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Synthetic ammonia production by the Haber–Bosch process revolutionized agriculture by making relatively inexpensive nitrogen (N) fertilizer widely available and enabling a rise in global food production1,2. The Haber–Bosch process relies on fossil fuels (known as grey ammonia production) and emits more than 450 Mt of CO2 annually3. Green ammonia, which is produced using renewable energy, offers a pathway to decouple ammonia production from fossil fuels and reduce CO2 emissions. As a carbon-free fuel, green ammonia could partially replace fossil fuels to decarbonize hard-to-abate sectors such as maritime shipping4. However, the widespread use of green ammonia could have complex environmental and social consequences, as it threatens to add reactive N into the biosphere3 and could disrupt fertilizer markets. In this Comment, we identify opportunities, barriers and open questions related to green ammonia production and usage as a fertilizer and beyond. We then recommend research priorities to avoid unforeseen consequences through research, monitoring and adaptation in real time. 

Abstract Quasi-periodic pulsations (QPPs) are frequently detected in solar and stellar flares, but the underlying physical mechanisms are still to be ascertained. Here, we show microwave QPPs during a solar flare originating from quasi-periodic magnetic reconnection at the flare current sheet. They appear as two vertically detached but closely related sources with the brighter ones located at flare loops and the weaker ones along the stretched current sheet. Although the brightness temperatures of the two microwave sources differ greatly, they vary in phase with periods of about 10–20 s and 30–60 s. The gyrosynchrotron-dominated microwave spectra also present a quasi-periodic soft-hard-soft evolution. These results suggest that relevant high-energy electrons are accelerated by quasi-periodic reconnection, likely arising from the modulation of magnetic islands within the current sheet as validated by a 2.5-dimensional magnetohydrodynamic simulation.