by Judith Curry

A burning question for the Paris negotiations: Are the INDCs sufficient to prevent 2 degrees of warming?

A provocative new paper addresses the feasibility of not exceeding the 2C target.

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

Yoichi Kaya, Mitsutsune Yamaguchi & Keigo Akimoto

Abstract. 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.

Published in Sustainability Science [link to abstract].

Excerpts:

By the end of June, 2015, the United States, the European Union, China and several other countries submitted their intended nationally determined contributions (INDCs) to the UNFCCC secretariat. This is a good start toward the coming Paris climate conference (COP 21). However, according to our estimate based on our global energy systems model DNE21+ and a simple climate change model MAGICC, 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 of the equilibrium climate sensitivity (ECS).

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.

Fig. 1: Estimated emission pathways toward 2050 by the DNE21+ – model (and MAGICC model): Black dotted line shows the emissions pathway under current policies, green line shows the emissions pathway that limits the temperature increase below 2 C through 2100 under a climate sensitivity of 2.5 C, which corresponds to the scenario of a slight temporal overshoot of 580ppm CO2- eq. 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 500ppmCO2 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.

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 ppmCO2-eq. This implies, with ECS equal to 2.5 C, that the 2 C target is still within reach.

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.

JC comments

In my previous post Climate sensitivity: lopping off the fat tail, I argued that it is becoming increasingly difficult to defend high values of ECS. However, the uncertainty is sufficiently large that we can’t really identify a meaningful ‘best value’ of sensitivity, or rule out really high values.

A key issue is that emerging estimates of aerosol forcing are considerably lower than what was used in the AR5 determinations of ECS, implying lower values of ECS than was determined by the AR5.

This uncertainty in ECS makes emission targets rather meaningless. It will be interesting to see how this uncertainty is factored into the Paris negotiations

Note, there are other papers on this general topic that are in the review process, I expect a spate of such papers to appear during the next month.

I have been corresponding via email with Yamaguchi and Kaya. Also, on his recent trip to the U.S., Mits Yamaguchi spent a day visiting with me, some very stimulating conversations and interesting insights into WG3. Below is a photo from this meeting.