Integrated assessment models (IAMs) are central to long-term climate and energy policy analysis, yet widely used global frameworks typically subsume Saudi Arabia within an aggregated Middle East region, obscuring country-specific features that are decisive for credible decarbonization analysis in a hydrocarbon-centric economy. This paper presents GCAM-KSA, the first country-specific implementation of the Global Change Analysis Model tailored to the Kingdom of Saudi Arabia. GCAM-KSA decouples Saudi Arabia from GCAM-Core v7.2 and introduces structural and parameter updates spanning the power sector, refining, buildings, transport, six industrial subsectors, water, agriculture and land-use, and the climate system. Methodological advances include explicit representation of hydrogen and carbon capture and storage (CCS) in refining, CCS-enabled cement and steel pathways, Saudi-specific renewable resource and capacity-factor characterization, a saturation-based cooling and floorspace formulation suited to hot–arid climates, and a desalination system in which combined water-and-power plants operate alongside dedicated reverse-osmosis facilities. The model is solved in 5-year steps from 2015 to 2100. We document the key mathematical formulations, calibration procedures, and data updates, and demonstrate the platform through a Current Policy (2025) and an economy-wide Net-Zero 2060 scenario. Results reveal substantial structural inertia in the Saudi energy system under existing policies, and, under the net-zero constraint, a coordinated multisector response featuring rapid power-sector decarbonization, broad end-use electrification, CCS deployment in refining and heavy industry, and engineered negative emissions via direct air capture. Residual emissions persist in data-aggregated subsectors, pointing directly to priority areas for future data collection and model development. GCAM-KSA provides a transparent analytical platform for cross-sectoral decarbonization analysis in Saudi Arabia and a template for country-specific IAM adaptations elsewhere.

Model-based analysis of fuel pathways is essential for informing energy and environmental policies. Two major model types are typically used: multisector dynamics models, which capture the broader energy economy, such as GCAM (Global Change Analysis Model), and life-cycle assessment models, such as GREET (Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation). Each has distinct strengths and limitations, and recent studies have increasingly adopted hybrid approaches to harness the advantages of both. However, such integration is often time-consuming and complicated by inconsistencies in the system boundaries and technology definitions. We present LC-GCAM, a new tool that enables estimation of life-cycle greenhouse gas emissions and primary energy use for any fuel pathway represented in GCAM. We apply LC-GCAM to 300 scenarios designed to explore key uncertainties affecting the life-cycle performance of future fuel options in the U.S. freight sector. To evaluate LC-GCAM, we compared its results with those from GREET for nine fuel types in a 2030 reference scenario. When input assumptions are modestly aligned, LC-GCAM and GREET estimates typically agree within 13% (aggregate absolute-sum error), although discrepancies can be larger for pathways involving large amounts of land use change emissions. LC-GCAM offers a flexible and efficient approach to generating life-cycle metrics within an integrated modeling framework, supporting robust policy analysis across a wide range of interacting energy system uncertainties.

This study analyzes how spatial resolution and cross-sectoral interactions shape future modeled outcomes for the United States energy system, using the Global Change Analysis Model (GCAM). The analysis of spatial resolution considers two configurations: one representing the United States as a single region and another distinguishing each state as a separate region, using harmonized assumptions for all shared technologies. We find consistent high-level results, but demonstrate how the national resolution can be more accommodating to deployment of selected technologies where the use of nationally aggregated supply curves can mask geographic mismatches between low-cost production and demand. This is illustrated with hydrogen produced by renewable electrolysis, where low-cost production sites (e.g., the Northern Great Plains) are often quite far from major population and industrial centers. Cross-sectoral impacts from accelerated technological advances are weakest for buildings, greater for transportation, and strongest for electricity generation. In buildings, advanced technologies tend to increase electricity demand while also improving its efficiency. In contrast, improvements in the electricity generation sector drive nearly 400~TWh per year of additional electricity demand, primarily in industry and transportation, highlighting comparatively strong sectoral responses here. The study provides commentary on the implications of these results for the roles of single-sector as opposed to energy-system-wide modeling and analysis.

