Geoengineering Using Oceanic Microbubbles

The most commonly studied solar radiation management schemes reduce the amount of solar radiation absorbed by reflecting more sunlight in the stratosphere (eg. stratospheric injection of SO2) or by making marine clouds more reflective (eg. injection of sea salt into the marine boundary layer). Both these schemes require technologies that do not currently exist and involve polluting the atmosphere. An alternative scheme which brightens the surface of the ocean without the use of chemicals has so far been largely overlooked. The technology already exists to efficiently create 1μ radius bubbles in water. Such bubbles could enhance the albedo of open sea by 0.2 and have a lifetime of the order of days (Seitz, 2010). The top of atmosphere radiative forcing produced by the wakes of existing large ocean going vessels has been estimated to be 0.14 mWm-2 (Gatebe et al, 2011). However, existing ships are not optimised to produce such small bubbles; their wake lifetimes are in the order of minutes and their albedo gains are of the order of 0.02. If fitted with existing bubble generation technology, the current fleet of cargo and merchant ships could provide a surface radiative forcing of around -1 Wm-2.We present results from a climate model simulation implemented with this enhanced albedo in current shipping lanes. Currently ships are more abundant in the Northern Hemisphere but a larger and more uniform forcing could be produced by sending out extra ships with bubble generators in the more sparsely populated Southern Hemisphere oceans. Our climate model simulation with a uniform open sea albedo enhancement of 0.03 had an effective radiative forcing of -2.6 Wm-2 and reduced global mean temperature by 1.6 K, enough to offset global mean warming under RCP4.5 for at least 40 years.