James Rising

Scientific assessments play a critical role in synthesizing evidence to support evidence‐based decision‐making in many sectors and topics. For climate change, large‐scale assessments like the Intergovernmental Panel on Climate Change (IPCC) and the United States (U.S.) National Climate Assessment (NCA) reports help many different users—from national government to local governments, Indigenous Peoples and communities, the private sector, the social sector, and the scientific community itself—navigate the complex field of climate risks, impacts, and integrated solutions. While co‐creation with users is essential to developing actionable assessments, this paper lays the groundwork for future processes by identifying many advancing and emerging areas of relevant knowledge ready for assessment or additional near‐term research. Drawing on the expertise of many scholars who were formerly tasked with producing the Sixth NCA and authors of other national or sub‐national climate assessment reports, this paper highlights select U.S.‐relevant topics in which research advances since the 2023 Fifth NCA make new syntheses particularly valuable for future national, sub‐national, and sectoral climate assessments. It also identifies critical research topics in those select areas that would be useful for future climate assessment efforts. Overall, this research priorities roadmap aims to catalyze collaborations, research, and a network of climate assessments to synthesize the state of climate impacts, risks, and responses and to provide widely accessible and foundational information for sectoral, regional, and cross‐cutting climate action plans and decisions in the U.S.

Scientific assessments play a critical role in synthesizing evidence to support evidence-based decision-making in many sectors and topics. For climate change, large-scale assessments like the Intergovernmental Panel on Climate Change (IPCC) and the U.S. National Climate Assessment (NCA) reports help many different users—from national government to local governments, Indigenous Peoples and communities, the private sector, the social sector, and the scientific community itself—navigate the complex field of climate risks, impacts, and integrated solutions. While cocreation with users is essential to developing actionable assessments, this paper lays the groundwork for future processes by identifying many advancing and emerging areas of relevant knowledge ready for assessment or additional near-term research. Drawing on the expertise of many scholars who were formerly tasked with producing the Sixth NCA and authors of other national or subnational climate assessment reports, this paper highlights select U.S.-relevant topics in which research advances since the 2023 Fifth NCA make new syntheses particularly valuable for future national, subnational, and sectoral climate assessments. It also identifies critical research topics in those select areas that would be useful for future climate assessment efforts. Overall, this research priorities road map aims to catalyze collaborations, research, and a network of climate assessments to synthesize the state of climate impacts, risks, and responses and to provide widely accessible and foundational information for sectoral, regional, and cross-cutting climate action plans and decisions in the United States. PLAIN LANGUAGE SUMMARY: Scientific assessments are critical in informing households, communities, businesses, institutions, and governments as they respond to intensifying climate hazards. As climate change impacts accelerate, the pace of assessment and integration of science with decision-making and action must accelerate as well. A network of U.S. climate assessment efforts could rapidly synthesize actionable science and case studies to inform climate response strategies. To support future processes, this paper presents a selection of foundational topics needing assessment and near-term research across climate impacts, risks, and actions from the perspective of the United States and some of its key regions, environments, and economic sectors. For example, assessment is needed for the increasing threats posed by compound extremes and cascading climate risks, which can overwhelm systems developed for independent events. The effectiveness of climate responses likewise requires continuous evaluation and improvement, with consideration of the broader impacts (e.g., economic, environmental, health) associated with those strategies. System-level understanding of integrated climate response strategies can be advanced by combining observations and modeling with mitigation, adaptation, climate interventions, disaster management, and resilience decision scenarios. Overall, these research priorities aim to encourage new research and future national, regional, local, and sectoral climate assessments that support climate-informed decision-making in the United States.

Snow and ice melt form significant contributions to river base flows throughout the Himalayas, but their contribution to seasonal and catastrophic flooding is unclear. As glaciers melt and winter accumulation and temperature patterns shift with climate change, this contribution will cause changes to flood risk. This paper applies a new physical model of streamflow to estimate the separate contributions of precipitation and melt to historical flows at the Bhakra Dam on the Sutlej river, in the Indus River Basin. At the Bhakra dam, 26% of the flow volume of historical floods within our dataset is attributable to melt. A multivariate copula-based probability model produces a similar result for the 10-year flood. Using the model parameters calibrated for the Bhakra inflow, we find that the contribution of melt to flood flows varies over the basin, being higher at high elevation and in smaller basins, and is between 10 to 30% for the larger Indus river basin. We also perform sensitivity checks across a range of increases in temperature and precipitation.

