A new study published on August 8 in Nature, a scientific journal, suggests that the positive effects of aerosols intentionally released into the atmosphere ― a form of solar geoengineering ― would be offset by the negative response associated with less sunlight resulting in reduced photosynthesis. In the newest study assessing the potential of solar radiation management, the authors used historical volcanic eruptions as a “natural experiment,” in the form of observational evidence, to determine the potential effects of releasing stratospheric sulfate aerosols on crop yields.
Solar engineering is an overarching term used to describe the various technologies proposed to reflect sunlight away from the earth in an effort to minimise global warming. Stratospheric aerosol injection is one such strategy that involves releasing aerosols, like sulphur dioxide, into the upper atmosphere. The aerosols produce particles that reflect sunlight back into space thereby cooling the earth. These so-called solar radiation management techniques, inspired by volcanic eruptions, are frequently considered, yet all the possible outcomes ― both geophysical and economic ― remain largely unknown, and no rigorous experimental testing has been done.
Past events leading to global cooling were caused by gases emitted during massive volcanic eruptions, such as Mt. Pinatubo in the Philippines in 1991, therefore, it is purported that purposely injecting sulfate aerosols into the upper atmosphere could artificially cool the earth’s surface and reduce the amount of global warming being caused by increased levels of greenhouse gasses such as carbon dioxide. Pinatubo released approximately 20 million tonnes of sulfur dioxide into the stratosphere, thereby reflecting of sunlight back into space (a 2.5 percent reduction) and dropping the global average temperature by approximately 0.5 degrees Celsius.
To study the impact on agriculture, the yields of vital crops, including maize, soy, rice, and wheat, were assessed between 1979 and 2009 in 105 countries and global satellite observations of volcanic aerosols were collected. The results were then combined within global climate models to estimate the potential loss of sunlight owing to a sulfate-based geoengineering program, as well as crop loss, which was then contrasted with the intended beneficial effects of protecting crops from the damage due to extreme heat.
The results suggest that damage caused by the scattering of sunlight by solar radiation management would be roughly equal to the purported benefits due to cooling. The researchers were able to show that crops yield were in fact negatively affected by the resulting reduction in sunlight since the reduced sunlight led to a direct influence on plant photosynthesis.
The findings provide important insights into the potential widespread impact of something like solar geoengineering. While the results shed some light on the need to consider all probable outcomes ― both good and bad ― solar geoengineering should not be written off completely. However, understanding the consequences of implementing some of the proposed methods for helping humanity manage the impact of global warming will crucial to their success.
The study did not address possible beneficial effects on human health and natural ecosystem functions, moreover, other forms of geoengineering, such as the capture and storage of carbon dioxide, may prove to be more useful.
(1) Proctor, J. et al. Estimating global agricultural effects of geoengineering using volcanic eruptions. Nature (2018). DOI: 10.1038/s41586-018-0417-3