Guest comment by Richard Tol
In 1995, the Scientific Council of the German Federal Government for Global Environmental Changes (WBGU) recommended that it would be appropriate to keep the global mean surface air temperature below a two degrees Celsius warming relative to the start of the industrial revolution. The “two degrees target” was adopted by the German government and the European Union; and has been endorsed by the G8 and noted by the United Nations.
Two degrees is a vague target. With or without natural variability? Can we exceed the target in the interim or not? For sure or with a 50% probability? Let us assume that the target is without natural variability, in the long run, and with something like a 50% chance. (Note that this makes the target softer.) This means that anthropogenic radiative forcing should not exceed some 2.5Wm-2.
The forcing from long-lived greenhouse gases is already at 2.6 Wm-2. Total anthropogenic forcing is 1.6 Wm-2 – but we want to get rid of aerosols because of concerns about health and environment. Therefore, the two degrees target is very tough.
The Energy Modeling Forum (EMF), hosted at Stanford University, organises model comparisons for climate economists. The latest exercise, EMF22, was about stringent climate targets. Fourteen models from around the rich parts of the world participated. The results are published here (open source). More accessible material is here.
The following results emerged on the two degrees target. Six out of fourteen models simply could not meet the target whatever was tried. The other eight impose the following conditions on meeting the target:
1. All countries will have to start reducing emissions by 2020;
2. The price of carbon in 2020 in all countries for all emissions should be at least $15/tCO2 (roughly what it is today in the regulated half of Europe), but may need to be as high as $263/tCO2;
3. Emissions from rich countries will have to be around 20% of their 2000 levels by 2050 and emissions from poor countries will have to be round 50% of their 2000 levels by 2050;
4. There are no constraints on the expansion of carbon-neutral energy, such as wind or nuclear power.
If any of these conditions are violated, then carbon-negative energy is needed. Carbon dioxide emissions in 2000 were some 25 GtCO2. In 2100, this would need to be -15 GtCO2 – that is a reduction of 160%. This would require providing a large share of the world’s energy needs by biomass (without affecting world food supply) and carbon capture and storage at a massive scale. This places a lot of faith in technologies that have yet to be demonstrated.
I would be surprised if any of these conditions are met, let alone all. I therefore think that the two degrees target is infeasible.