Cooling the Atmosphere? – Solar Radiation Management
On June 17, international experts and DLR researchers gathered at the Earth Observation Center (EOC) to exchange views on Solar Radiation Modification (SRM). SRM refers to potential measures aimed at cooling the Earth but which may also have far-reaching side effects. The meeting was organized within the framework of the ESA-funded project ACtIon4Cooling. This project investigates the potential impacts, side effects, and risks of SRM. Neither DLR, ESA, nor any of the ACtIon4Cooling partners are planning or advocating for climate intervention through Solar Radiation Management.
All measures interfere with processes that are not yet fully understood. Therefore, their consequences are to be analyzed and assessed using already observable interactions in the atmosphere. Earth observation data is used, for example, to analyze the effects of volcanic eruptions and "artificial" clouds, and global SRM effects are simulated using the ICON climate model.
The following measures are specifically being investigated: the injection of aerosols into the stratosphere, marine cloud brightening, and cirrus cloud thinning.
The Stratospheric Aerosol Injection (SAI) approach is based on the idea that aerosols in the stratosphere can block sunlight from entering the atmosphere, potentially cooling the Earth. Major volcanic eruptions in Earth’s history have had massive climatic consequences. The year 1816, for example, became known as the "Year Without a Summer," attributed to the eruption of Mount Tambora in Indonesia the year before. The cooling effect of sulfur dioxide on the atmosphere was first proposed as a geoengineering method by Chemistry Nobel laureate Paul Crutzen in 2006.
Marine Cloud Brightening (MCB), on the other hand, targets lower layers of the atmosphere. Overall, clouds have a cooling effect and could be artificially generated over the ocean. Today, clouds already form behind ships, as pollutants in their exhaust plumes provide condensation nuclei for water vapor to accumulate and form droplets or clouds. In MCB, sea salt spray could take over the role of pollutant aerosols in serving as condensation nuclei.
The Cirrus Cloud Thinning (CCT) technique aims to improve Earth's thermal radiation. Cirrus clouds are very high clouds made of ice particles. While they reflect some sunlight back into space and thus have a cooling effect, they also trap heat radiated from Earth, intensifying the greenhouse effect. Overall, the warming impact outweighs the cooling. The idea behind CCT is to introduce suitable aerosols into cirrus clouds to promote the rapid growth of larger ice crystals and prevent the formation of homogeneous, fine ice layers. As a result, the ice clouds could become more transparent to Earth's outgoing heat radiation.
What all these measures have in common is that they address the symptoms rather than the causes of climate change. They involve high technical and financial costs and interfere with a highly complex system that, despite significant scientific progress, remains only partially understood.
The meeting, attended by over 50 experts, was organized by the ESA-funded projects ACtIon4Cooling (led by researchers from the EOC) and Statistics (GRASP France). The event featured four keynote talks from researchers at ETH Zurich, University of Exeter, TU Delft, and the University of Chicago, along with two panel discussions including experts from GRASP France, ETH Zurich, the National Observatory of Athens, TU Delft, Cornell University, and Imperial College London.