China has announced its ambitious targets towards carbon neutrality by 2060. The Guangdong-Hongkong-Macau (GHM) region, as a pilot demonstration area for China's reform and opening-up, faces dual pressures on providing low-carbon electricity for meeting its surging demand while limiting carbon emissions. Here, we develop an energy system optimization model with high spatio-temporal resolution that integrates investment planning and operation optimization to explore transition pathways for the GHM power system under various decarbonization scenarios. Power system operations in hourly resolution are included to quantify spatio-temporal variability of generation and demand. We show that reducing carbon emissions by 70%, 85%, and 100% in 2050 requires a total system cost of 619.1, 628.3, and 653.1 billion USD (values in 2021 USD), with an average decarbonization cost of 4.79, 5.27, and 7.11 USD per ton, respectively. Accelerating transition to carbon neutrality by 2035 incurs a total system cost of 686.9 billion USD and an average decarbonization cost of 9.11 USD per ton. Moreover, nuclear, offshore-wind, and imported electricity cover over 80% of electricity demand when achieving carbon neutrality and thus serve as cornerstones for supporting the GHM power system transition. Furthermore, we observe that a high fossil fuel price benefits emission mitigation, while reinforcing transmission networks considerably reduces system transition costs. Combing low renewable and storage prices, a high electricity import ratio, and transmission network expansion delivers a lower bound for system transition and a negative average decarbonization cost for the GHM region.

As the decarbonization of the electric power sector gathers pace, electric power systems will need to evolve in multiple ways: larger amounts of intermittent renewables will need to be deployed, as will dispatchable power generators, transmission lines, and energy storage solutions. Some of these resources could provide the electric power system with much-needed dispatchability, but the value of each will evolve over time and space, as will the impact of their deployment on economic costs and carbon emissions. This study systematically compares the roles that different sources of dispatchability could play in a power system's decarbonization and the evolution of its topology over the period from 2020 to 2060, with China as a case study. Results show that adding these resources to a system's operation yields multiple benefits for decarbonization. These include higher levels of renewables integration (+46 000 TWh, +19%), cost reductions (-5800 billion RMB, -13%), and lower GHG emissions (-23 billion tonnes, -23%). The role of different resources changes throughout the transition: transmission always plays the dominant role; dispatchable coal and nuclear generation reduces renewable curtailment up to 2030; pumped-hydro storage reduces curtailment from 2030 to 2040; and battery storage and concentrating solar power become critical after 2040. This evolution could redraw the economic and environmental maps of China by redistributing infrastructure, renewables integration and carbon emissions to less-developed, remote areas. These results help prioritize efforts to facilitate power system decarbonization over several decades; they also yield insights regarding the regional and sustainable development impact of the low-carbon transition.

Oxygen deficient zones (ODZs) account for about 30% of total oceanic fixed nitrogen loss via processes including denitrification, a microbially mediated pathway proceeding stepwise from NO3- to N-2. This process may be performed entirely by complete denitrifiers capable of all four enzymatic steps, but many organisms possess only partial denitrification pathways, either producing or consuming key intermediates such as the greenhouse gas N2O. Metagenomics and marker gene surveys have revealed a diversity of denitrification genes within ODZs, but whether these genes co-occur within complete or partial denitrifiers and the identities of denitrifying taxa remain open questions. We assemble genomes from metagenomes spanning the ETNP and Arabian Sea, and map these metagenome-assembled genomes (MAGs) to 56 metagenomes from all three major ODZs to reveal the predominance of partial denitrifiers, particularly single-step denitrifiers. We find niche differentiation among nitrogen-cycling organisms, with communities performing each nitrogen transformation distinct in taxonomic identity and motility traits. Our collection of 962 MAGs presents the largest collection of pelagic ODZ microorganisms and reveals a clearer picture of the nitrogen cycling community within this environment.

As a key biogeochemical pathway in the marine nitrogen cycle, nitrification (ammonia oxidation and nitrite oxidation) converts the most reduced form of nitrogen - ammonium-ammonia (NH-NH3) - into the oxidized species nitrite (NO) and nitrate (NO). In the ocean, these processes are mainly performed by ammonia-oxidizing archaea (AOA) and bacteria (AOB) and nitrite-oxidizing bacteria (NOB). By transforming nitrogen speciation and providing substrates for nitrogen removal, nitrification affects microbial community structure; marine productivity (including chemoautotrophic carbon fixation); and the production of a powerful greenhouse gas, nitrous oxide (N2O). Nitrification is hypothesized to be regulated by temperature, oxygen, light, substrate concentration, substrate flux, pH and other environmental factors. Although the number of field observations from various oceanic regions has increased considerably over the last few decades, a global synthesis is lacking, and understanding how environmental factors control nitrification remains elusive. Therefore, we have compiled a database of nitrification rates and nitrifier abundance in the global ocean from published literature and unpublished datasets. This database includes 2393 and 1006 measurements of ammonia oxidation and nitrite oxidation rates and 2242 and 631 quantifications of ammonia oxidizers and nitrite oxidizers, respectively. This community effort confirms and enhances our understanding of the spatial distribution of nitrification and nitrifiers and their corresponding drivers such as the important role of substrate concentration in controlling nitrification rates and nitrifier abundance. Some conundrums are also revealed, including the inconsistent observations of light limitation and high rates of nitrite oxidation reported from anoxic waters. This database can be used to constrain the distribution of marine nitrification, to evaluate and improve biogeochemical models of nitrification, and to quantify the impact of nitrification on ecosystem functions like marine productivity and N2O production. This database additionally sets a baseline for comparison with future observations and guides future exploration (e.g., measurements in the poorly sampled regions such as the Indian Ocean and method comparison and/or standardization). The database is publicly available at the Zenodo repository: https://doi.org/10.5281/zenodo.8355912 (Tang et al., 2023).

Extreme climate events are becoming more frequent, with poorly understood implications for carbon sequestration by terrestrial ecosystems. A better understanding will critically depend on accurate and precise quantification of ecosystems responses to these events. Taking the 2019 US Midwest floods as a case study, we investigate current capabilities for tracking regional flux anomalies with "top-down" inversion analyses that assimilate atmosphericCO(2)observations. For this analysis, we develop a regionally nested version of the NASA Carbon Monitoring System-Flux system for North America (CMS-Flux-NA) that allows high resolution atmospheric transport (0.5(degrees )x 0.625(degrees )). Relative to a 2018 baseline, we find the 2019 US Midwest growing season net carbon uptake is reduced by 11-57 TgC (3%-16%, range across assimilated CO2 data sets). These estimates are found to be consistent with independent "bottom-up" estimates of carbon uptake based on vegetation remote sensing (15-78 TgC). We then investigate current limitations in tracking regional carbon budgets using "top-down" methods. In a set of observing system simulation experiments, we show that the ability of atmospheric CO2 inversions to capture regional carbon flux anomalies is still limited by observational coverage gaps for both in situ and satellite observations. Future space-based missions that allow for daily observational coverage across North America would largely mitigate these observational gaps, allowing for improved top-down estimates of ecosystem responses to extreme climate events.