Artificially cooling the Earth through technologies that reduce incoming sunlight would not be sufficient at preventing extreme warming over long timescales, if not coupled with cuts to greenhouse gases, a new study has confirmed.
The research uses modelling to examine what would happen if, hypothetically, greenhouse gases were left to spiral over the coming century while “solar geoengineering” was used to reduce global warming.
Solar geoengineering is a term used to describe a set of largely still-hypothetical technologies that would reflect sunlight away from the Earth in order to reduce global temperature rise and some of its associated impacts.
The most commonly proposed method for achieving this would be through the release of reflective particles, known as aerosols, into the stratosphere. Once released, the aerosols would form a protective sheath around the Earth, scientists expect, reflecting away incoming sunlight.
The idea has a natural analogue in volcanic eruptions, which in the past have temporarily cooled global temperatures. When a volcano erupts, it often sends an ash cloud high into the atmosphere, which can lead to the production of aerosols that reflect away sunlight.
Although solar geoengineering would theoretically be able to lower temperature rise, it would not be able to directly address its root cause, which is greenhouse gas emissions released by humans.
The new research, published in the Proceedings of the National Academy of Sciences, found that, hypothetically, solar geoengineering would not be effective at reducing warming if greenhouse gases were left to climb to extremely high levels.
This is because extremely high levels of greenhouse gases in the atmosphere would cause low-lying clouds, known as stratocumulus clouds, to thin and eventually break apart.
Stratocumulus clouds themselves cool the Earth by shading large portions of its surface from sunlight. If the straocomumlus clouds covering the subtropical oceans were to break up entirely, it could lead to additional 5C of global warming, the research says.
“Stratocumulus clouds thin under increased greenhouse gas concentrations and their reflection of sunlight diminishes,” study lead author Prof Tapio Schneider, a climate scientist at the California Institute of Technology, told The Independent.
“Solar geoengineering may not be fail-safe to prevent strong warming if it is prolonged for more than a century and greenhouse gas emissions continue to increase during that time.”
The research found that, in the simulations including solar geoengineering, subtropical stratocumulus clouds break up when levels of carbon dioxide in the atmosphere exceed 1,700 parts per million (ppm). This is more than four times the current level.
“I think the paper provides yet another argument for why solar geoengineering can’t be a ‘get out-of-jail-free’ card that lets us off the hook for the need to cut our CO2 emissions; we can’t just burn all the fossil fuels in the ground and solve the problem with solar geoengineering,” Dr Doug MacMartin, a senior research associate at Cornell University in New York who was not involved in the paper, told The Independent.
“It is equally important to stress, though, that the scenario they consider is really quite extreme.”
Levels of carbon dioxide would only reach 1,700ppm in a “super-extreme worst case scenario” where little is done to tackle emissions past the 21st century, added Dr Pete Irvine, a lecturer in climate change at University College London who was also not involved in the study.
So far, most research into solar geoengineering has been carried out using computer simulations, meaning little is known about its potential real-world implications.
However, in the future, a team of scientists from Harvard University plan to carry out one of the first solar geoengineering field tests.
The project, known as the Stratospheric Controlled Perturbation Experiment (SCoPEx), plans to use a high-altitude balloon to release a package containing aerosols 20km up into the stratosphere.
The amount of aerosols released will not be enough to have a cooling effect on the planet, the researchers say, but could allow the scientists to collect data on how aerosols particles interact with the air.
The scientists originally announced their intentions to carry out the experiment in 2017, but the project has so far been delayed by “technical issues”, according to the project’s blog.
Previous research has highlighted various social and ethical considerations associated with solar geoengineering.
One is, if the technology were to be developed, it could be perceived as a “quick fix” to the climate crisis, leading to countries stalling on their commitments to cutting their greenhouse gas emissions. Another is that the technology could be misused by single actors or states with malicious intentions.
“The question of whether to pursue solar geoengineering is not purely scientific, but it involves governance, ethical, and policy questions,” said Prof Schneider.