A deodorant to save planet Earth...no, really?!

Well, not exactly Lynx Chocolate (other products are available), but many important environmental scientists including Paul J. Crutzen and Philip J. Rasch have discussed injecting aerosols into the atmosphere in an attempt to prevent UV radiation from the sun from striking the Earth’s surface and warming the planet. A key concept with regard to controlling the Earth’s surface temperature is the planet's radiative balance. This is the balance between energy hitting the earth from the sun and outgoing thermal (longwave) and reflected (shortwave) energy. If one side of this balance outweighs the other, for example if there is an unbalanced increase in reflected shortwave energy, then effectively the Earth’s albedo will be enhanced causing a cooler planet. This then leads one to the thought that what if mankind could develop a way of controlling the earth’s radiation balance within the Earth's atmosphere and use this to tackle climate change? Enter, Solar radiation management (SRM). 

Who to thank for such an atmospheric rise (pardon the pun) in environmental thinking …Volcanoes!

The famous eruption of Mount Pinatubo (12/06/1991)
http://volquake.weebly.com/mt-pinatubo-1991.html

This type of geoengineering has long been inspired by volcanoes. During large volcanic eruptions, immense volumes of SO₂ emitted and are converted into Sulfate aerosols in the atmosphere, which then reflect and absorb infrared energy whilst emitting long-wave radiation. On 12^th June 1991, Mount Pinatubo in the Philippines erupted producing the greatest aerosol cloud of the century. Crutzen states that as a result of this eruption, 10TgS of SO₂ was emitted into the stratosphere. The stratosphere (the second layer of the atmosphere, encompassing area 12-50km above the Earth's surface) is the most significant layer of the atmosphere with regards to the planets radiative balance, as not only does this layer contain the ozone layer, but also in the stratosphere sulfate aerosols have long residence time of 1-2 years. In comparison, the troposphere (the first layer of the atmosphere, located below the stratosphere) only has a sulfate aerosol residence time of a few days. This volcanic event initiated the cooling of the Earth by 0.5ᵒC for the following year and thus offered greater hope for the success of SRM as a means for geoengineering.

This notion poses perhaps even more questions, such as: What type of aerosol would we inject into the atmosphere? What quantity of aerosol would be required and where should this be distributed?

Rasch et al state that the most favoured way of injecting sulfate aerosols into the atmosphere are via precursor sulfide gases, such as sulfuric acid, Hydrogen Sulfide (H₂S) and Sulfur Dioxide (SO₂). These precursors become oxidised to end products which importantly contain the Sulfate anion (SO₄^-2). The majority of stratospheric sulfate aerosols undergo further oxidation to form an assortment of Sulfuric acid, water and nitric acid hydrates. A perilous aspect of this task is determining just how much aerosol would be required to reduce global temperatures. Using the effects of the Mount Pinatubo eruption to model SRM, geophysicist James Hansen calculated that this eruption resulted in a radiative cooling of approximately -4.5W/m² caused by 6TgS that remained in the stratosphere six months after the first eruption. This gives a 75% cooling efficiency which offers a basis for suggesting an aerosol quantity that would be required to restrict the Earth's temperature.

This figure shows the widening gap between 'natural and human factors' and 'natural factors
only' for global temperature changes, expressing the clear relationship between human impacts
and global temperature change.
https://www.epa.gov/sites/production/files/2016-07/models-observed-human-natural.png

When analysing the difference between human and natural influences on climate, there is a clear temperature gap of approximately 1.2ᵒC. To reduce temperatures by 1.2ᵒC a radiative cooling of -10.8W/m² is required. When equating this to a sulfate aerosol volume, using a cooling efficiency of 75% we would require 14.4TgS to be emitted into the Stratosphere. However, this would be the upper limit of the amount of aerosol that would be required. This is because sulfur from anthropological aerosols differ to volcanic sulfur particles, they are finer and have a longer residence time. Thus, less than 14.4TgS would actually be required. However, one must also realise that by the time that a form of SRM could be implemented, global temperatures would be even greater than the present day and thus more Sulfate would actually be required. Although this is a huge amount of sulfur, it is still comfortably within the total amount of sulfate that is created every year throughout the world for various purposes. Therefore one can realise that little extra effort would actually be required to meet this increased sulfur demand.

Practical or impractical?...How do we actually inject aerosols into the stratosphere?

An initial idea for ejecting aerosol into the atmosphere was to release Carbonyl Sulfide (COS) from the earth’s surface. COS is thought to be a principle source of sulfur in the stratosphere that is emitted from low activity volcanoes. The stumbling block for this idea is exposed by Crutzen , where he points out that of the total amount of COS emitted into the atmosphere, approximately 74% is absorbed by plants, 21% removed by reactions with OH in the troposphere leaving only 5% of the emitted COS to reach the stratosphere. Therefore when reviewing this option, to make it a successful sulfur pre-cursor it would have to be developed photochemically in the stratosphere to produce sulfate aerosols. Bearing in mind that this gas would also need to have a long residence time whilst being nonreactive with OH^- it appears that this concept for aerosol distribution seems some what implausible at the moment.

Future testing for developing techniques to eject aerosol into the atmosphere seem to point towards the use of high altitude military jets which would have aerosol ejecting features attached to them. This option poses political issues, as questions such as who’s military equipment are to be used? As well as who would be funding this? Questions which are likely to be unanswered for a considerable time.

To conclude, there remains great potential for injecting sulfur aerosols into the environment as a means of increasing the earth’s albedo, as volcanic evidence proves that this model can work. Key difficulties which must be addressed include configuring a definitive sulfur gas pre-cursor to be emitted into the stratosphere as well as deciding how to best deliver the aerosol into the stratosphere. Nevertheless, the overriding factors determining whether this form of geoengineering will ever be introduced will likely be the predicted costs and environmental impacts. Both issues will be discussed in next weeks blog post…