A Hitch in Plans for "Sunshade Earth"

13 March 2009 (All day)

Randy Montoya/Sandia National Laboratories

Dimmer future? Dusting the skies to tame global warming could cut the efficiency of solar-energy-collecting mirrors.

It might make for gorgeous sunsets and help to cool the planet, but seeding Earth's stratosphere with dust particles called aerosols has a cost. A researcher has found that the aerosols could block part of the sunlight collected by solar panels for heat and electricity--in some cases by significant amounts--thereby compromising an important source of green energy.

Aerosol particles come from sources as diverse as industrial smokestacks and volcanoes. Even at low concentrations, they can scatter sunlight, lessening its intensity and therefore the amount of heat it delivers to the surface. Scientists were treated to a firsthand look at the effect in 1991, when Mount Pinatubo erupted. The Philippine volcano ejected about 15 million metric tons of sulfur-dioxide–laden dust into the air, cooling the planet's average temperature by about 0.6°C for nearly 2 years. Based on that event, some scientists have advocated replicating it artificially to help offset greenhouse-gas buildup in the atmosphere (ScienceNOW, 28 January). The idea would be to spread sulfur aerosols by high-flying jets, balloons, and even rockets on a periodic or regular basis.

But how effective would such a scheme be? That's what an atmospheric physicist at the National Oceanic and Atmospheric Administration's (NOAA's) Earth System Research Lab in Boulder, Colorado, attempted to quantify. Using well-established data on the light-diffusing effects of aerosol particles, Daniel Murphy calculated that the geoengineering scheme currently envisioned could reduce incoming sunlight by about 3%. That squares with data from the Mount Pinatubo eruption.

The geoengineering scheme would also mean 3% less sunlight reaching flat photovoltaic collectors that generate electricity. But the aerosols would cut the available solar radiation even more to dish- and tube-shaped collectors that use mirrors to concentrate sunlight. Murphy's research, reported in this week's issue of Environmental Science and Technology, shows that for every watt per square meter of sunlight diffused by the aerosols, as much as 5 watts per square meter would be made unavailable to mirrored collectors on the ground. The dimmed sunlight would also reduce the energy collected by solar water heaters, although Murphy's paper did not calculate the ratio specifically.

Such a reduction demonstrates the need to pay careful attention "to the potential unintended consequences of employing geoengineering concepts," says Murphy's NOAA colleague, atmospheric chemist A. R. Ravishankara.

The paper marks another possible strike against using the particles to geoengineer the climate, says atmospheric scientist Simone Tilmes of the National Center for Atmospheric Research in Boulder. The first is the evidence, studied by Tilmes and colleagues last year (ScienceNOW, 24 April 2008), that sulfur particles could damage the planet's already fragile ozone layer. "Perhaps we should concentrate on mitigation efforts," she says, and attempt geoengineering as "a last resort, until more research is done."