Scientists in the US have been cloud-spotting over shipping lanes and have noticed something more interesting than teddy-bear shapes and faces. They have detected that rising steam from passing ships has caused brightening in the clouds which they theorize alters the reflectivity of the cloud and prevents the energy from reaching the Earth. They propose that if this could be achieved artificially via geoengineering it could be an effective defense against global warming.

Previous research into clouds in shipping lanes showed that natural or man-made particles of dirt, water and gas known as aerosols change cloud characteristics by increasing the number of droplets while decreasing their size. Findings suggested that these droplets reflect more sunlight and make the cloud seem brighter. However, it also revealed the cloud seemed darker in places, and was unclear in conclusion whether the net effect led to brighter, more reflective clouds or whether in fact the two effects canceled out one another.

To address this question, the Pacific Northwest National Laboratory (PNNL) modeled the reflectivity of clouds in a detailed simulation that aimed to determine the net effect of increased aerosols on cloud reflectivity. Chief climate scientist Phil Rasch and his team at the US Department of Energy Office of Science's PNNL in Richland, Washington, simulated three ships chugging along in a 93-mile by 37-mile area of the Pacific Ocean, several hundred miles southwest of Los Angeles. Their findings suggest that introducing aerosols near the surface would, in fact, result in cloud brightening and reflectivity and therefore form an effective tool against global warming, except in clouds already drizzling which would be largely unaffected.

While the aerosols currently affecting clouds in shipping lanes are expelled from ships as polluting steam, the artificially-brightening aerosol could be seawater sprayed from ocean vessels. But there are still unanswered questions as to how safe, efficient or predictable such methods might be. Rasch and his team are also using the simulation to explore when might be the best time of day to spray, given further information as to how the aerosols are affected by climate and prevailing weather; what effect brighter clouds have on rain, whether aerosols are burned off by the rising sun and how long they maintain a brighter cloud.

This is one of a few circulating ideas to fight global warming with geoengineering and is not the first of these to suggest modification of Earth's atmospheric particles. Tim Flannery's slightly disturbing idea compounds global dimming with the proposed addition of sulfur to aircraft fuel that will change the color of the sky when dispersed but leave protective reflective particles in the atmosphere. Another suggestion that less brings to mind an apocalyptic sky is from Colorado's National Center for Atmospheric Research (NCAR) supporting white roofs in densely-populated warmer climate cities. It is clear that drastic times call for drastic measures and the sky is the limit.

Rasch's paper Climate effects of geoengineering using cloud seeding and stratospheric aerosols was presented in a symposium "Can geoengineering save us from global warming?" at the American Association for the Advancement of Science 2010 annual meeting at the San Diego Convention Center on Saturday. He also discussed the results from a Community Climate System Model study which found that increasing aerosols in the ocean air can either increase the amount of sea ice to previous levels, restore precipitation, or reduce temperatures, but not all at the same time. These were published in environmental research letters in December 2009.