Climate change has very wide-ranging economic impacts. Comprehensive economic assessments are therefore needed, yet existing estimates remain fragmented, incomplete, and deeply uncertain. This paper develops an emulation methodology to achieve a much more complete economic valuation of climate risks. We apply it to the United Kingdom, which provides a test case due to the availability of many modeled and expert elicitation estimates. The approach provides channel-specific estimates, quantifies climatic and socioeconomic uncertainties, disaggregates to 406 local areas, and crucially includes understudied impacts such as spillovers from other regions and missing risks. While current warming produces welfare losses equivalent to 2% of GDP [95% CI of 1%-4%], damages grow rapidly under the baseline scenario and reach 10% of GDP [2% to 20%] by the end of the century. Comprehensiveness matters: half of the expected total damages and the greatest uncertainties come from catastrophic and missing risks.

Climate change is expected to impose heterogeneous damages throughout the world due to varying levels of hazards, exposure, and vulnerability. The social cost of carbon (SCCO ₂ ), the net present value of welfare losses from an additional ton of CO ₂ , provides a useful synthesis of these damages globally, and marginal damages at the national level highlight the spatial pattern of these damages. This paper presents the PAGE-2025 integrated assessment model with 183 individual countries and five small country groups. We incorporate several critical improvements: a novel mapping of climate uncertainty to country-level warming patterns, an empirical approach to dynamic vulnerability using risk indices, updated abatement cost estimates using NGFS energy model scenarios, and an approximation of the effects of subnational heterogeneity in impacts. These changes result in a global SCCO ₂ distribution with a median of $359 per ton of CO ₂ . Damages are dominated by market damages, which are high in tropical regions and negative at high latitudes, and nonmarket damages, which are high in temperate regions that show the greatest rates of warming. We then decompose the spatial pattern of country-level SCCO ₂ into the role of hazard, exposure, and vulnerability, by calibrating a simple emulator with temperature, population, and income, which captures the country-level SCCO ₂ pattern with high precision. The spatial pattern roughly follows GDP, but with tropical countries at twice their GDP-only SCCO ₂ level and cooler countries below this level. We use this decomposition to evaluate the role of different modeling decisions, including different scenarios, downscaling, damage assumptions, abatement costs, and assumptions about sub-national vulnerability. Across modeling decisions, the greatest changes in spatial pattern are driven by the use of SSP scenarios (rather than the default RFFSPs) and the updating of nonmarket damages damages to reflect Howard and Sterner ( 2017 ).

The results of five Integrated Assessment Models are discussed in this concluding paper. All the scenarios measure the National Social Cost of Carbon (NSCC) across approximately 200 countries in a scenario with no mitigation. Despite assuming similar population and economic growth rates (RFF-SPs), the five models imply a wide range of forecasted climate impacts. Summing the mean NSCC estimates of all countries leads to a mean global Social Cost of Carbon (SCC) that varies from $33/tCO ₂ to $1373/tCO ₂ across the models. Most of the literature on climate IAMs has focused on these SCC values. This special issue, in contrast, is focused on the NSCC. However, the assumptions in each IAM model that led to different SCC values also determine the cumulative magnitude of the NSCCs from each model. The discount rate, the climate damage function, and the modeling of uncertainty strongly influence the magnitude of the NSCCs across models. What is unique to the analysis of the NSCCs is how they are distributed across countries. The NSCC values explain how climate damages and, therefore, the benefits of mitigation are distributed across nations. The IAMs predict that China and especially the United States are particularly exposed to climate change due to their large asset bases. About half of the global damage is assumed to happen in the 10 largest countries in the world, measured using GDP PPP. Most of the models predict every country has some damage, but one model predicts 29 countries — mostly in high latitudes — will benefit from near-term climate change. Regression analysis confirms that the distribution of climate damages across countries is primarily determined by the total size of the national economy. Because IAMs scale physical risks by economic exposure, they predict that two-thirds of the global climate burden is concentrated in the mid-to-high latitudes, where the majority of global economic activity is located. Finally, the IAMs predict a very wide range of NSCC values in 2100, ranging from a 40% increase to an eight-fold increase over current NSCC values, depending on structural assumptions regarding adaptation and catastrophic risk.