Making the wakes of ocean ships brighter could cool the Earth by 0.5°C and help combat global warming, according to a new modeling study. Other geoengineering studies have examined how greenhouse gas warming could be counteracted by making Earth’s atmosphere more reflective. But this is one of the first to look at using the bright, bubbly wakes of cargo ships as they crisscross the world’s oceans. Natural foaming agents in the sea—chemicals often derived from phytoplankton—help create bright, white microbubbles in ship wakes that persist for about 10 minutes. Now, climate scientists say that designer foaming agents could create even brighter wakes that last much longer. If these supercharged wakes were 10 times brighter and lasted 10 days instead of 10 minutes, they would cover 5.5% of the world’s oceans and cool the planet by 0.5°C by the year 2069, the researchers write in a 28 January publication of the Journal of Geophysical Research: Atmospheres. That’s enough to partially restore Arctic ice loss and offset the 2°C warming that could occur by then. Because most ship traffic is in the Northern Hemisphere, most of the cooling would be felt there, where it would be accompanied by a drop in precipitation.
Depending on the average hydrosol lifetime, offsetting doubled CO2 might thus require annual injection of only 100- 1000 kg of microbubbles per capita, a level of necessary effort within the limits of industrial precedent, as the mass requirements are smaller in scope than global CO2 emissions.
Solar radiation management schemes could potentially alleviate the impacts of global warming. One such scheme could be to brighten the surface of the ocean by increasing the albedo and areal extent of bubbles in the wakes of existing shipping. Here we show that ship wake bubble lifetimes would need to be extended from minutes to days, requiring the addition of surfactant, for ship wake area to be increased enough to have a significant forcing. We use a global climate model to simulate brightening the wakes of existing shipping by increasing wake albedo by 0.2 and increasing wake lifetime by ×1440. This yields a global mean radiative forcing of −0.9 ± 0.6 Wm−2 (−1.8 ± 0.9 Wm−2 in the Northern Hemisphere) and a 0.5°C reduction of global mean surface temperature with greater cooling over land and in the Northern Hemisphere, partially offsetting greenhouse gas warming. Tropical precipitation shifts southward but remains within current variability. The hemispheric forcing asymmetry of this scheme is due to the asymmetry in the distribution of existing shipping. If wake lifetime could reach ~3 months, the global mean radiative forcing could potentially reach −3 Wm−2. Increasing wake area through increasing bubble lifetime could result in a greater temperature reduction, but regional precipitation would likely deviate further from current climatology as suggested by results from our uniform ocean albedo simulation. Alternatively, additional ships specifically for the purpose of geoengineering could be used to produce a larger and more hemispherically symmetrical forcing.