Climate Engineering on Land: Comprehensive evaluation of Earth system impacts of terrestrial carbon dioxide removal (CE-LAND+)

陆地二氧化碳清除（tCDR）方法，包括植树造林和专用生物质种植园，有时被称为有效的、“绿色”和安全的气候工程（CE）选择，因为它们增加了生物圈作为CO2汇的自然功能，而且似乎在经济上是可行的。然而，CE-LAND项目第一阶段的证据表明，tCDR与其他CE选项一样具有争议性，因为它指出了困难的地球系统和伦理考虑。因此，在对tCDR方法进行单纯的CO2预算和经济考虑的同时，还需要对这些方法的环境限制、它们对地球系统的影响以及相关的不确定性、它们与其他土地和水的使用之间的权衡以及它们潜在的深远的伦理影响进行合理的分析。对项目第一阶段的假设情景的分析表明，要使tCDR有效，需要大量的土地面积，因此，将出现与其他土地用途的坚韧权衡。此外，研究结果表明，对气候系统（除了预期的全球平均温度降低）和陆地生物地球化学循环将产生重大的副作用。CE-LAND+将对tCDR生物圈改造的非经济成本进行更深入、定量和空间明确的评估。它将讨论潜在的权衡和影响沿着，并通过采用两个地球物理植被模型、一个地球系统模型和该项目新的动态生物多样性模型，系统分析其估计中的主要不确定性。具体而言，CE-LAND+将分别调查最大限度地扩大tCDR面积和生物多样性保护之间的权衡-迄今为止基本上没有量化-以及最大限度地扩大tCDR淡水供应与粮食生产和河流生态系统之间的权衡。此外，还将量化背景气候变化和与tCDR相关的土地利用变化对水资源短缺（基于不同的衡量标准，并假设灌溉管理水平不同）和生物多样性的（非）直接影响。权衡和影响将结合全球可持续发展目标，以及减缓气候变化，即生物多样性保护、淡水安全和粮食安全-优先方案中没有提供的背景。此外，该项目将通过研究模型结构差异、高温、高CO2条件下tCDR植物的生长和死亡率以及tCDR土地使用活动与气候之间的反馈，帮助更好地理解和量化tCDR在未来气候变化下的影响的关键不确定性。最后，CE-LAND+将在优先计划内与我们的一组代表性方案合作，从环境伦理的角度评估tCDR相关的权衡。

Terrestrial Carbon Dioxide Removal (tCDR) methods, comprising afforestation and dedicated biomass plantations, are sometimes referred to as effective, 'green' and safe climate engineering (CE) options, as they increase the biosphere's natural function as a CO2 sink and appear to be economically feasible. However, evidence from the first phase of the CE-LAND project indicates that tCDR is as controversial as other CE options as it points to difficult Earth-systemic and ethical considerations. Mere CO2 budgeting and economic consideration of tCDR methods thus needs to be supplemented by sound analysis of their environmental limits, their impacts on the Earth system with related uncertainties, their tradeoffs with other land and water uses, and their potentially far-reaching ethical implications. Analysis of hypothetical scenarios in the first project phase suggests that substantial land areas would be required for tCDR to be effective, such that tough tradeoffs with other land uses would emerge. Moreover, results indicate that significant side-effects on the climate system (besides the intended reduction of global mean temperature) and on terrestrial biogeochemical cycles would occur. CE-LAND+ will deliver a more in-depth, quantitative and spatially explicit evaluation of non-economic costs of biosphere transformation for tCDR. It will address potential tradeoffs and impacts along with a systematic analysis of major uncertainties in their estimation by employing two biogeophysical vegetation models, one Earth system model and, new to the project, dynamic biodiversity models. Specifically, CE-LAND+ will investigate the - hitherto largely unquantified - tradeoffs between maximising areas for tCDR and biodiversity protection on the one hand, and between maximising freshwater availability for tCDR and both food production and river ecosystems on the other hand, respectively. Also, the (in)direct impacts of background climate change and tCDR-related land use change on water scarcity (based on different metrics, also assuming different levels of irrigation management) and biodiversity will be quantified. The tradeoffs and impacts will be contextualised with a view on global sustainability goals besides abating climate change, namely biodiversity conservation, freshwater security and food security - a context otherwise not provided in the Priority Program. Furthermore, the project will contribute to a better understanding and quantification of key uncertainties of tCDR effects under future climate change by examining model-structural differences, growth and mortality of tCDR plants under high-temperature, high-CO2 conditions, and feedbacks between tCDR land use activities and climate. Finally, CE-LAND+ will contribute, in cooperations within the Priority Program and with a representative set of our scenarios, to the evaluation of tCDR-related tradeoffs from an environmental ethics perspective.