Interpretation of the Paris goals
In this section, we present an interpretation of the Paris goals in the light of our assessment described above (The ICA-RUS project was originally intended to provide alternatives in terms of long-term climate goals. However, in the Paris Agreement, the international community has already chosen long-term goals. Therefore, we changed the purpose of our project and made an interpretation of the Paris goals.)
First, we found in the above that temperature goals as low as 2 or 1.5 °C cannot necessarily be supported from a globally aggregated cost–benefit perspective, as far as we can tell within the scope of our assessment. Although maximizing the aggregated economic value is often seen as an objective principle to optimize a policy intervention, we should note that the adoption of this principle, which is a form of utilitarian approach, heavily involves value judgments. By value judgments, we mean judgments on a question like “what should be protected?”, “what should be avoided?”, or “what is acceptable/unacceptable?” To appreciate the grounds of the Paris temperature goals, values other than the globally aggregated economic value should be taken into consideration. As has been discussed in the literature (Okereke 2010), such values involve ethical dimensions, particularly a concept called “climate justice”, which draws attention to a structural social injustice that those who are least responsible (i.e., people in developing countries and future generations, who have emitted least amount of GHGs) suffer the most harm. The Paris temperature goals can be supported by an ethical position that such an inequality is unacceptable. Other grounds may come from concerns of crossing a “tipping point” of any sub-system of the Earth system (tipping element). Although the scientific understanding on tipping elements still has large uncertainties, and thus, our assessment of them is also limited; the lowest possible temperature goals might well be supported to reduce the risks of crossing any tipping point. This could also involve value judgments, as triggering a catastrophic change in the Earth system by the current generation, which entails grave consequences for future generations, is unacceptable by a certain ethical position. The issue of tipping elements is further discussed by Iseri et al. (2018) in this Special Feature.
Second, actions needed to meet a temperature goal are not very clear, given the uncertainty in climate system (including climate sensitivity, carbon-cycle feedback, and possibly others). On the other hand, the emission goal of the Paris Agreement (net zero global anthropogenic GHG emissions in the second half of this century) is a clearer one in terms of actions needed. In that sense, seen from the framework of our assessment, we appreciate the Paris emission goal as a more “actionable” goal. The two kinds of goals, the temperature and emission goals, are roughly, but not precisely, consistent with each other, and the relationship of the two should be made clearer. This conclusion of our echoes that of Geden (2016)’s, who also recommended a net zero emissions’ target as an actionable one, although his conclusion was based on his political science arguments.
Learning climate sensitivity
Practically, we should start taking actions with estimated likely values (or ranges) of climate parameters and then adjust our mitigation pathways in the course according to the future reductions of uncertainties in climate parameters (so-called “learning” or “act then learn” approach; Yohe et al. 2004; Manne and Richels 1992). Hereafter, for simplicity, we use “climate sensitivity” as a single parameter representing uncertain processes in the Earth system, keeping in mind that other parameters (including the one for carbon-cycle feedback) can be important, as well (Friedlingstein et al. 2014).
The estimate of climate sensitivity is expected to be improved through an advancement of science. However, even without such an advancement, thanks to the future accumulation of the observed surface air temperature data, the uncertainty range of global mean temperature changes can be reduced (Shiogama et al. 2016). We suggest that this gradual improvement of the estimate of climate sensitivity through learning plays an essential role when we deal with uncertainties lying between the temperature and emission goals. The economic value of scientific information in this context is discussed by Mori and Shiogama (2018) in this Special Feature.
If we find in this process that the climate sensitivity is substantially lower than the current central estimate [~ 3 °C, the middle of likely range from IPCC (2013)], we can more reasonably aim at the 1.5 °C goal, as the emission path for aiming at 1.5 °C goal with climate sensitivity of a little more than 2 °C is almost equivalent to that for the 2 °C goal with a climate sensitivity of 3 °C.
On the contrary, in case we find that the climate sensitivity is substantially higher than the central estimate, we should recognize that we are on the course of exceeding 2 °C even when the emission goal is achieved. This case represents a possible gap between the Paris temperature and emission goals and needs more discussion.
Alternatives left for the case of high climate sensitivity
We have identified the following three options that we could take if we found that climate sensitivity was substantially high and 2 °C would be exceeded even if the Paris emission goal was achieved.
Option A: Accepting and adapting to a warmer world
Option B: Boosting mitigation
Option C: Climate geoengineering.
Option A means taking no additional mitigation action and letting the temperature exceed 2 °C, accepting the risks of more severe climate change impacts, and possibly making efforts and paying costs for additional adaptation.
