Impacts of Geoengineering Using Stratospheric Aerosols

“地球工程”是指采取有目的的行动来减轻无意中造成的全球温室效应。该项目利用气候模型模拟“太阳辐射管理”（SRM）方案的效果，在该方案中，将硫酸盐（SO2）注入平流层以产生反射气溶胶，或者通过人工增强持续存在的海洋层云区域的云凝结核的数量和浓度，使海洋云变亮。在这两种情况下，目标都是通过将阳光反射回太空来冷却地球，以抵消人为温室气体造成的全球变暖。该项目的目标是了解SRM方案的潜在有效性和意想不到的后果。以前的工作，包括PI在以前的国家科学基金会奖下的工作，表明通过平流层气溶胶产生的SRM可能导致夏季降水减少，特别是在依赖夏季季风降雨来生产粮食的地区。在该奖项下进行的工作涉及四个问题：1)平流层地球工程将如何影响地球的水文循环，包括非洲和亚洲夏季季风的降雨？2)平流层地球工程将如何影响地球的海洋和长期气候？平流层地球工程造成的长期气候变化在很大程度上受海洋温度和环流的响应控制。在地球工程期间，El Niño和La Niña的频率将如何变化？冷却信号是否会像火山爆发后观测到的那样，传播到海洋深处？这将如何影响地球工程对长期气候的反应？3)是否存在向平流层注入硫的策略，具有特定的纬度和季节模式，可以产生理想的气候变化，同时避免不理想的气候变化？例如，春季和夏季高纬度注入是否可以在高纬度地区最大限度地发挥冷却效果，同时避免低纬度降水的影响，避免阳光较少的冬季注入造成的浪费？4)平流层地球工程导致的气候变化将如何影响粮食生产？例如，如果印度和中国的夏季降水减少，并伴有降温和日照减少，那么对作物生产的净影响是什么？这些问题是通过内部气候模式实验来解决的，这些模式能够表示气溶胶形成、生长、循环以及化学和辐射相互作用的排放和随后的运输和化学过程，直到它们从大气中消失。除了内部建模工作外，该研究还包括分析地球工程模型比对项目（GeoMIP）的模型输出，该项目是由研究人员在之前的国家科学基金会奖下组织的。GeoMIP由一组地球工程实验组成，这些实验在全球十几个建模中心进行。这些实验在假定的温室气体增加和抵消SRM努力的数量和时间上有所不同。GeoMIP活动将用于确定在以前的研究中发现的季风降水减少是否是一种强有力的行为，这种行为可以在各种具有不同物理、动力学和大气化学公式的气候模型中重现。将通过使用农业技术转让决策支持系统（DSSAT）模型的作物建模模拟来解决季风降雨减少可能导致的粮食减产问题，该模型将由气候模型的输出驱动。这个项目的更广泛的影响源于需要进行这样的研究，以便让决策者和公众了解地球工程方案的潜在成本和收益。

Abstract"Geoengineering" is the idea of taking purposeful action to mitigate the effects of inadvertent global greenhouse warming. This project uses climate models to simulate the effects of "Solar Radiation Management" (SRM) schemes in which sulfate (SO2) is injected into the stratosphere to produce reflecting aerosols, or marine clouds are brightened through artificial enhancements in the number concentration of cloud condensation nuclei in regions of persistent marine stratus clouds. In both cases the goal is to cool the earth by reflecting sunlight back into space to counteract the global warming produced by anthropogenic greenhouse gases. The goal of the project is to understand both the potential effectiveness of and the unintended consequences of SRM schemes. Previous work, including the PI's work under the previous NSF award, suggests that SRM through stratospheric aerosol generation could result in reductions in summertime precipitation, particularly in regions dependent on summer monsoon rainfall for food production. The work conducted under this award addresses four questions:1) How would stratospheric geoengineering affect the planet's hydrological cycle, including the rainfall of the African and Asian summer monsoons?2) How would stratospheric geoengineering affect the planet's ocean and long-term climate? Long-term changes of climate forced by stratospheric geoengineering would be controlled to a large extent by the responses of ocean temperature and circulation. How will the frequencies of El Niño and La Niña change during geoengineering? Will a cooling signal propagate deep into the ocean, as has been observed and modeled after volcanic eruptions, and how will this affect the long-term climate response to geoengineering?3) Are there strategies of sulfur injection into the stratosphere with particular latitudinal and seasonal patterns that can produce desirable climate changes while avoiding undesirable ones? For example, could spring and summer high-latitude injections maximize the cooling effect at high latitudes, while avoiding lower latitude precipitation impacts and wasteful winter injection when there is little sunlight?4) How would the changes in climate that result from stratospheric geoengineering affect food production? For example, if there is a reduction of summer precipitation over India and China, accompanied by cooling and less insolation, what would be the net effect on crop production?These questions are addressed using in-house climate model experiments using models capable of representing the emission and subsequent transport and chemical processes through which aerosols form, grow, circulate and interact chemically and radiatively until they are removed from the atmosphere. In addition to the in-house modeling effort, the research includes analysis of model output from the Geoengineering Model Intercomparison Project (GeoMIP), which the researcher organized under a previous NSF award. GeoMIP consists of a set of geoengineering experiments conducted at over a dozen modeling centers worldwide. The experiments differ in the amount and timing of assumed greenhouse gas increase and counteracting SRM effort. The GeoMIP activity will serve to determine whether the reductions in monsoon precipitation found in previous studies are a robust behavior that can be reproduced across a variety of climate models with different formulations of physics, dynamics, and atmospheric chemistry. The potential reduction in food production due to reduced monsoon rainfall will be addressed through crop modeling simulations using the Decision Support System for Agrotechnology Transfer (DSSAT) model, which will be driven by output from climate models.The broader impacts of this project stem from the need for studies such as this one to inform policy makers and the public regarding the potential costs and benefits of geoengineering schemes.