As the emissions of carbon dioxide have continued largely unabated over the past decade, a number of people have given thought to geoengineering, or changing the environment in a way that tweaks the planet's thermostat. Although people have suggested some exotic interventions—reflecting sunlight away from the Earth with orbiting mirrors—more serious consideration is being given to pumping sulfur dioxide into the stratosphere. But a new paper in Nature Climate Change suggests that focus might be keeping us from considering even better options.

Sulfur is a major focus in part because we know it will work, since major volcanic eruptions provide a natural test of it. The sulfur released in eruptions can reach the stratosphere, where it combines with water to produce aerosol particles that reflect sunlight back to space. It's estimated that the aerosols created in the wake of the eruption of Mount Pinatubo were large enough to drop the global temperature by half a Kelvin for two years.

Sulfur is also abundant and cheap, making the raw materials for this form of geoengineering relatively inexpensive. In fact, the whole process is expected to be so cheap that some have estimated that it might be within reach of a handful of wealthy individuals. But, even if you were committed to reflecting sunlight back to space, there are some downsides to using sulfur. The sulfur would have to be constantly replenished, and its constant presence at high altitude would trigger chemical reactions that could damage the ozone layer.

So, the authors of the new paper decided to look at alternate materials. We know how small a particle must be in order to remain smoothly distributed in the atmosphere while keeping gravity from pulling it back to earth, and it's possible to measure how much light will be scattered by particles of that size from various materials. As it turns out, there are a lot of materials that are more reflective than sulfur aerosols. In fact, tiny diamond fragments have a refractive index that's 1.6 times that of sulfur aerosols.

The authors, however, suggest that we might want to focus on a slightly cheaper material, like titanium dioxide, which is nearly as reflective. It would work so well that, to create the equivalent of a Pinatubo-scale cooling would require a third of the mass of the sulfur, and only one-seventh of the volume. They also note that silicon carbide has an even higher index of refraction.

The authors suggest we take this efficiency seriously. The stratosphere is a very different environment, so determining how these particles behave under the conditions prevalent there would be a key first step. If the particles end up aggregating and coming out of the air or causing any unfortunate side effects—say, for example, ozone destruction—the authors suggest that it might be possible to put a small layer of inert material on top of them.

And, of course, what goes up must come down. Although these will never reach the levels of dust and aerosols that fall out of the atmosphere naturally, their composition would be different. Environmental impacts would need to be examined.

There are lots of reasons to be hesitant about geoengineering. It does nothing to slow ocean acidification, and it would involve layering one massive uncontrolled experiment (geoengineering) on top of a second: the massive increase in greenhouse gasses we're currently generating. Still, if we eventually decide that geoengineering is necessary, it would be nice to think we'd use the best method for doing it... that we could identify of course.

Nature Climate Change, 2012. DOI: 10.1038/NCLIMATE1528 (About DOIs).