Can 2°C warming be avoided?

CO2 stabilisation paths Figure: A schematic representation of (a) fossil CO2 emissions, (b) CO2-equivalent concentrations, and (c) global mean temperature for two scenarios: Firstly, an “immediate stabilization” which implies rising CO2-equivalent concentrations up to around 415 ppm in 2015 and stable levels after that (red dashed line). This scenario is clearly hypothetical as the implied emission reductions in 2015 and beyond would hardly be feasible. Secondly, a peaking scenario (green solid line), which temporarily exceeds and then returns to a 415 ppm stabilization level. Both scenarios manage to stay below a 2°C target – for a climate sensitivity of 3.8°C or lower. This is roughly equivalent to a 4:1 chance of staying below 2°C3.

Indeed, avoiding concentrations of – say – 475ppm CO2 equivalent (see Figure a) will require quite significant reductions in emissions. However, as long as we reduce emissions decisively enough, concentrations in the atmosphere could lower again towards the latter half of the 21st century and beyond. The reasons are the relatively short lifetimes of methane, nitrous oxide, other greenhouse gases and some CO2 uptake by the oceans (as discussed here).

Now, what is going to happen to our cat, if we turn up the heat control of our oven to about 475 ppm and then reduce it again? If we react quickly enough, we might be able to save the cat from some irreversible consequences. In other words, if concentrations are lowered fast enough after peaking at 475 ppm, the temperature might not exceed 2°C. Basically, the thermal inertia of the climate system will shave off the temperature peak that the cat would otherwise have felt if the oven temperature reacted immediately to our control button.

Thus, to sum up: Even under the very likely scenario that we exceed 400 ppm CO2 concentrations in the very near future, it seems likely that temperatures could be limited to below 2°C with a 4:1 chance, if emissions are reduced fast enough to peak at 475 ppm CO2 equivalent, before sliding back to 400 ppm CO2-equivalent.

Peaking at 475 ppm CO2-equivalent concentrations and returning to 400 ppm certainly comes at a cost, though: Our oven will approach 2°C more quickly compared to a (hypothetical) scenario where we halt the build-up of CO2 concentrations at 400 ppm (see Figure c). Thus decisions would need to be different if we care more about the rate of warming than the equilibrium. In fact, some emission pathways and some models also suggest that peaking at 475 ppm CO2 equivalent and returning to 400 ppm might even slightly decrease our chances to stay below 2°C (see Chapter 28 in the DEFRA report). Depending on the actual thermal inertia of the climate system, the peak temperature corresponding 475 ppm might be very close to the 400 ppm equilibrium temperature. This points to a more fundamental issue: Rather than discussing ultimate stabilization levels, it might be more worthwhile for policy and science to focus on the peak level of greenhouse gas concentrations. By the time that we managed to peak concentrations we could still decide whether we want to stabilize at 400 ppm or closer to pre-industrial levels. We will most likely be able to make wiser decisions in the future given that we certainly have learnt something about the cat’s behavior in its current fever.

1. The equilibrium warming dT can be easily estimated from the CO2 equivalent stabilization level C, if one would know the climate sensitivity S, with the following little formula: dT=S*ln(C/278 ppm)/ln(2)

2. This is of course a probability that merely reflects the uncertainty in our knowledge. The climate sensitivity is not random, it is just unknown. It expresses our degree of belief in that outcome, and is of course subject to future modification in the light of new evidence.

Page 2 of 3 | Previous page | Next page