in which things go very badly, with a very high, relatively poor population, which uses a lot of coal. Our climate strategy needs to be consistent with both scenarios, which we will not really choose, and to adapt to the evolution of the world. Finally, the emissions will not decrease on their own so we need to take action. At the current pace of our emissions, we have 20 to 30 years' worth of emissions left to remain under the two-degree mark. After 30 years at best, we would need non-existent emissions that would then become negative. We would need to be able to capture what we emit.
