Scientists have known for decades that the particulate emissions from ships can have a dramatic effect on low-lying stratocumulus clouds above the ocean. In satellite images, parts of the Earth’s oceans are streaked with bright white strips of clouds that correspond to shipping lanes. These artificially brightened clouds are a result of the tiny particles produced by the ships, and they reflect more sunlight back to space than unperturbed clouds do, and much more than the dark blue ocean underneath. Since these “ship tracks” block some of the sun’s energy from reaching Earth’s surface, they prevent some of the warming that would otherwise occur.

The formation of ship tracks is governed by the same basic principles behind all cloud formation. Clouds naturally appear when the relative humidity exceeds 100 percent, initiating condensation in the atmosphere. Individual cloud droplets form around microscopic particles called cloud condensation nuclei (CCN). Generally speaking, an increase in CCN increases the number of cloud droplets while reducing their size. Through a phenomenon known as the
Twomey effect, this high concentration of droplets boosts the clouds’ reflectivity (also called albedo). Sources of CCN include aerosols like dust, pollen, soot, and even bacteria, along with man-made pollution from factories and ships. Over remote parts of the ocean, most CCN are of natural origin and include sea salt from crashing ocean waves.

Satellite imagery shows “ship tracks” over the ocean: bright clouds that form because of particles spewed out by ships.Jeff Schmaltz/MODIS Rapid Response Team/GSFC/NASA

The aim of the MCB Project is to consider whether deliberately adding more sea salt CCN to low marine clouds would cool the planet. The CCN would be generated by spraying seawater from ships. We expect that the sprayed seawater would instantly dry in the air and form tiny particles of salt, which would rise to the cloud layer via convection and act as seeds for cloud droplets. These generated particles would be much smaller than the particles from crashing waves, so there would be only a small relative increase in sea salt mass in the atmosphere. The goal would be to produce clouds that are slightly brighter (by 5 to 10 percent) and possibly longer lasting than typical clouds, resulting in more sunlight being reflected back to space.

Solar climate intervention is the umbrella term for projects such as ours that involve reflecting sunlight to reduce…

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