Stratospheric solar geoengineering

The increasing dismal prospects for climate change mitigation propel research on stratospheric solar geo-engineering (SSG). An important focus has been on SSG by sulfate aerosols. However, this technique would lead to a number of adverse side effects including impacts on the ozone layer. Recent studies found that compared to SSG by sulfate aerosols, the injection of solid aerosols such as calcite and alumina particles might result in more effective cooling, while simultaneously reducing some of the negative side effects due to better optical, chemical and microphysical aerosol properties. However, these studies relied on highly simplified model approaches or unconfirmed assumptions, which makes it difficult to assess risks and potential of this SSG technique.

We propose a holistic assessment of SSG by these two particle types. We will do this by (1) performing laboratory experiments to test optical and chemical properties of these substances, and (2) chemistry-climate modeling to assess impacts and risks. The experiments (1) will be performed under stratosphere like conditions to study the hypothesized dissolution of alumina particles in sulfuric acid under persistent UV exposure, as well as to detect the formation of calcium nitrate, chloride or sulfate on the calcite particle surfaces, their morphology and growth rate. For (2), we will develop the representation of the optical properties of these solid particles in our chemistry-climate model SOCOL-AER. This will require significant modification and extension of the existing model code, to account for the treatment of calcite and alumina, including their interactions with acidic gases and H 2 SO 4 -H 2 O droplets, which may lead to either complete coatings or distinct crystalline “islands” with different optical and chemical consequences. We will analyze the impacts of these aerosols on stratospheric chemistry, in particular the ozone layer, as well as heating rate changes, and the resulting climate feedbacks. This will be the first study that interactively couples a global 3-D chemistry-climate model with solid and liquid aerosol microphysics. We expect to provide a reliable scientific basis for judging SSG approaches, with implications for the upcoming assessments by CCMI, WMO/UNEP and the IPCC. In summary, this project will:

- Determine the physicochemical and optical properties of solid calcite and alumina particles under stratosphere-like conditions.
- Model the use of such particles for SSG, assessing their impact on global climate and atmospheric chemistry, including the associated environmental risks.
- Explore the long-term efficiency of this type of geoengineering and investigate feedbacks in future scenarios including ocean coupling.

Ethical aspect:

Detailed assessments of risks and benefits need to be conducted, before the feasibility of certain SSG techniques can be assessed. The National Research Council in the US (NRC, 2015), the British Royal Society (The Royal Society, 2009), and the European EuTRACE initiative (Schäfer et al., 2015) presented international assessments of the potential and risks of SSG in detailed reports, while a global, e.g. UN- based perspective is still missing. However, these reports recommend conducting research on SSG, with the aim of gaining further understanding of the climate system, and its human dimensions. The work proposed here does not suggest actual deployment of SSG, not even investigate its technical feasibility. Such investigations would ethically not be viable, given the lack of governance and knowledge on intended and unintended environmental impacts (Preston, 2013). Rather, we aim to assess the environmental and climatic risks facing SSG, in comparison with the impact of anthropogenic climate change on our lives.