In Chapters 3-5 the committee provides recommendations and conclusions for both near-term and long-term improvements to SC-CO2 estimation. In this chapter, the conclusions that focus solely on long-term research needs as a guide for research in the relevant fields are repeated. The committee notes that neither the IWG nor any other single entity has responsibility for identifying and supporting research in these fields.1 Thus, these conclusions about what is needed are intended for all interested researchers, institutions that support research, and policy makers. For each component of the SC-CO2 analysis discussed below, research topics are listed in order of priority for developing updates to the SC-CO2 framework.
The committee structured its work, conclusions, and recommendations around four components of analysis that are involved in estimating the SC-CO2—socioeconomic and emissions projections, climate modeling, estimation of climate damages, and discounting net monetary damages—which are identified as modules. Each module comprises conceptual formulations and theory, computer models and other analytical frameworks, and each is supported by its own specialized disciplinary
1 Recognizing that the IWG is itself not a research funding agency, we encourage the IWG to communicate these research priorities to the key research programs of its member agencies, as well as the U.S. Global Change Research Program (USGCRP), the National Science Foundation, and other funding agencies of the USGCRP, and other relevant research and research funding institutions both within and outside of government.
expertise. Among the committee’s research recommendations, the highest priority is placed on research relevant to the climate damages module followed by the socioeconomic module.
Estimation of the SC-CO2 involves the integration of these four modules, while taking account, when possible, of the feedbacks and interactions among them. Research in climate impacts and damages, integrated assessment, economics, and Earth system modeling could explore interactions in and feedbacks among the components of SC-CO2 estimation that go beyond the potential feedback of climate impacts on socioeconomic projections, or that may even suggest changes in the structure of the four-module system. In combination, these physical and economic changes might yield regional and global interactions large enough to affect the overall operation of the economic or climate system in ways that have not yet been considered. It is not clear in what ways the structure and implementation of the SC-CO2 estimates might be refined to capture changes in understanding, but accounting for new knowledge will be important to future updates of the SC-CO2 estimates.
In addition, three of the committee’s recommended modules—socioeconomic, climate, and damages—require advances in the models that contribute to their component of SC-CO2 estimation. For the fourth module, on discounting, the committee’s recommendations rely on applying existing techniques to the SC-CO2 estimation process, so we do not offer any specific research needs in this area. However, the committee does not mean to imply that the issue of long-term discounting would not benefit from further research.
CONCLUSION 2-3 Research to identify and explore the magnitude of various interactions and feedbacks within the human-climate system, which are relationships not currently well represented in the SC-CO2 estimation framework, will be an important input to longer-term enhancements in the SC-CO2 estimation framework. Areas of research that are likely to yield particular benefits include:
Exploration of methods for representing feedbacks among systems and interactions within them, such as:
- feedbacks between climate, physical impacts, economic damages, and socioeconomic projections, and
- interactions between types of impacts or economic damages within and across regions of the world.
- Assessment of the relative importance of specific feedbacks and interactions in the estimation of the SC-CO2, perhaps using an existing detailed structure model of the world economy.
- Assessment of existing analyses that integrate socioeconomic, climate, and damage components to assess their suitability for use in estimating the SC-CO2, particularly with respect to feedbacks and interactions, while recognizing the computational requirements for such analyses.
The IWG process is committed to updating the SC-CO2 estimates as the science and economic understanding of climate change and its impacts on society improve over time. There are therefore many advantages to encouraging research that supports the construction of a dedicated socioeconomic projection framework and, considering its unique objectives, a detailed-structure economic model, as recommended in Chapter 3.
CONCLUSION 3-1 Research on key elements of long-term economic and energy models and their inputs, focused on the particular needs of socioeconomic projections in SC-CO2 estimation, would contribute to the design and implementation of a new socioeconomic module. Interrelated areas of research that could yield particular benefits include the following, in rough order of priority:
- Development of a socioeconomic module to support damage estimates that depend on interactions within the human-climate system (e.g., among energy, water, and agriculture, and between urban emissions and air pollution).
- Use of econometric and other methods to construct long-run projections of population and gross domestic product (GDP) and their uncertainties.
- Quantification of the magnitude of feedbacks of climate outputs and various measures of damages (e.g., on consumption, productivity, and capital stocks) on socioeconomic projections, based in part on existing detailed-structure models.
- Development of detailed-structure economic models suited to projections that are consistent over very long time horizons, in which functional form and levels of regional and sectoral
detail in inputs and outputs may differ between the nearer term (e.g., to 2100) and the more distant future.
