The uncertainty of climate sensitivity and its implication for the Paris negotiation

Uncertainty of climate sensitivity is one of the critical issues that may affect climate response strategies. Whereas the equilibrium climate sensitivity (ECS) was specified as 2–4.5 °C with the best estimate of 3 °C in the 4th Assessment Report of IPCC, it was revised to 1.5–4.5 °C in the 5th Assessment Report. The authors examined the impact of a difference in ECS assuming a best estimate of 2.5 °C, instead of 3 °C. The current pledges of several countries including the U.S., EU and China on emission reductions beyond 2020 are not on track for the 2 °C target with an ECS of 3 °C but are compatible with the target with an ECS of 2.5 °C. It is critically important for policymakers in Paris to know that they are in a position to make decisions under large uncertainty of ECS.

these pledges are nowhere near sufficient to limit the temperature increase to less than 2°C since pre-industrialization if we apply 3°C as the best estimate 1 of the equilibrium climate sensitivity (ECS).
ECS is defined as an increase in global mean surface temperature caused by a doubling of the atmospheric CO 2 concentration. The uncertainty of climate sensitivity poses one of the greatest challenges in planning strategies on how and to what extent we should cope with risks of climate change.
Throughout IPCC's 1st to 3rd Assessment Report, the likely range of ECS was estimated as 1.5-4.5°C with its best estimate at 2.5°C. The 4th Assessment Report (AR4) specified a likely range (greater than 66 % probability) of ECS as 2-4.5°C with its ''most likely value'' or ''best estimate'' of 3°C, but the 5th Assessment Report (AR5) lowered the figure to 1.5-4.5°C. In addition, no best estimate was given by AR5 because of the difference of methodologies of estimating ECS as explained below (IPCC 2013).
According to the estimates of atmosphere-ocean general circulation models (AOGCMs) in the AR5, the mean value of ECS is 3.2°C and the ranges is 2.0-4.5°C, close to that in the AR4. Climate sensitivity can also be estimated from observations of surface temperature and climate forcing data. The values of ECS thus estimated are rather lower as pointed out by Rogelj et al. and others (IPCC 2013, Lewis and Curry, 2014, Otto et al. 2013 and Lewis and Curry (2014)  There are several criticisms of the observation-based methods, though, including one arguing that the observed warming is likely biased low (Durack et al. 2014).
We would like to examine the effect of uncertainty of ECS on emissions targets. Rogelj et al. (2014) admitting recent estimates based on the observed warming trends tend to show lower values of climate sensitivity, argues that '' [T]here are several climate policy implications that can be drawn from recent ECS estimates. The most important, however, is that they do not change the big picture if all available evidence is taken into account. (…) Even the lowest ECS estimate assumed in this study only results in a delay of less than a decade in the timing of when the 2°C threshold would be crossed when emission trends from the past 10 years are continued.'' This conclusion, what we found, comes from the Supplementary Material of the paper. There, two ECS distributions, among others, were used for comparison; one was named as IPCC AR4 consistent and the other as IPCC AR5 consistent. By comparing those two, Rogelj et al. concluded that the differences of climate sensitivity 'do not change the picture'. What matters is that the median values of those two were almost the same, i.e., 3°C for the former and 3.1°C for the latter, though distribution itself is a little bit flat in AR5 consistent.
