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Stratospheric Aerosol Injection (SAI) aims to offset some climate hazards by injecting aerosols into the stratosphere to reflect solar radiation. The lifetime of injected particles influences SAI's radiative efficacy—the ratio of radiative forcing to particle mass flux. We employ a Lagrangian trajectory model with particle sedimentation to simulate how background circulations influence the transport of passive particles (without microphysical growth) in the stratosphere and quantify sensitivities of particle lifetime to injection locations. At 20 km, optimizing injection locations can increase particle lifetime by >40%. Injection strategies can be constrained to maintain an interhemispheric balance of particle lifetime without significantly decreasing total lifetime. Generally, increasing injection altitude increases particle lifetime while also increasing costs and environmental impacts of deployment aircraft. Optimizing injection latitude and longitude can relax this altitude‐lifetime trade‐off by increasing lifetime without needing to increase altitude, which warrants further testing in global climate models with aerosol microphysics. Plain Language Summary: Stratospheric Aerosol Injection (SAI) aims to reduce climate change by increasing the amount of aerosols in the stratosphere. These additional aerosols can reflect additional sunlight to partially offset the energy imbalance caused by greenhouse gases. The lifetime of injected particles in the stratosphere is one of the important factors that can influence the cooling effects of SAI, as particles that stay longer in the stratosphere can reflect more sunlight over their lifetime. We use observed stratospheric winds to simulate the transport of injected particles and then calculate the particle's lifetime in the stratosphere, aiming to understand how lifetime is related to the location and season at which the particles are injected. For particles injected at 20 km altitude, we can increase particle lifetime by >40% by optimally choosing injection locations. Increasing injection altitude can increase particle lifetime while also increasing the costs and environmental impacts of deployment aircraft. Our results suggest that optimizing injection latitude and longitude to increase lifetime can relax the trade‐off between altitude and lifetime by increasing particle lifetime without needing to increase injection altitude. Key Points: Injection longitudes influence particle lifetime because of zonal asymmetry of poleward winds, especially in the lower stratosphereOptimizing injection latitude and longitude can increase stratospheric lifetime of injected particles without increasing injection altitudeInjection strategies can be developed to maintain an interhemispheric balance of particle lifetime without compromising total lifetime [ABSTRACT FROM AUTHOR]

Over its multibillion‐year history, the Earth has experienced a wide range of climates. The long‐term climate is controlled by the atmospheric carbon dioxide concentration, which is regulated by marine sequestration through chemical weathering. This chemical weathering sink is strongly linked to the distribution and composition of the continents. However, the effect of continental distribution has never been studied within a general framework. Here we show that the global weathering rate is sensitive to the size and shape of the continents, but is not well explained by the amount of land in the tropics. We construct synthetic continental configurations and use an ensemble of global climate model simulations to isolate the expected effect of continental arrangement on weathering and carbon burial. Runoff patterns are complex, sensitive to detailed features of continental geometry, and poorly predicted by continental latitude. These results help explain the long‐term variability and irregularity of Earth's climate. Plain Language Summary: Chemical weathering of the continental crust draws down atmospheric carbon dioxide and regulates the global climate on geological timescales. The weathering process is thought to be controlled primarily by temperature and runoff. Therefore, a concentration of continental landmasses in the tropics has long been considered a factor leading to higher weathering rates and global cooling. We rigorously test this hypothesis by running a large ensemble of climate simulations with random continental configurations. Surprisingly, we find that tropical land fraction and mean continental latitude are poor predictors of global weathering rates. We also find that although the size of the continents is important, it is not a dominant factor either. A significant fraction of weathering variability is driven by precipitation and runoff patterns, which are sensitive to detailed aspects of continental geometry. This sensitivity indicates that factors other than the latitudinal position of the continents (such as continental breakup or crustal composition) are important controls on the Earth's long‐term climate. Key Points: We simulate climate and weathering with an ensemble of randomly generated continental configurationsTropical land fraction and continent latitude are poor predictors of weathering rates, but degree of continental breakup is betterHalf of the weathering variability is produced by complex, hard to predict interaction between continent geometry and atmospheric dynamics [ABSTRACT FROM AUTHOR]