Could cloud brightening save the world?

Currently, around 30% of the Sun’s rays that reach Earth are reflected back into space by white surfaces, largely by the polar ice. Indeed, sea ice reflects sunlight better than any other known natural surface, sending about 90% back – by contrast, the ocean reflects just 6% of the sunlight, absorbing the remaining 94%. As Arctic Sea ice is declining, a reduction in ice and an increase in water is expected to see global warming accelerate. However, this bodes an interesting question – what if there was some other surface that could be used to reflect the Sun’s rays? For scientists, the answer might be manipulating the clouds. 

   The origin of this idea dates back to June 1991 and the volcanic eruption of Mount Pinatubo in the Philippines. It spewed around 15-17 million tonnes of white ash and sulphates into the stratosphere which spread into a haze that covered much of the globe. However, in the 15 months that followed, scientists discovered that the particle cloud had formed a sun-shield of sorts, reflecting a significant proportion of the Sun’s rays back into space, leading to an average global temperature drop of 0.6°C. As a result, scientists realised the possibility of developing other such shields – many of them were artificial, but there was also much discussion over brightening the clouds, which naturally reflect the Sun anyway. 

What if there was some other surface that could be used to reflect the Sun’s rays?

   The concept of geoengineering – employing aerosols and other devices to reduce how much sunlight hits the Earth – dates back to the 1970s, but it has picked up some major proponents in recent years in response to the climate crisis. The most advanced geoengineering concept is known as a stratospheric aerosol injection, in which sulphur dioxide would be deliberately released into the stratosphere to gather around the poles. The idea is that these aerosols would have a similar effect to volcanic ash clouds, reaching a high altitude and reflecting sunlight. There are other such proposals, including using emissions from shipping lanes to make stratocumulus clouds whiter, but many of them are in their infancy. 

   Is it a good idea? Frankly, even conceptualising it is difficult. Ben Kravitz, atmospheric scientist at Pacific Northwest National Laboratory, says: “It’s difficult for me to say whether I’m for or against stratospheric aerosol injection because it’s hard for me to even understand what I’m giving an opinion on. The effects will vary depending on how much aerosol injection is done, where, when and what material is used. And then we need to translate effects (such as changes in temperature, precipitation, etc) into impacts (such as food and water security). And then all of that needs to be compared to what would happen under climate change without stratospheric aerosol injection. So it’s too hard for me to say whether geoengineering is a good or bad idea at this stage.” 

   And that’s exactly the point – there’s so much that’s unknown about this science. While it works in theory, the kind of stratospheric manipulation required to achieve the desired reduction in temperature could irreversibly change weather systems around the world, and there’s no sense of what the long-term consequences may be. There are political and moral questions about altering the planet in such a way – it would be necessary to involve local communities in the first place, and then there would be countless legal issues about who controls and who is responsible for such technology, particularly as it comes back to Earth. 

The kind of stratospheric manipulation required to achieve the desired reduction in temperature could irreversibly change weather systems around the world

   Nonetheless, there are tentative steps towards exploring cloud brightening, whether or not it is ultimately the answer. A selection of scientists, including some major names in climate science, have called for further study of these possible strategies in an open letter. It states: “While reducing emissions is crucial, no level of reduction undertaken now can reverse the warming effect of past and present greenhouse gas emissions. Significant uncertainties remain around how any of these SRM [solar radiation modification] interventions would affect climate risk under different scenarios of greenhouse gas and background aerosol concentrations. Yet as the impacts of climate change grow and become more tangible, there will be increasing pressure to reduce climate warming using one or more SRM approaches.” Meanwhile, in a recent speech, billionaire philanthropist George Soros called for an embrace of marine cloud brightening, in order to “refreeze” the Artic. 

   The conversations around geoengineering are likely to continue, even as scientists acknowledge that there are many other, better ways to reduce climate change. Voices are calling for radical solutions to help mitigate, or even solve, this global problem – innovation will be required as well as reduction, and brightening clouds may be one part of the solution.