Hydrogen could help to decarbonize hard-to-electrify end uses in future energy systems. While increased hydrogen production, transmission, distribution, and use may lead to an increase in hydrogen emissions, both greenhouse gas (GHG) and non-GHG emissions could decline as hydrogen displaces incumbent fuels and energy carriers. The full suite of potential climate forcing changes from hydrogen deployment has not been fully examined, in part because it requires combining information from different fields. This study addresses this gap by using a well-known integrated assessment model (the Global Change Analysis Model) to combine (1) credible hydrogen deployment scenarios that illustrate which fuels and energy carriers could be displaced by hydrogen; (2) information about emissions of hydrogen and other forcers by technology, sector, region, and time; and (3) a simple climate model capable of translating relevant emissions into changes in radiative forcing. Across all scenarios considered, when compared to a scenario without expanded hydrogen deployment, reduced forcing from lower CO2 emissions is larger than other forcing changes by 2050 even after accounting for hydrogen and other indirect forcers. Forcing attributable to methane emissions may increase or decrease depending on how much hydrogen is produced with natural gas and how much natural gas is displaced as a fuel. Lastly, the net forcing change from changes in emissions of the indirect forcers CO, NOx, NMVOC, and H2, including their impact on methane's lifetime, is always small and in most cases negative at midcentury. These findings raise important questions for technology assessment regarding the treatment of indirect forcers and aerosols.

This paper presents the Global Change Analysis Model for Saudi Arabia (GCAM-KSA), the first country-specific implementation of the Global Change Analysis Model tailored to the Kingdom of Saudi Arabia.

Previous studies on the use of hydrogen (H _2 ) in the future energy system typically find a limited role for the energy carrier; however, these studies have not accounted for geologic H _2 (gH _2 ), which by some estimates may be abundantly available at low cost. Consequently, hydrogen’s role in the energy system could shift from a minor contributor, deploying mostly in difficult-to-decarbonize sectors, to a much more prominent element of the global energy system. In this paper we examine the role of gH _2 for global energy and industrial systems with different scenarios of future radiative forcing. We find that if favorable conditions exist, where gH _2 is abundant and inexpensive ($1/kg), it could lead to expanded hydrogen deployment in a wide variety of energy applications, with power generation the most notable. However, deployment of gH _2 is also found to be sensitive to its production cost, with its use declining rapidly as its production cost approaches the cost of manufactured hydrogen.

Food systems are a major contributor to exceeding planetary boundaries1-3 and poor quality diets are a key mortality risk globally4. Projected population and income growth could exacerbate these challenges5. In response, there are calls for transformation towards healthy and sustainable food systems6-8. However, the scale and distribution of the impacts of this transformation on agriculture are underexplored. Here we show that, by 2050, the transformation of food systems towards healthy diets (adoption of the EAT-Lancet reference diet), improved productivity and halving of food waste results in a fundamental restructuring of global agriculture, aspects of which break with historical trends. Scenario simulations using a multimodel ensemble of ten global economic models show a 6% median decrease in agricultural land (+1% to -26%) compared with 2020 levels. By 2050, agricultural production would be 17% lower than business-as-usual projections (-2% to -32%) and, economically, the value of this production is US$1.6 trillion (26%) lower (+8% to -58%). Within this, the value of livestock production would be substantially lower than current 2050 projections (-49% to -83%), while vegetable, fruit, nut and legume production value would increase by 23% (-33% to +106%). Results are dependent on the assumed policies to achieve the transformation scenario. We highlight a more active role for food policy to consider the benefits of such a transformation (improved population health and reduced environmental pressures) and navigate the political economy of its impacts.

The planetary boundaries framework sets precautionary limits to keep humanity within a safe operating space, aiming to maintain a stable, Holocene-like Earth system. Current methods for estimating these limits, however, create an imbalance by overstating biogeochemical risks relative to climate change. Here we propose a revised, flow-based, method for estimating the climate change boundary, aligned with the other biogeochemical flow boundaries. We find that under a consistent approach, climate change is in greater violation than nitrogen and phosphorus. This is consistent with the widely accepted view that greenhouse gas emissions constitute one of the most pressing biogeochemical issues in environmental protection. The planetary boundaries framework has emerged as a powerful tool for assessing the sustainable habitability of our planet. Reassessing these boundaries from a flow-based perspective demonstrates the critical threat posed by climate change relative to other biogeochemical risks.

