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The authors designed and executed the integrated assessment modeling for the United States Long-Term Strategy. They bring diverse expertise to the modeling and analysis of United States decarbonization. Russell Horowitz, Matthew Binsted, and Haewon McJeon are scientists at the Joint Global Change Research Institute, a partnership between Pacific Northwest National Laboratory and the University of Maryland. Allen Fawcett, James McFarland, and Morgan Browning are economists at the Environmental Protection Agency's Climate Economics Branch. Claire Henly is White House Fellow at the Office of the U.S. Special Presidential Envoy for Climate. Nathan Hultman is the Director of the Center for Global Sustainability at the University of Maryland.

This paper explores potential future implications of climate change on building energy expenditures around the globe. Increasing expenditures result from increased electricity use for cooling, and are offset to varying degrees, depending on the region, by decreased energy consumption for heating. The analysis is conducted using a model of the global buildings sector within the GCAM integrated assessment model. The integrated assessment framework is valuable because it represents socioeconomic and energy system changes that will be important for understanding building energy expenditures in the future. Results indicate that changes in net expenditures are not uniform across the globe. Net expenditures decrease in some regions, such as Canada and Russia, where heating demands currently dominate, and increase the most in areas with less demand for space heating and greater demand for space cooling. We explain these results in terms of the basic drivers that link building energy expenditures to regional climate.

Energy system transformation scenarios satisfying particular climate stabilization objectives, such as 2°C, suggest that a wide range of fossil energy outcomes could be consistent with such objectives. The underlying drivers of variability in these outcomes cannot, however, be easily separated. This paper attempts to shed light on such drivers using a single, state-of-the-art global integrated assessment model (GCAM) to evaluate the energy system implications of climate stabilization near 2°C. We focus specifically on the role of fossil primary energy through midcentury under different assumptions about carbon capture and storage (CCS) and bioenergy supply, as well as about the extent of end use electrification and variable renewable energy (VRE) costs. In our scenarios, coal and natural gas primary energy are most sensitive to the availability of CCS, with bioenergy supply, end use electrification and VRE costs having a secondary impact on these sources. Primary energy from coal declines by midcentury relative to 2015 in all GCAM mitigation scenarios considered here, whereas primary energy from natural gas increases by midcentury relative to 2015 in scenarios in which CCS is available but decreases in scenarios in which CCS is not available. Changes in oil primary energy by midcentury relative to 2015 are generally smaller in magnitude than changes in coal or natural gas primary energy over the same period. We explain these results in terms of fundamental energy-economic relationships and discuss the implications of these findings for broader energy policy and planning.

Typically in human-earth system modeling studies, feedbacks between the earth and human systems are analyzed by passing information between independent models. The reliance on existing Earth System Model outputs limits the ability to explore feedbacks under arbitrary scenarios and equally important limits the ability to explore large-scale uncertainty in these interactions. In this study we explore a wide range of climate uncertainties and incorporate the implications of increased cooling hydrofluorocarbons emissions. We implement a statistical relationship between global mean temperature change and heating and cooling degree days that allows us to produce changes in building energy demands within GCAM at every time step and every region. While there is a general agreement in the literature that increasing temperatures will increase cooling energy demands and decrease heating energy demands, there has been no fully-coupled analysis of this dynamic that would, for example, account for the feedbacks on hydrofluorocarbons from increased cooling demands. The variation in the spatial distribution of temperatures leads to substantial variation in the change in cooling and heating energy across regions, with regions like USA, India and Sub-Saharan Africa experiencing a factor of two difference in cooling demands. While the feedbacks between building energy demand and global mean temperature are modest by themselves, this study prompts future research on coupled human-earth system feedbacks, in particular in regards to land, water, and other energy infrastructure.

In the 2015 Paris Agreement, nations worldwide pledged emissions reductions (Nationally Determined Contributions-NDCs) to avert the threat of climate change, and agreed to periodically review these pledges to strengthen their level of ambition. Previous studies have analyzed NDCs largely in terms of their implied contribution to limit global warming, their implications on the energy sector or on mitigation costs. Nevertheless, a gap in the literature exists regarding the understanding of implications of the NDCs on countries' Energy-Water-Land nexus resource systems. The present paper explores this angle within the regional context of Latin America by employing the Global Change Assessment Model, a state-of-the-art integrated assessment model capable of representing key system-wide interactions among nexus sectors and mitigation policies. By focusing on Brazil, Mexico, Argentina and Colombia, we stress potential implications on national-level water demands depending on countries' strategies to enforce energy-related emissions reductions and their interplays with the land sector. Despite the differential implications of the Paris pledges on each country, increased water demands for crop and biomass irrigation and for electricity generation stand out as potential trade-offs that may emerge under the NDC policy. Hence, this study underscores the need of considering a nexus resource planning framework (known as "Nexus Approach") in the forthcoming NDCs updating cycles as a mean to contribute toward sustainable development.

The concept of the energy, water, and land (EWL) nexus encompasses a growing concern on the availability of vital resources derived from these intertwined systems, and how to manage the nexus resources to respond to the challenges posed by future human demands, aggravated by the perspective of climate change. This paper explores how the Paris Pledges might influence the EWL nexus in Latin America. In particular, it explores the near-term and long-term implications of the Paris pledges on the EWL nexus in four of the major countries/economies in Latin America: Argentina, Brazil, Colombia and Mexico. For this purpose, we employ the Global Change Assessment Model (GCAM), a state-of-the-art integrated assessment model of human and natural processes that captures the national-level EWL synergies and tradeoffs and allows the understanding of the key drivers of the EWL sectoral interactions and the role of climate policies such as the Nationally Determined Contributions (NDCs) that are included in the Paris pledges. Our findings indicate that under the emissions mitigation scenarios explicitly modeled to represent the Paris pledges framework, potential conflicts regarding the use of nexus resources in the four focus Latin American countries may be exacerbated by the induced changes in the energy and land sectors that would impinge upon the water sector of those nations. Despite the differential implications of the Paris pledges on each country, increased water demands for crop and biomass irrigation and for electricity generation were identified as the potential sources of the nexus conflicts that may emerge under this climate policy scenario. Hence, this study underscores the need to refine national climate policies within a EWL nexus resource planning framework such that a balance between the rational use of the nexus resources and stringent climate policies can be found.

The increasing data requirements of complex models demand robust, reproducible, and transparent systems to track and prepare models’ inputs. Here we describe version 1.0 of the gcamdata R package that processes raw inputs to produce the hundreds of XML files needed by the GCAM integrated human-earth systems model. It features extensive functional and unit testing, data tracing and visualization, and enforces metadata, documentation, and flexibility in its component data-processing subunits. Although this package is specific to GCAM, many of its structural pieces and approaches should be broadly applicable to, and reusable by, other complex model/data systems aiming to improve transparency, reproducibility, and flexibility. Funding statement: Primary support for this work was provided by the U.S. Department of Energy, Office of Science, as part of research in Multi-Sector Dynamics, Earth and Environmental System Modeling Program. Additional support was provided by the U.S. Department of Energy Offices of Fossil Energy, Nuclear Energy, and Energy Efficiency and Renewable Energy and the U.S. Environmental Protection Agency.