Chemtrails: a not-so-distant future?

Included in the Royal Society's (2009) Geoengineering the climate: Science, governance and uncertainty report is a table and figure replicated in Figure 1 which compares the relative effectiveness, affordability, safety, and timeliness of various geoengineering methods, granting them a score between 1 and 5.

Figure 1: A comparison of the effectiveness, affordability, timeliness, and safety of various geoengineering techniques. (Source: Royal Society, 2009).

BECCS and surface albedo enhancement (urban) have been examined on this blog and they are two of the safest methods, although neither score highly for timeliness, affordability or effectiveness. Building from David Keith's TED talk in which he claimed that stratospheric aerosols could be used to plunge the Earth into an ice age extremely quickly for about 0.01% of global GDP, I wanted to explore it in depth as it appears to be the SRM method with the most research behind and most potential.

Forming the basis of the 'chemtrail' conspiracy theory, it is also one of the most controversial methods. GeoengineeringWatch.org claim that stratospheric aerosols are already being dispersed by jet aircraft alongside other harmful chemicals and indeed an international survey of 3015 people showed that 17% of respondents believe this to be true or partly true (Mercer et al., 2011). A survey of 77 atmospheric scientists found that 76 (98.7%) had not uncovered any evidence to support this theory and believed that the 'evidence' used by sites such as GeoengineeringWatch.org could be explained scientifically (Shearer et al., 2016).

The reality of developments in stratospheric aerosol injection

Although aeroplanes have been suggested as a potential method of dispersing aerosols into the stratosphere, alongside high-altitude balloons, artillery guns, towers, and space elevators (Caldeira et al., 2013), very few studies on stratospheric aerosol injection have conducted outdoor experimentation. These include:

• A Russian study which tested the refractivity of aerosols in the lower troposphere by generators aboard helicopters (Izrael et al., 2010).
• The British Stratospheric Particle Injection for Climate Engineering (SPICE) project which aimed to test the potential of a tethered balloon to disperse aerosols but the field experiment was cancelled due to issues with patents (Cressey, 2012).
• The Harvard University Stratospheric Controlled Perturbation Experiment (SCoPEx) project, previously mentioned on this blog, plans to launch a balloon to 20 km altitude and spray up to 1 kg of aerosols to create an air mass which can be tested by equipment on board the balloon (Dykema et al., 2014).

How does it work?

In 1991, Mount Pinatubo erupted and created eruption columns containing approximately 17 megatons of sulphur dioxide (SO2) reaching 40 km in altitude, with the resulting aerosol cloud attaining global coverage within a year and causing a global surface cooling in excess of 0.5°C in 1992 (Dutton and Christy, 1992; Self et al., 1993). Literature praising sulphur aerosols as a potential geoengineering technique often point to the fact that they are created naturally and their impacts have been studied.

Figure 2: The 1992 Mount Pinatubo eruption. (Source: https://pubs.usgs.gov/fs/1997/fs113-97/)

SO2 oxidises in the atmosphere to form H2SO4 (sulphuric acid) which then undergoes a process called nucleation, which is the condensation of this gaseous precursor to create aerosols; solid or liquid particles suspended in a gas. Depending on their physical properties, different aerosols possess varying scattering or absorbing properties: sulphate aerosols reflect almost all radiation they encounter whereas black carbon aerosols, the subject of an upcoming lecture, absorbs radiation and warms the atmosphere as well as reducing surface albedo.

There are two methods to instigate this nucleation to form a stratospheric sulphate aerosol cloud: the release of SO2 (the precursor gas) or the release of H2SO4 directly. H2SO4 is more costly than SO2 to release (you reading this can purchase 1 tonne of SO2 on Alibaba for as little as $350) but allows the potential for control over the size of the aerosols created and the location of the cloud formed; smaller particles are more effective at reflecting light and have longer lifespans.


What are the potential issues?

Any global SRM method is accompanied by a wealth of political and governance issues (touched upon here and explored in-depth here), but according to MIT research (2009) and Robock (2008) stratospheric sulphate aerosols have numerous direct physical impacts as well, including:

• Regional Precipitation Changes: studies suggest that the Mt Pinatubo eruption did not only cause global cooling but it also severely impacted the hydrological cycle, decreasing precipitation over land and river discharge; causing an increase of drought occurrences in 1992.
• Increased Acid Deposition: if sulphuric acid (or a precursor) is regularly injected into the stratosphere, some will inevitably pass through the troposphere and deposit on the Earth's surface through acid rain which creates a plethora of ecological and health impacts. 
• Ozone Depletion: sulphate aerosols increase the surface area available for the reactions creating the Antarctic ozone hole, which the Mount Pinatubo eruption exacerbated as well as causing mid-latitude ozone concentrations to fall to the lowest levels ever recorded in the year following the eruption (Self et al., 1993).