|
|
| Fig. 1: Predicted effect on warming with sulfate injections equivalent to Mount Pinatubo's eruption. (Image Source: R. Nemawarkar, after Llanillo et al. [6]) |
Albedo modification, or solar geoengineering, is a theoretical technique that aims to reflect sunlight away from the Earth's surface in order to induce cooling effects that can help counteract temperature increases from global warming. Albedo is the fraction of light a surface reflects, so albedo modification in the context of warming refers to modifying the amount of sunlight the Earth's surface is reflecting. Geoengineering is a method that involves deploying technology at large scales to modify Earth's natural systems to counteract climate change. The form of albedo modification investigated in this report is stratospheric aerosol injection, a process of injecting sulfur dioxide (SO2) into the stratosphere to form sulfate aerosols that reflect sunlight. [1]
Warming can be attributed to an imbalance in the Earth's energy budget. This energy imbalance comes from greenhouse gases absorbing thermal radiation that would've been emitted to space otherwise, which has led to an accumulation of energy in the Earth's system. [2] Since clouds and aerosols have a large impact on radiative forcing, injecting stratospheric aerosols can have a large impact on the imbalance in the energy budget. The IPCC The IPCC calculates the Earth's energy imbalance, caused by increased greenhouse gases, to be approximately 0.79 W/m2, reflecting the net energy retention in the system. [2]
Since this technology hasn't been deployed on large and measurable scales as of yet, researchers look towards volcanic eruptions to see the effects of stratospheric aerosols at similar concentrations, like the eruption of Mount Pinatubo in 1991. The eruption caused a veil of debris in the stratosphere that led to a decrease in absorbed solar radiation. [3] This was due to an increased global albedo of 0.007 due to the reflection of increased stratospheric aerosols. [3] Researchers theorized that the increase in reflection of solar radiation due to the particles resulted in a cooling effect on the surface of the Earth by 0.5°C in the year after the eruption through data received from a microwave sounding unit, which is a satellite device that measures temperatures in the lower stratosphere and troposphere. [4,5]
In order to have a significant impact on reversing the Earth's energy imbalance while also working within the confinements of the environmental effects we have from past volcanic eruptions, Glasow et al. calculated that there needs to be an injection of 5.3 Tg of sulfate (TgS), or 5.3 × 109 kg, in the stratosphere to counteract warming due to CO2 doubling. [6] Using the data from Pinatubo's eruption, the IPCC theorized through modeling that injecting 1 TgS in the stratosphere would lead to a global average vertical optical depth of 0.007, which is a measure of how opaque the layer of reflective aerosols would be in relation to radiation passing through it. [4] Further extrapolating the data observed from Pinatubo, Hansen et al. calculated that there was a radiative cooling effect of 4.5 W/m2 caused by 6 TgS, which was the amount of S that remained in the stratosphere six months following the eruption, which initially introduced 10 TgS in the stratosphere. [4,7]
Glasow et al. modeled the cooling effects of potentially injecting sulfate into the stratosphere equivalent to the 6 TgS found after Pinatubo's eruption, as shown in Fig. 1, and found that warming could be delayed by 53 years. [6] Fig. 1 shows the implementation of stratospheric aerosol injections within IPCC's A1B scenario, and it's influence on the Surface Air Temperature (SAT), the temperature of the air near the Earth's surface. The A1B scenario is a model that assumes a future of rapid economic growth, technological innovations involving increased clean energy usage and other strategies, international cooperation on warming issues, and a continuation of moderate global emissions. [2]
Based on information from the National Academy of Sciences in 1992, Crutzen calculated the estimated cost of injecting 1 TgS into the stratosphere to be $25 billion. [4] Thus, maintaining a sulfate aerosol layer through annual injections of 5.3 TgS would be approximately $150 billion annually, with the need for this number to be scaled to current costs. [4]
Using the timeframe of the effects after Pinatubo as an example, if the aerosols were not annually injected, their effect would last around 3 years. [6] Therefore, if this strategy were to be deployed, it would have to be a part of a long-term plan to continuously administer the injections. [6]
Since this technology has not been deployed at a large scale as of yet, it is impossible to know the potential side effects this could have on the climate or other ecological processes, thus making it a risky solution. After Pinatubo's eruption, the climate system was not drastically changed, so researchers theorize that as long as the injection concentration is kept similar to Pinatubo's levels, the disruption should be minimal. [6] However, Pinatubo did not have a consistent concentration being injected over years. Trenberth and Dai did find that there was a decrease in precipitation after the eruption, though, calculating that in the year following the eruption the precipitation was 3.12 standard deviations below normal - the largest variation within the 55-year period they were examining. [8] This consequence on precipitation levels could lead to an overall decrease in global precipitation, possibly leading to droughts, famine, and hunger.
While there are additional potential effects to the climate that haven't been thoroughly investigated yet, such as acid rain, secondary effects on pH and aragonite saturation (the carbonate ion concentration), ocean acidity, etc., the biggest risk is the impact of uncertainty surrounding this technology's deployment. Uneven deployment due to error or failed coordination between countries could lead to adaptation difficulties of natural ecosystems due to the oscillations between climates. [6] Additionally, there is a risk for aerosol injections to cause damage to the ozone layer, as evidenced by Pinatubo's eruption that had a global column ozone loss of 2.5%. [4]
While there is not much research surrounding the deployment of stratospheric aerosol injections in order to modify the Earth's albedo, thereby making it a risky option for counteracting climate change, it could be useful as a last resort option. The effects of cooling seem to be rapid, as evidenced by Mount Pinatubo's eruption, which is a good reference point for the effect of sulfate aerosols at the concentration that would theoretically be used. Using this over the long term can have side effects relating to decreasing precipitation, increasing ozone layer damage, or other unexpected effects from disrupting natural ecological processes.
© Ruhee Nemawarkar. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] Climate Intervention: Reflecting Sunlight to Cool Earth (National Academies Press, 2015), p. 29.
[2] P. Forster et al., "The Earth's Energy Budget, Climate Feedbacks and Climate Sensitivity," in Climate Change 2021: The Physical Science Basis: Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge University Press, 2021).
[3] B. A. Wielicki et al., "Changes in Earth's Albedo Measured By Satellite," Science 308, 525 (2005).
[4] P. J. Crutzen, "Albedo Enhancement by Stratospheric Sulfur Injections: A Contribution to Resolve a Policy Dilemma?" Clim. Change 77, 211 (2006).
[5] V. Aquila et al., "Impacts of the Eruption of Mount Pinatubo on Surface Temperatures and Precipitation Forecasts With the NASA GEOS Subseasonal-to-Seasonal System," J. Geophys. Res. Atmos. 126, 2021JD034830 (2021).
[6] P. Llanillo, P. D. Jones, and R. Von Glasow, "The Influence of Stratospheric Sulphate Aerosol Deployment on the Surface Air Temperature and the Risk of an Abrupt Global Warming," Atmos. 1, 62 (2010).
[7] J. Hansen et al., "Potential Climate Impact of Mount Pinatubo Eruption," Geophys. Res. Lett. 19, 215 (1992).
[8] K. E. Trenberth and A. Dai, "Effects of Mount Pinatubo Volcanic Eruption," Geophys. Res. Lett. 34, L15702 (2007).