Exploring the Potential and Side Effects of Climate Engineering

Considering the ambitious climate targets of the Paris Agreement to limit global warming to 2°C, with aspirations of even limiting it to 1.5°C, questions arise regarding how to achieve this. Geoengineering has been proposed to minimise global harm from anthropogenic climate change, in combination with mitigation. In this project, we used the Norwegian Earth System model, NorESM1-ME, to evaluate the climate response when transferring from a high CO2 forcing scenario, RCP8.5, to a middle-of-the-road forcing scenario, like RCP4.5, using aerosol-based geoengineering. Three different techniques were considered: stratospheric aerosol injections (SAI), marine sky brightening (MSB) and cirrus cloud thinning (CCT). The climate states appearing when using aerosol-based geoengineering are found to be closer to the RCP4.5 climate pathway although CO2 emissions are high as in RCP8.5, and many anthropogenic global warming symptoms are alleviated. All three techniques resulted in similar lowered global mean temperature evolutions. However, there were some notable differences in other climate properties due to the nature of the forcings applied. CCT acts mainly on the infrared terrestrial part of the radiation budget, as opposed to MSB and SAI blocking sunlight. This yields differences in the climate response, particularly in the hydrological cycle. We found also other climate responses due to geoengineering such as changes in natural variability modes including the North Atlantic Oscillation, sea ice, sea level, ocean heat and circulation, as well as the carbon cycle. We found in our experiments, that all three geoengineering forms alleviate Arctic summer sea ice conditions. On the other hand, ocean acidification rates could be exacerbated and the affected area in the interior North Atlantic broadened, which could have detrimental impact on benthic biodiversity. Our results emphasize the uncertain changes to the projected ocean productivity when geoengineering is implemented and therefore advocate caution at any deliberate attempt involving large-scale perturbation of the Earth system. Sudden termination of large-scale geoengineering deployment showed in our model, that the climate very rapidly, within two decades, reverts to the path of RCP8.5, questioning the sustainable nature of such types of geoengineering, and urging the need for simultaneous mitigation during any such form of geoengineering.

Facing the steadily increasing risk of the human society failing to meet the 2-degree target, scientists have recently started exploring the possibility of manipulating the climate by deliberate measures that go beyond mitigation. In particular, Climate E ngineering (CE), or geoengineering, has been proposed as a possible future option in the event of a climate emergency. In order to meet the main objective, the project will carry out dedicated simulations using the Norwegian Earth System Model, exploring and comparing aspects of three different climate engineering techniques, followed by a careful statistical analysis of the results. An important aspect in clarifying the potential future role of climate engineering is our ability to separate the effect of climate engineering (signal) from natural (and man-made) climate variability (noise). This will be addressed in the project. Also, special attention will be given to the response of biogeochemical interactions, both on land and in the ocean. The purpose of this project is not only to learn more about aspects of climate engineering per se, but also to gain new insight into interactions between forcings, internal variability and response in the climate system. A large emphasis will be put on open publicati on of all results as well as outreach. The project addresses several topics highlighted in the call, such as internal vs. external influences on climate variability, biogeochemical cycles, and the relationship between human influence and natural variabil ity. The project applies innovative research ideas to a new research topic in Norway, and builds a new team of scientists with pre-existing expertise and a strong international network in an emerging field of climate research.