[MUSIC] The future of fossil fuels can be projected with some amount of reason by sort of extrapolating from what is going on today and how we think things might change into the future. There are lots of different ways that you can do this sort of scenario forecasting. Some of them very complicated that take into account individual countries and individual situations, but you can also get there from a broader perspective using this formula which is known as the Kaya identity. It takes the rate of carbon emission can be calculated as the product of the population as you expect it to grow in the future times the GDP per capita, because fossil fuel use is so closely tied with economic activity. And if we look at the units that we're building up here, population is people. GDP per capita is dollars per person per year and so the units of people cancels out there. The third term is the energy intensity. And so this is basically how much energy it takes to make a dollar, to do a dollar's worth of economic activity. And this depends on whether you're making a dollar's worth of steel or a dollar's worth of coffee or something like that, it makes a difference. It also depends on energy efficiency. So now we've got watts per year and the last term has to do with this energy source, how you're making the energy. The amount of carbon per watt of energy. So nuclear power would have less carbon per watt than coal production. The units now cancel, so we have gigatons of carbon per year. So extrapolating the present day trends in these terms, we need to guess how high we think Earth's population is going to plateau at. Hopefully, it will plateau and stop growing. It looks like it's doing that. The average income on the Earth is increasing as a function of time, so we can put the rate of change of that into the equation. The energy use is getting more efficient. So the energy intensity is decreasing through time and then there are also changes in the carbon source that have to do with efficiency, but also a there's been a recent trend away from coal, because of lower prices for natural gas brought about by improvements in the ability to extract natural gas known as fracking. But you put all these things together and you end up with the projection that under business as usual, that means with no efforts to avoid climate change, the rate of carbon emission. Today, about 10 gigatons a year could grow to about 20 gigatons a year by the year 2100. [MUSIC]