ACtIon4Cooling

Summary

The ACtIon4Cooling project is making use of existing data, algorithms, models and adaptations to propose monitoring capabilities of different Solar Radiation Management (SRM) approaches, including Stratospheric Aerosol Injection (SAI), Marine Cloud Brightening (MCB) and Cirrus Cloud Thinning (CCT).

The project is investigating natural analogues, the building of novel datasets and analysing study cases for the assessment of potential SRM deployments. Synergistic measurements of volcanic aerosols, marine clouds affected by ship-track emissions and aviation-relevant cirrus clouds will also be studied.

ACtIon4Cooling is an ESA-funded project of the German Aerospace Center (DLR), National Observatory of Athens (NOA ReACT) and University of Leipzig aiming at indoor research on Solar Radiation Management (SRM).

Background

The project addresses a critical scientific and policy challenge: understanding and quantifying aerosol-cloud interactions (ACI) to reduce uncertainties in climate projections and assess the feasibility, risks, and governance of solar radiation modification (SRM) technologies as a supplementary climate intervention strategy.

ACtIon4Cooling tackles the persistent uncertainties in effective radiative forcing due to aerosol-cloud interactions, particularly those related to aerosol-induced modifications of cloud micro- and macrophysical properties. Despite decades of research, estimates of total aerosol forcing still vary by over 50%, largely due to limitations in observing and modelling aerosols and their interactions with clouds. Key challenges include insufficient vertical resolution in passive satellite observations, poor characterisation of cloud condensation nuclei (CCN) and ice-nucleating particles (INP), and retrieval dependencies among cloud properties.

Integrated, high-resolution, and vertically-resolved aerosol and cloud measurements, using both passive and active remote sensing, are required to constrain ACI processes. A better understanding of these interactions is essential not only to quantify current climate drivers but also to evaluate the potential of SRM approaches such as stratospheric aerosol injection (SAI), marine cloud brightening (MCB), and cirrus cloud thinning (CCT).

ACtIon4Cooling aims to contribute to the emerging discourse on SRM as a potential emergency or transitional response to rapid climate change, particularly if mitigation efforts fail to meet the Paris Agreement targets. While SRM methods could provide rapid cooling by increasing Earth's albedo, they also pose significant scientific, environmental, and geopolitical risks, including stratospheric ozone depletion, shifts in precipitation patterns, and ethical concerns over governance and deployment. The project seeks to inform responsible SRM research through improved scientific evidence, particularly on the timing, regional effects, and unintended consequences of such interventions.

By advancing the observational and theoretical understanding of ACI, the ACtIon4Cooling supports both climate science and policy. It aims to reduce critical uncertainties in climate modelling, evaluate the physical plausibility and climate impacts of SRM technologies, and provide evidence-based input for governance frameworks on the responsible research and potential deployment of geoengineering solutions.

Aims and objectives

ACtIon4Cooling aims to advance our understanding of aerosol-cloud interactions (ACI) and their implications for climate forcing and geoengineering strategies. Its primary scientific objectives include:

1. Enhancing Measurement and Retrieval Techniques:
   The project seeks to improve the observational capabilities for aerosols and clouds by integrating high-resolution, vertically-resolved remote sensing data from both passive and active instruments. This is intended to overcome current limitations in cloud masking and aerosol retrieval, particularly in terms of capturing critical microphysical properties such as cloud droplet number concentration (Nd), cloud optical thickness (COT), and liquid water path (LWP).
2. Reducing Uncertainties in Radiative Forcing Estimates:
   A key goal is to minimize the large uncertainties—over 50% spread—in estimates of aerosol-induced effective radiative forcing. This will be achieved by refining models of aerosol absorption and the processes governing CCN and ice-nucleating particles, thereby providing a more accurate quantification of aerosol-cloud interactions and their impacts on cloud albedo and lifetime.
3. Supporting Solar Radiation Modification (SRM) Research:
   The project is designed to generate robust scientific evidence to assess the feasibility, timing, and potential efficacy of various SRM approaches, including stratospheric aerosol injection (SAI), marine cloud brightening (MCB), and cirrus cloud thinning (CCT). By elucidating the mechanisms that control aerosol-induced adjustments in cloud properties, the project will help evaluate how these SRM techniques could alter local and global climates, particularly in terms of mitigating rapid warming and extreme weather events.
4. Informing Policy and Governance:
   Beyond its scientific pursuits, the project aims to contribute to the policy discourse on geoengineering by providing detailed assessments of the environmental risks and governance challenges associated with SRM deployments. This includes evaluating potential negative side effects, such as ozone depletion and unintended alterations of regional climate regimes, to support the development of comprehensive, evidence-based guidelines for responsible SRM research and implementation.

Project plan

ACtIon4Cooling is structured into a sequence of interlinked scientific and technical tasks aimed at enhancing the observational and modelling capabilities required to assess and monitor Solar Radiation Management (SRM) techniques, including Stratospheric Aerosol Injection (SAI), Marine Cloud Brightening (MCB), and Cirrus Cloud Thinning (CCT).

The first phase focuses on data acquisition and preparation. This involves collecting satellite (e.g., CALIPSO, EarthCARE, Aeolus, TROPOMI) and ground-based (e.g., ACTRIS, BSRN, AERONET) observations to characterise natural analogues such as volcanic eruptions (e.g., Pinatubo, Calbuco) and ship-track aerosols. These datasets provide essential inputs to constrain microphysical and optical properties of aerosols and clouds relevant to SRM analogues.

The second phase consists of data exploitation and analysis, where advanced radiative transfer modelling (using tools like DOME and pyDOME) will simulate radiation fields at the top and bottom of the atmosphere. These simulations will incorporate realistic aerosol and cloud properties, validated against satellite and ground-based measurements. Special attention will be given to the vertical profiling of aerosols and clouds, particularly in the stratosphere and marine boundary layer.

The third phase focuses on development and validation of SRM observational proxies. This includes investigating satellite-derived metrics (e.g., aerosol layer height, UV absorbing aerosol index) and their sensitivity to SRM-relevant perturbations. The impact of aviation-induced cirrus modification will also be analysed using lidar-based polarization data (PLDR) and linked to cirrus cloud formation processes.

The fourth phase is dedicated to sensitivity studies and scenario simulations. The radiative impact of SRM scenarios will be tested under a range of atmospheric conditions using advanced sensitivity analyses and phase function corrections. These outcomes feed into global and regional climate simulations using the ICON atmospheric model, aligned with GeoMIP/CMIP6 protocols, to evaluate broader climatic consequences such as temperature anomalies, teleconnection shifts, and extreme weather impacts.

A final integrative activity will synthesise all results, facilitating the development of monitoring and attribution requirements for a dedicated SRM satellite mission. Cross-cutting actions include validation campaigns (e.g., PANGEA/ASKOS) and the assessment of uncertainties associated with SRM interventions.

Overall, ACtIon4Cooling combines high-resolution observations, radiative transfer theory, and climate modelling to build a robust scientific basis for assessing the feasibility, effectiveness, and risks of SRM strategies.