Option B means accelerating mitigation actions exceeding the Paris emission goal, which possibly means that we should aim at substantially net negative GHG emissions in the second half of this century, accepting additional costs and risks due to possible side-effects. For example, bio-energy with CCS (BECCS) is one of the negative emission technologies that could be adopted then (and is often adopted in many scenarios to achieve even the Paris emission goal), but its large-scale deployment requires huge land areas and might compromise food security and/or ecosystem services (Fuss et al. 2014; Kato and Yamagata 2014). See also Yamagata et al. (2018) in this Special Feature.
Option C basically refers to the so-called solar radiation management, which is to cancel the warming effect of increased GHGs by the reduction of incoming solar radiation due to stratospheric aerosol injection or other means (Royal Society 2009). The adoption of this option means to accept risks due to possible side-effects and governance failure associated with this techno-fix solution (Preston 2013).
Obviously, no option is easy to adopt, as any of them entails additional risks, which can be disastrous. However, the international community should prepare to choose one or any combination of them, in case we find that climate sensitivity is high. That decision will require the deepest understanding and most careful deliberation of scientific and moral aspects of risks associated with each option.
Remarks on the achievement of the Paris emission goal
The above discussion is based on the assumption that we are able to achieve the Paris emission goal, that is, net zero global anthropogenic GHG emissions in the second half of this century. However, it is by no means an easy goal to achieve.
One remark regarding the achievement of the Paris emission goal that we can make based on our assessment is that the mitigation action needed to achieve the goal heavily depends on the socio-economic pathways, as has been observed in past assessments (IPCC 2014b), and is smaller for a more sustainable pathway, i.e., the smallest for SSP1, followed by SSP2, and the largest for SSP3, in our assessment. Different socio-economic pathways could be interpreted as representations of the uncertainty range of socio-economic development that we cannot manage. They are certainly external factors when we only think of “climate policy” in a narrow sense, which is represented in IAMs as introducing more expensive mitigation options by setting a higher carbon price. However, when we think of a wider “sustainability policy”, policy interventions should be able to affect SSPs to some extent. Although we did not clarify or identify within the scope of our project what such sustainability policies were, we can suggest that policies to shift the socio-economic condition of the world to a more sustainable one (e.g., from SSP2 toward SSP1) are as important as the climate policy itself to achieve the Paris emission goal, and they should be seriously discussed and implemented.
Another remark is about the limitation of scenarios illustrated by IAMs and the need for a careful interpretation of them. In principle, IAMs cannot represent structural changes (or “transformation”) of socio-economic and technological systems that cannot be foreseen at present, especially when they are applied to illustrate scenarios until the end of this century. Therefore, mitigation scenarios are not something that we have to follow exactly and expected economic damages are not necessarily what we have to accept. Instead, what really is necessary is creating policies to relax constraining conditions that are assumed in models and facilitating transformation of the systems. This point might be obvious in some research communities (Geels et al. 2016), but not necessarily in others. It was not drawn directly from our assessment, but was brought up in our inter-disciplinary discussion. At the same time, we should not forget that IAMs cannot represent either possible political failures that might result in insufficient implementation of mitigation actions (Mabey et al. 2011). Continuous efforts to avoid such a failure are needed.
Social process of risk decision on global and long-term climate change
The decision to choose global and long-term goals or options as described in Sect. 5.3 cannot be made based solely on science. It involves uncertainties and value judgments. Thus, social processes play an essential role in such a decision-making, possibly involving citizen participation to democratize the process.
However, we found in our survey that climate change, as compared with other risk issues, has a characteristic that an average person typically feels that it is difficult to be engaged in, as it is complex, uncertain, long-term, and hard-to-feel self-efficacy (Moser 2010; Wolf and Moser 2011). Nonetheless, citizens do not necessarily want authorities to make the whole decision. There is a gap between the understanding of climate risk issues by experts and that by citizens. Experts’ understanding should be translated into contexts of citizens’ everyday lives. In addition, citizens may want to know the ethical dimensions and value judgments that are embedded in seemingly objective experts’ arguments.
To bridge this gap, we propose that an “intermediate layer” of experts be designed to mediate among scientific experts, citizens, and stakeholders to deal with ethical dimensions, value judgments, and translations. This layer will consist of experts from various disciplines particularly including the humanities and social sciences, who can translate scientific results into the contexts of everyday life and can explain how value judgements are embedded in certain scientific statements by revealing the hidden power and knowledge structures of society. The selection process of the members of this layer should be done by the government as well as by the public. The members of this layer should have an open meeting day with concerned people in public. The quality control of this layer will be ensured by continuous discussion. This proposal is discussed in detail by Fujigaki (2018) in this Special Feature.