That the Earth is warming as a result of emissions of CO₂ to the atmosphere is unequivocal.  Earth’s clouds provide a cooling mechanism on climate by reflecting sunlight back to space.  Clouds form on particles in the air, and it is well established that more numerous airborne particles lead to clouds with more numerous droplets.  Clouds with more numerous droplets, all else being equal, reflect more sunlight back to space and cool the Earth even more.  Along with CO₂, the concentration of particles in the air has increased since preindustrial time.  This increased level of airborne particles is thought to lead to a larger cooling component that offsets a portion of the CO₂-induced warming.  The actual amount of cloud cooling that has resulted from the increase in airborne particle levels since preindustrial time is quite uncertain because actual particle levels at that time are unknown.  Still, a leading scientific problem associated with climate is the actual physical mechanisms by which changing aerosol levels impact clouds.  Marine stratocumulus clouds, the low-level clouds found over most of the Earth’s oceans, provide, of all Earth’s clouds, the greatest amount of climate cooling, so these clouds have received the most attention in terms of how they respond to changes in airborne particle levels. 

Aircraft measurement studies have been carried out over areas of the ocean where stratocumulus clouds are prevalent and where particle levels vary due to both marine and anthropogenic influence.  In studies of this type, the aircraft must seek areas where the clouds are influenced by varying aerosol levels.  In the NSF project, Collaborative Research:  Using Controlled Aerosol Perturbations to Improve Understanding of Cloud Responses for Climate, Award Numbers AGS-1013423, AGS-1013381, AGS-1008848, and AGS-1013319, a team of researchers from the University of California, San Diego Scripps Institution of Oceanography, the California Institute of Technology, the University of Arizona, the University of Miami, the Georgia Institute of Technology, and the Naval Postgraduate School employed the Research/Vessel Point Sur to serve as a platform for emitting well-characterized organic smoke to produce a uniquely identifiable cloud signature.  The project, call the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE), carried out in July and August 2011 off the coast of California, also measured the effect on clouds of exhaust from container ships transiting across the study region as well as salt particles released from the aircraft on which the airborne particle and cloud measurements were being made.  The investigators quantified the effect of the tailor-made particles, as well as those from the exhaust of container ships, on the properties of the overlying clouds and compared the measured effect with that predicted by cloud physics models.  This collaboration has contributed to 14 peer-reviewed publications (10 accepted as of 9/15/14, and 4 additional papers submitted or in preparation) by the four participating investigators that helped improve the representation of aerosol-cloud behavior in climate models.

Because making the Earth cloudier is one way to offset CO₂-induced global warming, it has been proposed that fleets of ships emitting sea spray into the air could intentionally thicken marine clouds, in activities termed climate engineering or geo-engineering.  While the present NSF project was not specifically directed at geo-engineering, data from it could be useful if geo-engineering by brightening of marine clouds were to be seriously considered.

Last Modified: 09/17/2014
Modified by: Lynn M Russell