As pointed out previously, the likely range of ECS was lowered to 1.5-4.5°C (in AR5) from 2 to 4.5°C (in AR4), and experts were unable to agree on the value of the best estimate in AR5 though it was agreed as 3°C in AR4. In addition, the value of 2.5°C had been used as best estimate (most likely value) throughout IPCC's 1st to 3rd assessment reports where the likely range of climate sensitivity had been 1.5-4.5°C. Under the above situation, it is only natural to assume the best estimate (median) for AR5 will be lower than 3°C. Therefore, we chose the best estimate value of 2.5°C for the purpose of comparison to explore the impact of difference in ECS on climate negotiations. The point at issue here is whether INDCs submitted by major countries are consistent with the 2°C target under different climate sensitivities. Note that it is not the authors' intention to argue 2.5°C is the correct value. Rogelj et al. (2012), Schaeffer et al. (2015) and IPCC (2014) calculated the temperature by MAGICC with the probabilistic mode for climate sensitivity by assuming its probability density function (Table 1). However, as the probabilistic mode of MAGICC is not accessible for (1.4-3.6) The above table is an extract from Table SPM.1 from AR5 WG3 that shows information including the relationship between CO 2 eq. concentrations and temperature changes in 2100 Temperature change in 2100 is provided for a median estimate of the MAGICC calculations, which illustrates differences between the emissions pathways of the scenarios in each category The range of temperature change in parentheses includes, in addition, the carbon cycle and climate system uncertainties as represented by the MAGICC model Note that the above figures are calculated based on a climate sensitivity of 3°C (most likely value) outsiders, we calculated the temperature for different emission pathways by using our global energy systems model DNE21? and MAGICC without the probabilistic mode under certain climate sensitivities, i.e., 3.0 and 2.5°C, instead of the median value under the probabilistic mode assuming the probability density function. Figure 1 shows three emission pathways of which only figures toward 2050 are shown: The black line shows an emissions pathway with current policies (BAU), the green line shows an emissions pathway that limits temperature increase below 2°C over the twenty first century under a climate sensitivity of 2.5°C, which corresponds to the scenario of temporally, though slightly, overshooting 580 ppm CO 2 -eq. in AR5, and the orange line shows an emissions pathway that limits temperature increase below 2°C over the twenty first century with a climate sensitivity of 3°C, which corresponds to the scenario where concentration stays below 500ppmCO 2 eq. through 2100. The red line shows an emissions pathway until 2030 on the assumption that the INDCs of countries that have been submitted by June 30 will be implemented. We estimated China's emissions will peak out in 2030 at 16.7 GtCO 2 -eq. based on its upper range of CO 2 /GDP improvement ratio of 65 % with annual GDP growth ratio of 6.2 % through 2015-2030. For the calculation of the 2030 emissions of United States, refer to the legend to Fig. 1.
The outcome of our model shows global total emissions under major countries' INDCs (red line) in 2030 will not be on track to attain the 2°C target if climate sensitivity is 3°C (orange line). On the other hand, the red line emissions are in line with the green line that is consistent with the 2 degree target if climate sensitivity is 2.5°C, and if we allow a temporal overshoot of 580 ppmCO 2 -eq. This implies, with ECS equal to 2.5°C, that the 2°C target is still within reach.
The authors would like to show, based on DNE21? model, the difference of marginal abatement cost (MAC) to attain 2°C target due to the difference of ECS. As shown in Fig. 2, MAC in 2050 is estimated to be as high as $318/ tCO 2 under ECS of 3°C, but it is merely $24/tCO 2 if ECS is 2.5°C. This implies that 2°C target would still be a feasible target.
It is clear from the above explanations that the impact of a mere 0.5°C difference in climate sensitivity is of critical significance for policy objectives, which is especially significant given the large uncertainties over climate sensitivity. through 2100 under a climate sensitivity of 2.5°C, which corresponds to the scenario of a slight temporal overshoot of 580ppm CO 2eq. concentration in AR5. Temperature is expected to stabilize below 2°C in the long run. Orange line shows the emissions pathway that limits the temperature increase to below 2°C through 2100 under a climate sensitivity of 3°C, which corresponds to the scenario in which the concentration stays below 500ppmCO 2 eq. through 2100 in AR5. Temperature is expected to stabilize below 2°C even under a climate sensitivity of 3°C. The red line shows emissions until 2030 based on the assumption that individual country's INDCs submitted at the end of June will be implemented. We assumed China's emissions in 2030 to be 16.7GtCO 2 -eq. This almost corresponds to its BAU emissions. The US pledge covers only until 2025 and comprises two targets, i.e., 26 and 28 % emissions reduction relative to 2005. We assumed here that the 28 % emission reduction will be implemented by 2025, thereafter with a linear interpolation to 80 % reduction in 2050. (Source Research Institute of Innovative Technology for the Earth) Sustain Sci (2016) 11:515-518 517 It is scientific community's vital role to narrow the uncertainty range of ECS. At the same time it is critically important for policymakers in Paris to know that they are in a position to make decisions under large uncertainty of ECS. Fig. 2 Difference of Marginal Abatement Cost due to difference of ECS. The figure shows the difference of marginal abatement cost (MAC) due to difference of ECS. Both orange and green lines show MAC necessary to achieve 2°C target (temperature stays below 2°C since pre-industrialization through 2100). Orange line is based on the assumption that ECS would be 3°C whereas green line is drawn on the assumption it would be 2.5°C. MAC in 2050 in the former case is $318/tCO 2 and the latter is $24/tCO 2 . (Source Research Institute of Innovative Technology for the Earth)