CAJ - Stratospheric Sulphate aerosols

Stratospheric sulphate aerosols

The last Geoengineering technique in the group of solar radiation management is the spraying of sulphate aerosols into the upper atmosphere, also called the stratosphere.

So far, the strategies to reduce greenhouse-gas emissions have not shown any effective results. The rate of emissions is actually even beyond the level which we thought is too dangerous a couple of years ago. The Intergovernmental Panel on Climate Change foresees a rise of 2.0 to 5.2 or 11.5 degrees in 2100. Scientists expect this rate to become even higher since developing nations such as India or China will burn more coal and use more vehicles. Even if we all stop creating carbon emissions immediately, which will obviously not happen, the problem of global warming would still persist. A great deal of the heat-trapping gas is expected to remain for decades, even for centuries, in the atmosphere. Doomsayers among scientists say that whatever we do with the existing carbon emissions, we will still face the threat of a worsening of global warming. To avoid the planetary warming getting even worse than it is now, scientist keep on saying how important it is to finally take effective revolutionary measures, a plan B so to speak. This is where the geoengineering technique of spraying sulphate aerosols becomes attractive. By implementing this technique, we could easily solve the problem in a short period of time. The best thing about this technique is that the costs would be minimal. To be precise, it would not cost more than military spendings which means a few billion dollars annually. With this in mind, would it not be more useful to invest this money in something important such as in measures to prevent global warming? Shouldn’t humanity have already reached the point where we can evaluate what really is crucial for the future of the planet? Why throw money out of the window? Shouldn’t we do anything to prevent long-term consequences of global warming such as rising sea levels, increasingly severe storms and droughts, melting glaciers as well as permafrost? In fact, the idea of countering global warming with airplanes in the stratosphere, burning sulphur to make aerosols, goes back to the mid-1970s. How and why did a shift in our thinking about global warming occurred?

Let’s get back to the technique itself:

Sulphate aerosols are considered an important part of the Earth system in the stratosphere as well as in the underlying troposphere. These particles have been studied for a long time since they play an essential role in the chemistry of the the lower stratosphere and, when it occurs, after a volcanic eruption. Sinks of aerosols are so much stronger in the troposphere and they only remain in the troposphere for a few days. However, stratospheric aerosol stay in their layer for approximately a year. The fact that their lifetime is longer means that only a few aerosols need to be introduced per unit of time to create an aerosol burden and to have them dispersed over a large area. This dispersion has a global, not a local, effect which is a very important aspect of geoengineering techniques. Sulfate aerosols occur naturally in the stratosphere because of the transport of natural sulphur compounds from the troposphere. Much higher concentrations are introduced when a volcanic eruption occurs, such as what happened with the Mt. Pinatubo. This eruption results in a temporary cooling of the Earth’s system. The eruption causes sulfur dioxide (SO2) to convert to sulfuric acid (H2SO4) that condenses rapidly in the stratosphere to form fine sulfate aerosols. Winds in the stratosphere spread the aerosols until they cover the globe. Once these aerosols are formed, they remain in the stratosphere for approximately two years. They reflect sunlight and thus reduce the amount of energy that reaches the lower atmosphere, the troposphere, and the Earth's surface. On the other hand, they also absorb heat which is radiated up from the Earth. 

The relative coolness of 1993 is thought to have been a response to the stratospheric aerosol layer that was produced by the Mt. Pinatubo eruption in 1991. The volcanic introduction of sulphate aerosols, therefore, serves as a natural simulation to the geoengineering technique. The only difference is that the volcanic aerosols remain in the stratosphere for a few years, whereas the injected particles will have to be introduced regularly to balance the global warming temperatures. 

Just to give you an idea of how many aircrafts you will see flying  in the future if it really comes to the implementation of this geoengineering technique: For a total aerosol mass injection of particles per year, one million flights  and several thousand aircrafts need to be active continuously in the future. 

How can these aerosols be delivered into the stratosphere?

The methods and ways for how to actually introduce sulphate aerosols into the stratosphere have been neglected in the current geoengineering proposals. There are many challenges when it comes to creating a delivery system that can inject the right amount of aerosols. Earlier papers have suggested some delivery system that inject sulphur using artillery shells, high flying jets or balloons. However, among the dangers that exist with these delivery systems is an unequal and inadequate distribution of the aerosols .This could result in the formation of larger particles which are known to rain out quickly and have shorter stratospheric lifetimes. However, factors like particle size, location as well as lifetime of aerosols need to be taken into account when inventing delivery systems.