The Paris Agreement grants countries flexibility in designing their pathways to net-zero emissions, yet most have focused on economy-wide, cost-effective approaches without clearly defining the role of sectoral emission reductions and/or carbon dioxide removal (CDR). These blanket strategies prioritize low-cost sectors, leaving significant residual emissions and relying on uncertain, largely unproven CDR technologies to bridge the gap—an inherently risky approach. In this study, we introduce a new framework that incorporates the explicit role of sector decarbonization. We examine three variations of sector-specific policies: selective (SECT), universal (SECT-AMB), and equity-informed (SECT-FAIR), and compare them with a conventional economy-wide carbon pricing scenario (CONV), all aligned with limiting warming to 1.5°C. Our findings reveal that by 2060, sector-specific policies could reduce residual GHG emissions by 6–12 GtCO₂/year and lower gross CDR requirements as well by 6–12 GtCO₂/year compared to CONV. They also achieve slightly lower peak warming (by 0.006–0.01°C) and cut air pollution (PM2.5) by over 50%. However, these gains are accompanied by trade-offs, including higher transition costs, increased demand for biomass, water, uranium, and fertilizer, and potential risks to biodiversity from forest loss and land-use shifts. To maximize the climate benefits of sector-specific policies with no or limited sustainability impacts, it is crucial to carefully design and implement these policies with a focus on minimizing resource demands, protecting biodiversity, and addressing potential trade-offs, while also ensuring that they complement, rather than hinder, efforts to achieve net-zero emissions and climate stability.

Hydrogen deployment is projected to expand in energy transition scenarios to decarbonize hard-to-electrify end uses. Hydrogen is an indirect climate forcer, and increased hydrogen production and use may lead to an increase in hydrogen emissions, which could occur during production, delivery, and/or final consumption. At the same time, when hydrogen deploys in the energy system, other energy carriers such as liquid fuels, natural gas, coal, and electricity would be displaced, affecting both CO2 and non-CO2 emissions, including CH4, SO2, NOx, CO, NMVOC, and BC. To our knowledge, the full suite of potential climate forcing changes from hydrogen deployment has not been examined in existing studies, in part because it requires combining information from different fields. This study addresses this gap by using a well-known integrated assessment model (GCAM) to combine (1) credible hydrogen deployment scenarios that illustrate which energy carriers could be displaced by hydrogen; (2) information about hydrogen emission rates and emission factors of other climate forcers by technology, sector, region and time; and (3) a simple climate model capable of translating all relevant emissions, including hydrogen emissions, into changes in climate forcing. Across all scenarios considered, when compared to a scenario without hydrogen deployment for energy, we find that reduced forcing from CO2 emissions dominates all other forcing changes. In addition, the net forcing change excluding CO2 and methane, as well as the net indirect forcing change from CO, NOx, NMVOC, and H2 is negative and small relative to the total forcing change. These results raise important questions for technology and policy assessment regarding the treatment of indirect and aerosol effects.

Saudi Arabia's ambitious goal to achieve a net-zero economy by 2060 offers a unique opportunity to diversify away from fossil fuels while fostering long-term economic resilience and sustainability. Crucial to this transition are energy policies that guide the Kingdom from a fossil fuel-based economy toward carbon neutrality. This study uses GCAM-KSA, a multi-sectoral integrated assessment model tailored to Saudi Arabia's economic and energy systems, to evaluate the impact of early energy transition initiatives on the policy costs of achieving the Kingdom's net-zero target. These initiatives include ongoing and proposed energy efficiency measures, renewable energy deployment, and fuel displacement targets. The study highlights that early implementation of these initiatives can significantly reduce barriers to adopting low-carbon technologies, ultimately lowering the economic burden of achieving the net-zero goal. Compared to a delayed implementation scenario, early action reduces long-term policy costs by 38-72% over the period from 2025 to 2060, driven by accelerated energy system transformation. These findings provide valuable insights into how Saudi Arabia's energy policies can mitigate economic challenges, promote economic diversification, and contribute to global emission reductions, reinforcing the Kingdom's transition to a sustainable net-zero economy.

