The COVID-19 pandemic is impacting human activities, and in turn energy use and carbon dioxide (CO₂) emissions. Here we present daily estimates of country-level CO₂ emissions for different sectors based on near-real-time activity data. The key result is an abrupt 8.8% decrease in global CO₂ emissions (−1551 Mt CO₂) in the first half of 2020 compared to the same period in 2019. The magnitude of this decrease is larger than during previous economic downturns or World War II. The timing of emissions decreases corresponds to lockdown measures in each country. By July 1st, the pandemic’s effects on global emissions diminished as lockdown restrictions relaxed and some economic activities restarted, especially in China and several European countries, but substantial differences persist between countries, with continuing emission declines in the U.S. where coronavirus cases are still increasing substantially.

To prepare for an urban influx of 2.5 billion people by 2050, it is critical to create cities that are low-carbon, resilient, and livable. Cities not only contribute to global climate change by emitting the majority of anthropogenic greenhouse gases but also are particularly vulnerable to the effects of climate change and extreme weather. We explore options for establishing sustainable energy systems by reducing energy consumption, particularly in the buildings and transportation sectors, and providing robust, decentralized, and renewable energy sources. Through technical advancements in power density, city-integrated renewable energy will be better suited to satisfy the high-energy demands of growing urban areas. Several economic, technical, behavioral, and political challenges need to be overcome for innovation to improve urban sustainability.

Hydrogen has been promoted as a revolutionary fuel for 50 years, yet usage is confined to oil refining and fertilizer production. For hydrogen to advance global decarbonization, many barriers must be overcome. In this Perspective, we examine the challenges hydrogen faces from production to usage, assessing its environmental and economic credentials, controversies and uncertainties. We provide the evidence base for companies and governments to assess clean hydrogen’s current and potential future competitiveness. Fuel cell cars and space heating are among the least promising applications owing to rapid advances in direct electric alternatives. Hydrogen holds potential in industry, long-duration energy storage and long-haul transport, but its competitiveness depends on large-scale deployment yielding substantial cost reductions. Current production cost estimates range by a factor of five and suggest that targets for 2030 will be difficult to achieve, especially once costs for transport and storage are included. The climate impacts of hydrogen production are also uncertain, with production from electrolysis or methane gas with carbon capture potentially increasing system-wide or upstream emissions, alongside water scarcity and persistent organic pollution. Future research must resolve these uncertainties, with strategic focus on deploying hydrogen in priority areas where it is most competitive.