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Energy systems support technical solutions fulfilling the United Nations’ Sustainable Development 2 Goal for clean water and sanitation (SDG6), with implications for future energy demands and greenhouse 3 gas emissions. The energy sector is also a large consumer of water, making water efficiency targets in4 grained in SDG6 important constraints for long-term energy planning. Here, we apply a global integrated 5 assessment model to quantify the cost and characteristics of infrastructure pathways balancing SDG6 tar6 gets for water access, scarcity, treatment and efficiency with long-term energy transformations limiting climate warming to 1.5 ◦ 7 C. Under a mid-range human development scenario, we find that approximately 8 1 trillion USD2010 per year is required to close water infrastructure gaps and operate water systems consistent with achieving SDG6 goals by 2030. Adding a 1.5 ◦ 9 C climate policy constraint increases these costs by up to 8 %. In the reverse direction, when the SDG6 targets are added on top of the 1.5 ◦ 10 C policy 11 constraint, the cost to transform and operate energy systems increases 2 to 9 % relative to a baseline 1.5 ◦ 12 C scenario that does not achieve the SDG6 targets by 2030. Cost increases in the SDG6 pathways 13 are due to expanded use of energy-intensive water treatment and costs associated with water conserva14 tion measures in power generation, municipal, manufacturing and agricultural sectors. Combined global spending (capital and operational expenditures) in the integrated SDG6-1.5 ◦ 15 C scenarios to 2030 on water and energy systems increases 92 to 125 % relative to a baseline scenario without 1.5 ◦ 16 C and SDG6 17 constraints. Evaluation of the multi-sectoral policies underscores the importance of water conservation 18 and integrated water-energy planning for avoiding costs from interacting water, energy and climate goals.

Global-mean temperature increase is roughly proportional to cumulative emissions of carbon-dioxide (CO2). Limiting global warming to any level thus implies a finite CO2 budget. Due to geophysical uncertainties, the size of such budgets can only be expressed in probabilistic terms and is further influenced by non-CO2 emissions. We here explore how societal choices related to energy demand and specific mitigation options influence the size of carbon budgets for meeting a given temperature objective. We find that choices that exclude specific CO2 mitigation technologies (like Carbon Capture and Storage) result in greater costs, smaller compatible CO2 budgets until 2050, but larger CO2 budgets until 2100. Vice versa, choices that lead to a larger CO2 mitigation potential result in CO2 budgets until 2100 that are smaller but can be met at lower costs. In most cases, these budget variations can be explained by the amount of non-CO2 mitigation that is carried out in conjunction with CO2, and associated global carbon prices that also drive mitigation of non-CO2 gases. Budget variations are of the order of 10% around their central value. In all cases, limiting warming to below 2 °C thus still implies that CO2 emissions need to be reduced rapidly in the coming decades., Environmental Research Letters, 10 (7), ISSN:1748-9326, ISSN:1748-9318