- Development of probability distributions of uncertain parameters used in detailed-structure models, with a particular focus on the differences among developed, transitional and low-income economies. Examples of uncertain parameters include key elasticities of substitution (e.g., between labor and capital inputs to production, between energy and nonenergy demand, and among fuels in total energy use), energy technology costs and rates of technology penetration, and rates of capital turnover.
In this area, the committee’s identified research needs cover both the near term and the long term.
CONCLUSION 4-5 Research focused on improving the representation of the Earth system in the context of coupled climate-economic analyses would improve the reliability of estimates of the SC-CO2. In the near term, research in six areas could yield benefits for SC-CO2 estimation:
- coordinated research to reduce uncertainty in estimates of the capacity of the land and ocean to absorb and store carbon, especially in the first century after a pulse release, applied to a range of scenarios of future atmospheric composition and temperature;
- coordinated Earth system model experiments injecting identical pulses of CO2 and other greenhouse gases in a range of scenarios of future atmospheric composition and temperature;
- the development of simple, probabilistic sea level rise models that incorporate the emerging science on ice sheet stability and that can be linked to simple Earth system models;
- systematic assessments of the dependence of patterns of regional climate change on spatial patterns of forcing, the relationship between regional climate extremes and global mean temperature, the temporal evolution of patterns under conditions of stable or decreasing forcing, and nonlinearities in the relationship between global means and regional variables;
- systematic assessments of nonlinear responses to forcing in Earth system models and investigations into evidence for such responses in the geological record; and
- the development of simple Earth system models that incorporate nonlinear responses to forcing and assessments of the effects of such nonlinear responses on SC-CO2 estimation.
In the longer term, more comprehensive climate models could be incorporated into the SC-CO2 estimation framework. However, the major focus of current model research is on increasing resolution and comprehensiveness, rather than on expanding the ability of comprehensive models to be used for risk analysis. SC-CO2 estimation would be advanced by an expanded focus on probabilistic methods that use comprehensive Earth system models, including the use of comprehensive models to represent low-probability, high-consequence states of the world, as well as the use of decision support science approaches to identify and evaluate key decision-relevant uncertainties in Earth system models.
Finally, the committee outlines in Chapter 5 a set of desirable characteristics of a damages module that could be developed in the long term and would improve the reliability of estimates of the SC-CO2. The committee’s conclusions cover the research tasks that would support the development of such a module.
CONCLUSION 5-1 An expansion of research on climate damage estimation is needed and would improve the reliability of estimates of the SC-CO2.
In the near term, initial steps that could be undertaken include:
- a comprehensive review of the literature on climate impacts and damage estimation, the evaluation of adaptation responses, and regional and sectoral interactions, as well as feedbacks among the damage, socioeconomic, and climate modules; and
- a comparison of methods for estimating damages, including characterizations of their differences, synergies, uncertainties, and treatment of adaptation.
In the medium to long term, several research priorities could yield particular benefits for SC-CO2 estimation:
- physical, structural economic, and empirical estimation of climate impact relationships for regions and sectors not currently covered in the peer-reviewed literature;
- structural and empirical studies of the efficacy and costs of adaptation;
- calibration of damage functions using empirical and structural models operating at sufficiently high temporal and spatial resolution to capture relevant dynamics;
- the development of systematic frameworks for translating estimates of impacts into welfare costs; and
- empirical observation-based and structural modeling studies of interregional and intersectoral interactions of impacts, as well as of feedbacks among damages, socioeconomic factors, and emissions.
In the long term, research priorities that could yield particular benefits for SC-CO2 estimation would include omitted critical thresholds in natural and socioeconomic systems:
- development of simple Earth system model or full complexity Earth system model scenarios in which potential critical thresholds of tipping elements (e.g., Atlantic meridional overturning circulation, monsoonal circulation patterns, sea ice, polar ice sheets) are crossed, and the use of the physical changes in these scenarios to drive models that assess impacts and damages;
- empirical observation-based and structural modeling studies of the potential for climate change to drive the crossing of critical thresholds in socioeconomic systems and of their ensuing damages; and
- expert elicitation studies of the likelihood of different tipping element scenarios, in order to allow tipping elements and their critical thresholds to be represented probabilistically in the SC-CO2 framework.
Overall, the committee’s long-term recommendations on an integrated approach to estimating the SC-CO2, as well as the socioeconomic, climate, and damages modules, requires a significant advance in the scientific literature. It is important that the IWG continue to engage with the scientific community to produce the research identified above. As noted in the committee’s recommendation for a regularized updating process in Chapter 2 (Recommendation 2-4), research, scientific advances, and peer review are central elements to improving the reliability and transparency of the SC-CO2 estimates.