A fundamental mismatch between countries’ carbon dioxide removal (CDR) responsibilities and their domestic capacities to fulfil them poses a major challenge to achieving the Paris Agreement’s long-term temperature goal. Interregional CDR trade offers a solution, yet there has been no quantitative assessment of how such trade could reshape the economies of exporting regions and impact their economy–food–energy systems. Here we address this gap by integrating country-level CDR trading into a global integrated assessment model, enabling Global South countries to export carbon removal credits to the Global North in exchange for financial transfers. We find that by 2060, the Global South could export approximately 5 GtCO₂ per year in international CDR credits, generating US$3.1 trillion annually in financial transfers and creating 17 million jobs in the CDR sector. However, by 2060, imports of biomass, natural gas, beef, and corn in the Global South could rise by 36%, 18%, 3%, and 2%, respectively

BACKGROUND:Current food systems leave one in ten individuals at risk of hunger while driving unsustainable environmental impacts. Inaction risks further exacerbating negative impacts on both human and planetary health. These challenges emerge from complex system interactions, requiring approaches that engage with this complexity and consider how transformation measures interact across food systems. We aimed to quantify the magnitude and uncertainty of the impacts of key food systems transformation measures both individually and in a bundle using an ensemble of global economic models. METHODS:In this global multimodel assessment, we applied an ensemble of ten state-of-the-art global economic models to evaluate the potential of four key measures in transforming food systems: increasing agricultural productivity, halving food loss and waste, shifting towards healthier diets, and economy-wide climate mitigation policies aligned with limiting warming to 1·5°C. The scenarios used a middle-of-the-road shared socioeconomic pathway for population and gross domestic product growth, climate impact data from Jägermeyr and colleagues, Thornton and colleagues, and Nelson and colleagues, and dietary targets based on the EAT-Lancet healthy reference diet, with model simulations conducted from 2020 to 2050. We then assessed the effect of these measures in isolation and in combination in a bundled scenario. To further understand the interactions between these measures, we conducted a decomposition analysis that distinguishes between the individual effects of a measure (effect when implemented alone), total effects (its contribution within the bundle), and interaction effects (the difference between total and individual effects). This approach aimed to show complementarities and trade-offs that emerge when multiple measures are implemented simultaneously. FINDINGS:Our analysis showed that individual measures in isolation are insufficient to achieve high-level environmental objectives and might generate unintended consequences. In contrast, bundling measures produces co-benefits: avoiding 50% of projected agricultural greenhouse gas emissions by 2050 and almost 20% of anticipated land conversion, while moderating food price increases associated with ambitious climate change mitigation policies. Our decomposition analysis further shows that measures can have varying effects across different dimensions. Although dietary shifts and climate mitigation policies are the largest drivers of environmental benefits (each contributing to a median decline of >10 percentage points in non-CO2 emissions and 5 percentage points in agricultural land use globally), productivity improvements and reducing food loss and waste play essential roles in moderating price increases (each contributing to a median decline of >5 percentage points in average prices). INTERPRETATION:This study highlights the importance of implementing coordinated approaches to food system transformation and climate change mitigation rather than relying on isolated interventions. Comprehensive transformation requires understanding how supply-side and demand-side changes can interact with climate mitigation policies, enabling policy makers to design intervention packages that maximise benefits while minimising trade-offs across environmental, economic, and social dimensions. FUNDING:Bill & Melinda Gates Foundation; Cornell Atkinson Center for Sustainability; Environment Research and Technology Development Fund; the Asahi Glass Foundation; CGIAR Initiative on Foresight; the CGIAR Science Program on Policy Innovations; US Department of Agriculture, Economic Research Service; and the ClimateWorks Foundation, European Union.

