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

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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.