There is no clearer evidence of how controversial geo-engineering (altering the atmosphere so as to reduce global warming, perhaps by lofting a haze of sulfate aerosols into the stratosphere to reduce incoming sunlight) is than the tempest stirred up when White House science adviser John Holdren told the Associated Press that the administration was discussing it. To lots of people concerned about global warming, merely mentioning geo-engineering detracts from the urgency to reduce emissions of greenhouse gases, conveying a message of, “oh, no problem; we can keep making this mess but deploy a techno-fix when we need to.” But let’s leave politics aside for a nanosecond. The risk of geo-engineering is, to put it bluntly, that we’re not smart enough to know what climate effects it will produce.
A 2007 study has already shown that using sulfate aerosols to reduce incoming sunlight, and thereby cool the planet, would also mess up precipitation patterns enough to trigger serious droughts. Now a new study pinpoints what can only be described as a truly ironic side-effect: lofting sulfates into the stratosphere would also reduce the effectiveness of solar power, the renewable, zero-carbon energy source that has the best chance of averting catastrophic climate change.
Writing in Environmental Science & Technology, Daniel Murphy of the National Oceanographic and Atmospheric Administration explains that while enhancing the stratospheric aerosol layer makes some of the incoming sunlight bounce back to space (producing cooling), it scatters even more of the sunlight. For every 1 watt of sunlight reflected away from the Earth, another 3 watts of direct sunlight are converted to diffuse sunlight. Unfortunately, it is direct, rather than scattered, sunlight that large concentrated-solar-energy facilities need in order to reach maximum efficiency; they can’t use diffuse light. (Concentrating systems focus sunlight onto photovoltaic cells, to produce electricity directly, or onto tubes to produce steam or a hot fluid.) Put another way, every 1 percent reduction in sunlight due to aerosols causes a 4 to 10 percent loss in output from concentrated-solar collectors.
Flat solar hot water and photovoltaic panels would experience less of a loss, because they can use diffuse sunlight. But “the performance loss will still exceed the reduction in total sunlight because a tilted panel does not capture diffuse sunlight as efficiently as direct sunlight,” notes Murphy. Bottom line: “a significant reduction in the efficiency of solar power generation systems.”