The Food Balance Sheets (FBS), compiled by the Food and Agriculture Organization (FAO), serve as a cornerstone dataset for studies on agricultural development, food security, and dietary health, providing a broad overview of global and regional food systems. However, its limited transparency and scalability hinder its application in empirical analysis and multisector dynamic modeling. Here, we present a traceable Food Balance Sheets (T-FBS) dataset, developed from detailed Supply Utilization Accounts (SUA) using a novel Primary Commodity equivalent (PCe) aggregation approach. This framework enables the aggregation of commodity flows along supply chains while ensuring consistency and balance across multiple dimensions. The T-FBS dataset includes 57 PCe commodities across 195 regions for the period 2010–2022, consolidated from over 500 SUA products. While T-FBS closely aligns with FAO-FBS at aggregate levels for dietary energy and macronutrients, it identifies key uncertainties in other elements (e.g., feed, trade, stocks). By enhancing methodological transparency, traceability, and scalability, T-FBS strengthens the robustness of food system studies and fosters future research and collaboration within the open-source community.

The Energy Modeling Forum 37 study is organized around carbon dioxide (CO2) mitigation scenarios reaching net-zero CO2 emissions by 2050 in the United States. This paper summarizes the potential contribution of bioenergy use in the electric power, transportation, industrial, and buildings sectors toward meeting that target based on model results. Thirteen modeling teams reported bioenergy consumption in the Reference and Net Zero scenarios. Consumption of bioenergy increased over time in the Reference scenario, from an average across models of 3.2 exajoules (EJ) in 2020 to 3.8 EJ in 2050. Average bioenergy consumption in 2050 increased further to 7.3 EJ in the Net Zero scenario. All scenarios that reach net-zero emissions required some form of carbon dioxide removal to offset emissions that are difficult to reduce. Carbon dioxide removal using bioenergy with CO2 capture and storage (BECCS) varies widely across models, up to 1000 Mt CO2 in 2050. Some models rely instead on direct air carbon capture and storage (DACCS), up to 2200 Mt CO2, and others use a combination of BECCS and DACCS. Model results show a strong inverse relationship between the amounts of BECCS and DACCS deployed. All modeling teams assumed a carbon sink from land use, land use change, and forestry, further offsetting a portion of emissions from fossil fuels and industry that are expensive to eliminate. Bioenergy consumption in 2050 decreased by an average of 1.5 EJ across eight models in a Net Zero+ scenario relative to the Net Zero scenario, due in part to a lower equilibrium carbon price resulting from optimistic cost assumptions for all energy technologies.

Green ammonia production could contribute to decarbonization and the decentralization of fertilizer production, but it brings critical challenges and risks. Assessing and addressing these challenges in real time will help advance technology and avoid unintended consequences.

Renewable-based ammonia production (hereafter, green ammonia) could present a transformative opportunity for agricultural systems, offering a pathway to decentralize and decarbonize fertilizer production. Modular green ammonia units that can be deployed on-farm are emerging around the world, with individual annual production capacities of 100 to 500 tonnes. Decentralized production is poised to increase ammonia availability and fertilizer accessibility, while decarbonizing production, lowering transport emissions, and enhancing farm resilience to supply chain disruptions. However, high capital and operating costs for modular green ammonia units, as well as access to water and renewable energy, pose significant adoption barriers for farmers. Safety concerns and mismatches between typical green ammonia products ( e.g., anhydrous ammonia) and existing fertilizer practices further complicate integration into the agricultural sector. Critically, widespread green ammonia availability could also risk fertilizer overuse, undermining environmental benefits associated with decarbonized ammonia production. This talk will explore opportunities, challenges and critical considerations for integrating green ammonia into agricultural systems.

