Modeling how human beings behave is not easy. How can we know the most likely energy mix and distribution of wealth in 2074? We cannot. Instead, the CMIP5 and CMIP6 models are run several times, each time with a different increase in the greenhouse effect.
Increasing the greenhouse effect by releasing CO2 and other climate gases, means that the Earth system keeps more of the energy that it gets from the Sun.
Picture any little patch on the surface of the Earth. Every second this patch receives energy from the sun and from the natural greenhouse effect of air molecules, aerosols and clouds. It also emits energy; there is a balance. Imagine that your little patch of land begins to receive more energy, as if you started a small heater or switched on a lightbulb – or increased the greenhouse effect.
Assume that by 2100, an average square meter of the Earth receives 8.5 watts more than it used to. Run the climate model, and see what happens. This is one scenario, called the RCP 8.5. An increase of 8.5 watts per square meter is what we will get if we keep emitting as much CO2 and other gases as we do today. That is why the RCP 8.5 is often called the “business as usual” scenario.
The CMIP models are run for four such scenarios. The 8.5-watt increase – RCP 8.5 – is the one with the strongest emissions. The others are RCP 6.0, RCP 4.5 and RCP 2.6 – with increases of 6.0, 4.5 and 2.6 watts per square meter of the Earth by 2100.
The more CO2 we emit, the stronger that extra lightbulb will be. The RCP 2.6 scenario, the one with the lowest emissions, is the only scenario that can avoid warming the Earth more than two degrees from the industrial revolution to the year 2100 – the “two degree target”. This would require significant reductions in the emission of greenhouse gases.
It is not possible to say that either of these scenarios is more likely than the others. Each is linked to a specific increase of CO2 in the atmosphere, an increase which depends on the choices of human beings.