While traveling to the World Economic Forum meeting in China, I came across a new paper that piques my interest. The paper is by David Keith at the University of Calgary (published in the Proceedings of the National Academies of Science), and is a theoretical investigation of how injecting large quantities of precisely engineered particles into the upper atmosphere might provide a cost-effective tool for climate intervention - geoengineering.
The idea of using aerosol particles for messing with climate change isn’t a new one; the idea of injecting sulfate aerosols into the stratosphere to reflect more sunlight away from the earth has been around for a while. But there were a couple of novel aspects of David’s paper that caught my attention.
The first was that he proposes engineering particles as disks a few micrometers wide and around 50 nanometers thick, that are designed to automatically congregate where they are most useful in the atmosphere - in other words, this is a beautiful case of nanotechnology meets geoengineering.
The second aspect of the paper that caught my attention was that I was working with precisely engineered particles not too dissimilar from those that David described back in the 1990s, which got me wondering whether techniques being used then for fabrication of silicon particles could be used for the more complex particles being proposed here.
In a nutshell, David’s idea is to engineer discs around 10 micrometers across and 50 nanometers thick, with a core of aluminum, a top layer of aluminum oxide, and a bottom layer of barium titanate. Injected high enough into the atmosphere (so Brownian motion didn’t muck things up) the discs should align with the lighter aluminum/aluminum oxide side facing up, and the heavier barium titanate side facing down. This is important, because the way these two surfaces interact with air molecules when the particles heat up - as they would do in sunlight - means that there would be a net force pushing the discs up (photophoresis). In effect, the particles would levitate to a stable position in the atmosphere, while keeping their shiny side to the sun - thus reflecting sunlight away from the earth (or increasing albedo).
This approach is a lot more sophisticated than dumping huge quantities of sulfates into the atmosphere, as in principle more could be achieved with less material, and in a more controlled manner. By engineering nanoparticles appropriately, it might also be possible to control where they go even further - by introducing a magnetic component for instance, so they follow the Earth’s magnetic field.
The idea is an intriguing one. The science that David Keith outlines - which admittedly is broad brushstrokes science - is plausible. The forces on discs the size he suggests should be sufficient to keep them aligned in the upper atmosphere - even when the Sun isn’t present for short periods of time. And if sufficient quantities could be produced, they should have a measurable cooling effect. The neat thing of course is that this is a concept that can be tested reasonably easily in the lab, using simulated atmospheres and prototype particles. And with advances in materials manufacturing in recent years, it shouldn’t be too hard to make small batches of the discs.
Which brings me to the second reason the paper caught my eye. Back in the 1990”²s I was interested in how non-spherical airborne particles - including discs - behaved in aerosol samplers. One particular source of particles I played around with was precisely engineered uniform discs, just a few micrometers in diameter, formed using micromachining techniques more usually used to manufacture semiconductor chips.
This was a technique described by Mark Hoover (a good colleague from NIOSH) and colleagues, and developed in the UK by Paul Kaye. By using suitable templates, precisely shaped particles could be etched on the surface of a silicon wafer, then floated off and aerosolized. The result was an airborne cloud of precisely engineered discs.
Of course, Mark and Paul were using silicon as their main material. But with modern Chemical Vapor Deposition techniques, it would be easy to use a similar technique to manufacture the particles described by David Keith. The question then is, how expensive would they be?
In his paper, David estimates that around 10 billion kg of these nano-discs would be needed. That’s a lot - but probably economically viable with large-scale investment in production and if the benefits were deemed important enough (David runs the figures assuming the cost of manufacture is less than 1% the cost of abating CO2 emissions, and arrives at a cost of less than $60/kg).
There is another question though, and that is the question of environmental and human health impact. If the use of such particles was ever explored seriously - even at the laboratory scale - it goes without saying that parallel studies would be needed to understand how they might interact with the atmosphere, environment and people in less than helpful ways, and how adverse impacts might be avoided.
Here again, though, David Keith comes up with a thought-provoking idea: What if the particles were engineered to have a finite lifespan, so that potential adverse impacts were minimized? This might be done, he suggests, by designing particles that degrade over time under UV radiation and a constant assault from oxygen radicals in the atmosphere. Safety by design in other words - an idea that has been discussed in nanotechnology circles for a while (including in the 2006 Safe Handling of Nanotechnology commentary in Nature) - but it’s good to see it being explored in this context.
At present, geoengineering the climate using engineered nanoparticles is just an idea - but it is a plausible one, and shows what can happen when different technologies and ideas begin to converge. One to watch in the future I suspect.