Climate engineering is generally divided into two pieces: carbon dioxide removal (CDR) and solar climate engineering (SCE; also called climate intervention or solar radiation management). CDR focuses on approaches to removing CO2 from the atmosphere or from the energy production stream and is generally viewed with less concern than SCE. SCE focuses on increasing the shortwave (solar) reflectivity of Earth, which then acts to cool the climate. Once can, in principle, maintain a globally constant surface temperature in the face of rising greenhouse gas concentrations by reducing solar energy absorption to counteract the enhanced greenhouse warming. This balancing act has been investigated in climate models for a number of possible scenarios by the GeoMIP group.
From a practical standpoint, there are two approaches to reducing solar absorption at the global scale: one is marine cloud brightening (MCB) and the other is stratospheric aerosol injection (SAI). Both approaches have strengths and weaknesses. Our research group at the University of Washington has chosen to focus our research efforts on MCB. The basic science of MCB is reasonably well understood. Stratus and stratocumulus clouds at the top of the marine boundary layer (typically 800 to 1000 meters above the ocean surface) are starved for cloud condensation nuclei (CCN), small particles that serve as the starting point for the growth of all cloud drops. In seminal papers in 1974 and 1977, Sean Twomey described how the addition of CCN particles to a polluted marine cloud layer increases the number of cloud droplets, whichare, on average, then smaller in size and therefore more reflective of solar radiation. John Latham published an article in 1990 suggesting that such an approach could be used to cool Earth by increasing the reflectivity (brightness) of marine clouds. Evidence that this approach works is demonstrated by “shiptracks”, long bright streaks that occur in marine clouds under certain environmental conditions when cargo ships pass beneath marine clouds. Our research goal is to investigate the mechanisms by which this brightening occurs, the changes in cloud circulation and solar brightness that follow from a change in the cloud droplet number and size distributions, and whether these changes are predictable and controllable. We propose to meet this goal by a combination of detailed numerical modeling of cloud processes and local atmospheric experiments where sea salt particles are injected into clouds.