Transportation is currently the largest source of U.S. anthropogenic CO2 emissions, at about a third of the total. Achieving net-zero emissions by mid-century will require substantial reductions in transportation emissions across passenger and freight travel. Here we leverage a model intercomparison study to explore the role of transportation in scenarios achieving net-zero economy-wide CO2 emissions by 2050. We find the transport sector is poised to play the most significant role in reducing demand-side emissions, mostly driven by technology substitution, as modeling results suggest a limited role for mode shifting and for reduced use of personal car travel in the U.S. Among various technology solutions, models show agreement that passenger on-road vehicles will largely transition to electric vehicles (EVs), while solutions to decarbonize heavier travel modes are more diverse and include greater use of liquid biofuels and hydrogen. Research should continue to investigate the evolution of on-road electrification, the role of biofuels and hydrogen across heavier travel modes, and the role of mode shifting and travel behavior change to support personal transportation decarbonization at national and regional scales to temper the rapid growth in clean fuel and electricity demand.

The invention of the Haber–Bosch process, which converts inert dinitrogen gas into ammonia, revolutionized agriculture by enabling the large-scale production of nitrogen (N) fertilizers. This innovation has powered global increases in crop production and is essential for feeding a growing population. However, conventional ammonia production is heavily reliant on fossil fuels, contributing approximately 1% of global annual greenhouse gas emissions. Green ammonia—ammonia produced using renewable energy—has emerged as a transformative alternative with significant potential to decarbonize both agriculture and the energy sector. Yet, this shift also brings potential unintended environmental consequences. In the transportation sector, green ammonia is a viable and promising option to decarbonize marine shipping. One projection finds that substituting green ammonia for 44% of fossil fuels in marine shipping would reduce CO 2 emissions by up to 0.38 Gt CO 2 -eq yr -1 but would require an increase in new N synthesis of 212 Tg N yr -1 . Even a modest leakage of un-combusted ammonia or unintended end products, such as nitrogen oxides (NO x ) and nitrous oxide (N 2 O), could exacerbate coastal pollution, disrupt oceanic N cycling processes, and increase emissions of N 2 O, which is the third most important greenhouse gas and the most abundantly emitted stratospheric ozone depleting substance. In the agricultural sector, green ammonia technology could lead to decentralization of fertilizer production, which stands in contrast to the current centralized, carbon-intensive production methods. This shift could enhance fertilizer use and bolster food production in countries where N fertilizer accessibility has been limited, thus improving crop production, economic prosperity, nutrition and food security. However, the current end products of distributed green ammonia production facilities are limited to a few types, such as anhydrous ammonia and aqueous ammonia, which are not widely used in crop production due to concerns for safety and machinery requirements. Ongoing innovation may enable farmers to improve the timing and dosing of fertilizer to better match crop needs and thereby reduce N losses. However, cheap and abundant N fertilizer could also exacerbate the current severe environmental problems of N losses to air and water from overuse and inefficient use of N fertilizers. Overall, many environmental impacts of green ammonia are still largely unknown and poorly quantified. This presentation will offer a framework and initial quantification of these environmental impacts. Along with the pursuit of decarbonizing the economy with green ammonia, it is critical to improve our understanding of its environmental impacts and establish necessary monitoring networks to ensure positive outcomes from its production and utilization.

Food systems exert significant stress on planetary boundaries1–3 while healthy diets are currently unaffordable for billions worldwide4. These challenges are expected to continue under global population trends, projected to reach 9.6 billion by mid-century5. Food systems must therefore transform in pursuit of health and sustainability goals6–8. However, the scale and distribution of this transformation on agriculture is underexplored. Here we show that, by 2050, an EAT-Lancet style food systems transformation results in a fundamental restructuring of global agriculture, aspects of which break with historical trends. Scenario simulations using a multi-model ensemble of 10 global economic models show a 6% median decrease in agricultural land of 274Mha (+1 to -26%, +48 to -1257Mha) compared to 2020 levels. By 2050, agricultural production would be 2 to 32% (-0.2x109 to -3.7x109 tonnes) lower than business-as-usual projections, and economically, the value of this production shows a 26% median relative decline of $1.6tn in USD2020 (+8% to -58%, +$0.5tn to -$2.9tn USD2020) Within this, the value of livestock production would fall substantially (-$1tn to -$2.2tn, -49% to -83%). These results reinforce the need for a more active role for food policy and stakeholder dialogue to catalyse such a transformation and navigate the political economic consequences of its impacts.