Scientific Method / Science & Exploration

Weighing the pros and cons of stratospheric geoengineering

One of the first cost-benefit analyses of stratospheric geoengineering looks …



The science behind sulfate aerosols

To grasp how such a "global cooling" scheme might work, it helps to first understand the chemistry behind sulfate aerosols. At their most basic level, aerosols are very fine suspensions of solid and liquid particles in gaseous form that range in size from 0.1 to 1 micrometer (�m) in diameter. These aerosols normally persist in the lower stratosphere (the second highest layer of the atmosphere) at small background concentrations. They're typically concentrated above industrial areas, and most are washed out by the rain every few days, though some endure for longer periods. 

Natural aerosols like dust have always been present, but their growth has been greatly outpaced by that of anthropogenic aerosols over the last half-century. These are produced when various precursor gases emitted from natural and human activities, such as sulfur dioxide (SO2) and dimethyl sulfide (DMS), are oxidized through a series of reactions in the stratosphere to sulfuric acid (H2SO4) gas. (At high enough concentrations and under the right conditions, they can cause acid rain.) 

The resulting aerosol particles tend to be long-lived (though not in comparison to greenhouse gases), often lasting for several years. This is largely due to the stratosphere's stability: warmer layers lie on top of the cooler layers, and there is very little mixing between the two. By contrast, aerosols in the troposphere, the lowest level of the atmosphere, are subject to a high degree of mixing (troposphere derives from, tropos, the Greek word for "turning" or "mixing") and therefore have much shorter lifetimes, on the order of a few days. The stratosphere's inherent stability, in addition to its naturally occurring layer of sulfate aerosols, thus makes it the more attractive choice for geoengineering.

Aerosols exert their cooling effect through two mechanisms: the aerosol "direct effect," and the aerosol "indirect effect." They act primarily by reflecting solar radiation back into space (the “aerosol direct effect”), thus increasing the planet’s albedo and lowering temperature levels. They can also act as cloud condensation nuclei (CCNs), providing a scaffolding around which cloud droplets can coalesce and form clouds (the “aerosol indirect effect”). The higher the number of particles, the more reflective the clouds tend to be.

Volcanic aerosols in the stratosphere
Image credit: USGS

Volcanic eruptions and "natural" geoengineering

The closest natural analogs to stratospheric geoengineering that researchers have been able to study are volcanic eruptions. The 1982 El Chich�n and 1991 Mt. Pinatubo eruptions pumped over 20 million tons of sulfur dioxide into the stratosphere, producing a haze that enveloped large areas of the world and significantly lowered temperatures by scattering incoming solar radiation.

Following the Mt. Pinatubo eruption, global temperatures dropped 0.5�C (0.9�F) and remained depressed for over a year. At the current rate of emissions growth, the equivalent of one Mt. Pinatubo eruption every 4 to 8 years would be needed to block, or at least slow, the current warming. To simulate the cooling effect of volcanic eruptions, Robock and his colleagues analyzed the costs of several proposed methods to inject the aerosol precursor gases into the stratosphere, including aircraft and balloons.

Flying military aircraft into the stratosphere

Existing US military planes like the F-15C Eagle or KC-135 Stratotanker could be used to dump sulfur gas or other sulfuric acid precursors (1 to 1.5 tons each) into the lower stratosphere. Smaller aircraft, some of which can reach altitudes of 20 km, would be deployed above the tropics, while larger aircraft, which can't fly as high, would be sent to the Arctic.

While many issues on the logistics end would first have to be resolved—the size of the payload, the method by which it would be dispensed, and the potential risks to the pilots, among others—the authors argue that, because climate change has national security implications, the military would have a direct stake in seeing this happen.

Furthermore, because the military has already built more aircraft than would be required for this scheme to work, it would help keep maintenance and equipment costs down. Assuming 167 F-15Cs capable of carrying 8 tons of payload are sent out 3 times a day on 2-hour flights, 250 days a year (for a combined payload of 1 teragram (Tg) of sulfur gas per year), the scheme would cost around $4.175 billion a year. A fleet of 15 KC-135s, each capable of carrying up to 91 tons of payload, would do the same for roughly $375 million a year.

The authors briefly turn their focus to naval rifles, which could be used to fire artillery shells filled with aluminum oxide (Al2O3) dust into the stratosphere (this would also lead to the formation of stratospheric aerosols). To make this scheme work, 40 10-barrel stations operating 250 days a year would be required—at a hefty cost of $30 billion a year. Though they do not state it outright, it seems clear that the plan’s high cost and impracticality make it one of the more unrealistic options.

Blowing up weather balloons

They spend the last part of their study investigating the potential of weather balloons, which have several advantages over planes. Balloons do not require any fuel and are already being sent up into the atmosphere on a daily basis to monitor the weather. They can be made out of plastic to better withstand the stratosphere’s low temperatures and can be filled with hydrogen gas, which is both less costly and more buoyant than helium.

One of the options they consider is mixing hydrogen and hydrogen sulfide (H2S) inside a balloon and letting it rise into the stratosphere. (Hydrogen sulfide is preferable to sulfur dioxide in this case because it has a lower molecular weight.) The balloon would eventually burst at the lower pressure, releasing the hydrogen sulfide, which would then oxidize and form sulfate aerosols.

With this method, a total of 9 million balloons, or roughly 36,000 per day, 250 days a year, would be required to put one Tg of sulfur gas into the stratosphere. Including the necessary investments in personnel and infrastructure, this plan would cost somewhere between $21 and $30 billion a year. Another option would simply be to load each balloon with a set amount of payload, but that plan would be much less cost-efficient. One problem with both of these plans, the authors wrily note, is the amount of plastic pollution they would create—about 100 million kilograms of falling plastic every year.

Balancing the risks and benefits

Once one comes down on the ethical implications of geoengineering, however, it seems clear that while it may offer some benefits over the short-term, its long-term impacts remain highly risky and uncertain, a point the authors readily acknowledge. (And, lest you think otherwise, Alan Robock is hardly a geoengineering booster: in an article published last year in The Bulletin of Atomic Scientists, he laid out 20 reasons for why we should oppose it.)

More importantly, as several recent studies have demonstrated, stratospheric geoengineering could accelerate ozone depletion, produce more severe droughts, and significantly curtail the world’s solar energy potential. On a more practical level, the cooling effects of sulfate aerosols would be very localized and short-lived. Quantifying these risks, and weighing them against the possible benefits, will be necessary for society to determine whether or not stratospheric geoengineering is worth pursuing.

One of the major challenges of geoengineering research remains figuring out how to set up an experiment on a large enough scale in order to study its impacts without also triggering a number of these less desirable side effects on an equally large scale. Small-scale experiments, at best, would only allow researchers to investigate the initial formation of aerosols or tweak certain parameters in their schemes. In the meantime, the authors conclude, scientists should focus on improving their climate models and finding alternatives.   

Geophysical Research Letters, 2009. DOI: 10.1029/2009GL039209

The Bulletin of Atomic Scientists, 2008. DOI: 10.2968/064002006

Listing image by U.S. Air Force Academy

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