Approaches to slowing the rate of global climate change take many forms. One potential way to buy a bit of time while nations work on the more permanent solution of reducing greenhouse gas emissions is to employ various geoengineering methods. Such methods include recapturing carbon compounds already in the air, shading Earth’s surface from the Sun’s rays, and making Earth’s surface more reflective to reduce surface warming. Because any such efforts could have wide-ranging effects, mathematical models and simulations are necessary to evaluate the possible outcomes of proposed geoengineering efforts before they can even be considered for deployment.
A group of approximately 30 scientists met last summer to discuss the latest science and future directions in the Geoengineering Model Intercomparison Project (GeoMIP). This international effort, an official part of the Coupled Model Intercomparison Project Phase 6 (CMIP6), is the largest source of information about the robust climate model response to solar geoengineering: a collection of proposed technologies designed to quickly, temporarily offset the effects of global warming.
A major focus of the meeting was GeoMIP’s contribution to CMIP6. Attendees raised questions about the ability of the participating Earth system models to simulate cirrus clouds and potential aerosol-cloud interactions associated with cirrus thinning. The meeting produced a recommendation to demote the currently proposed cirrus thinning experiment to a tier 2 experiment, effectively making it an optional simulation. In its place, the attendees decided to substitute an overshoot simulation, which models geoengineering deployment as a temporary means of reducing temperatures quickly while mitigation and negative emissions (also known as carbon dioxide removal) efforts ramp up.
There was also discussion of papers to be submitted to the GeoMIP special issue of Atmospheric Chemistry and Physics. Currently, 24 papers have been published, 2 more papers have been accepted for publication, and 4 papers have been submitted to this special issue, which prepublishes papers as soon as they become available. Two GeoMIP papers on sea spray geoengineering are among those submitted to the special issue, containing analyses of the most recently available GeoMIP simulations.
The GeoMIP Testbed continues to provide a platform on which new ideas can be vetted before being adopted by a large number of models. Attendees discussed a recent proposal to the GeoMIP Testbed that involves simulations of direct injection into the stratosphere of sulfuric acid droplets all of similar size, as opposed to the customary simulation of injecting gaseous sulfur dioxide into the stratosphere. This proposal, which meeting participants agreed needs to be vetted in climate model simulations, is designed to understand whether modifying the stratospheric injection strategy can overcome the problem of sulfate aerosol particle growth, which makes the sulfate species less reflective and more prone to falling out of the atmosphere.
Existing proposals to the GeoMIP Testbed, such as G4Foam (brightening the marine surface in select locations) and G4SSA (stratospheric sulfate aerosol geoengineering simulations with specified, internally consistent aerosol surface area density and stratospheric chemical constituents) have been vetted with single models. Meeting participants agreed that they are ready to be considered by the broader GeoMIP community.
Attendees identified a continuing need to involve new communities, particularly focusing on interactions with researchers planning small-scale field tests. There are ongoing efforts to integrate the impact assessment and social science communities into GeoMIP. This project continues to pursue geographic diversity, partnering with many of the major worldwide geoengineering research projects, for example, in Europe and China.
—Ben Kravitz (email: [email protected]), Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, Wash.; and Alan Robock, Department of Environmental Sciences, Rutgers University, New Brunswick, N.J.