By David Stanway
As the world struggles to renounce its burning of fossil fuels, scientists are studying whether atmospheric geoengineering could help limit warming and avert climate catastrophe.
One potential method, solar radiation management (SRM), seeks to reflect the sun's rays back into space, with the most well-known proposal being to blast sulphur dioxide - a coolant - into the higher reaches of the atmosphere.
Debates about its efficacy abound, with the United States, Europe and several environmental groups speaking out about the opportunities and risks. The discussion is largely theoretical at present, with only a handful of small-scale projects in operation.
WHAT STAGE IS THE TECHNOLOGY AT?
The idea of injecting sulphur dioxide (SO2) into the atmosphere is not new. The U.S. National Academy of Sciences proposed the idea as early as 1992, while scientists have documented that volcanic eruptions, which spew huge amounts of SO2 into the air, have a cooling effect on the planet.
Efforts to banish SO2 as a harmful air pollutant in China and elsewhere over the last decade have dampened its cooling effect and "unmasked" heat caused by greenhouse gases, thereby contributing to rising global temperatures.
U.S.-based start-up Make Sunsets, one of the few commercial ventures involved in the sector, released two weather balloons containing sulphur dioxide in Mexico last year, prompting the Mexican government to ban the activity in January.
Company founder Luke Iseman told Reuters that it was more "straightforward" to start projects in the United States and 30 launches had already taken place, funded by selling "cooling credits" to customers.
But apart from Make Sunsets, only a small number of other research projects have been conducted so far, including the launch of a high-altitude weather balloon in southeast England in 2022 to test the viability of aerosol injection equipment.
Some other projects have been cancelled as a result of public opposition, including a Harvard University and Swedish Space Corporation venture in 2021.
Research has been conducted into other potentially less dangerous SRM technologies, including marine cloud brightening, which involves the spraying of seawater from ships to make clouds more reflective.
While these methods were less intrusive and less potentially damaging than stratospheric aerosol injection, they could prove more expensive and too energy-intensive, said Benjamin Sovacool, Professor of Earth and Environment at Boston University, who has studied their potential deployment at the Great Barrier Reef.
WHAT ARE CRITICS OF SRM WORRIED ABOUT?
Dozens of scientists are calling for "a comprehensive international assessment" into the use of SRM in order to understand the risks involved and the regulations that might be required to deploy the technologies on a wider scale.
They said in a letter published in February that it was unlikely that carbon emissions could be reduced or removed quickly enough to keep temperature increases below 2 degrees Celsius and that SRM interventions could be made available when necessary to avert climate tipping points.
Opponents of the method say that while the injection of sulphate aerosols might cool the planet, the side effects could prove even more destructive. One group of 60 scientists launched a global initiative last year aimed at persuading governments to ban outdoor solar geoengineering experiments.
The group warned that the risks of SRM were too great and that it could impact weather patterns, agriculture, and "the provision of basic needs of food and water".
Critics point to models that show SRM could disrupt monsoons and cause droughts in Africa and Asia. Others say it could also slow the recovery of the ozone layer or lead to a dangerous spike in acid rain.
The technology could even be weaponised by "rogue states" or unscrupulous private companies and create new geopolitical and security threats, the United Nations Environment Program (UNEP) warned in a report published this year.
Opponents also worry the technology could serve as an excuse to delay the shift towards net-zero greenhouse gas emissions. Crucially, even if SRM interventions successfully keep temperatures down, they will not fix other consequences of rising CO2 levels, like ocean acidification.
"It is important people understand that SRM technologies ... do not solve the climate crisis because they do not reduce greenhouse gas emissions nor reverse the impacts of climate change," said Andrea Hinwood, the UNEP's chief scientist.
Its impact will also only be short term, raising the possibility that countries would be forced to deploy SRM for centuries.
"Once you've committed to it, you've got to keep doing it," said Laura Wilcox, a climate expert at Britain's University of Exeter. "If you stop, then you're going to see all of that warming that you've missed, essentially on climate timescales overnight. So it's a dangerous game."
(Reporting by David Stanway; Additional reporting by Jake Spring; Editing by Pravin Char)