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.

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.

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.

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.