One idea is to release reflective particles (aerosols) in the upper layers of the atmosphere – the stratosphere (15 to 50 kilo- metres altitude) – to reflect short-wave solar radiation back into space. Less solar radiation then reaches the Earth’s surface, thus reducing global warming.
The discussion of reflective aerosols centres on sulphate aerosols. Volcanic eruptions naturally emit large quantities of sulphate, a compound of sulphur, into the stratosphere. It has long been known that these ash particles reflect short-wave sunlight. For large volcanic eruptions, this can lead to global cooling. The idea of slowing down global warming by spreading sulphate aerosols in the stratosphere was first proposed as early as the 1970s. As the stratosphere has no precipitation to wash away the sulphate, the particles remain at high altitude for a relatively long time – estimated to be one to two years.
As large volcanic eruptions show, spreading sulphate aerosols in such quantities in the stratosphere could have a cooling effect of several tenths of a degree Celsius. However, recent studies underscore that the quantities of sulphate needed for substantial cooling are larger than previously assumed. Also, at excessively high concentrations, the particles would clump together and fall out of the sky more rapidly. There is also the fundamental question of how spreading the necessary quantities of sulphate could be technically implemented. A patent has existed for some years for sulphate to be spread by commercial aircraft by way of an additive in aviation fuel. US researchers are pursuing the idea of spraying aerosols using a hose lifted into the stratosphere by balloon.
The eruption of the Philippine volcano Pinatubo in 1992 caused an estimated 15 to 20 million tonnes of sulphur dioxide to be carried into the stratosphere. This resulted in a global cooling of, on average, around half a degree Celsius in the months that followed. Although a volcanic eruption – unlike deliberate radiation management – is a singular event that carries a lot of material into the stratosphere at one stroke, this example illustrates the scale on which sulphate aerosols would have to be spread in the stratosphere in order to have a significant impact on the climate.
Application readiness and research needs
This RM method remains hypothetical for the time being because as yet there is no mature, economically viable technology for transporting the required quantities of sulphate to altitudes of 20 to 25 kilometres. Also, despite a number of studies on the subject, the risks of this global measure are still unclear. It is largely unknown, for example, to what extent the shading effect at high altitudes might change the evaporation of water at the Earth’s surface and thus the global water cycle. Another unknown is whether the sulphate could increase stratospheric ozone depletion. The sulphate particles would also reduce the quantity of incident solar radiation reaching the Earth’s surface in general and so to a certain extent darken the sky. It is largely unknown what net effect that would have on, for example, plant growth or solar power generation.