Greenhouse gases, such as CO2, warm the planet . Is there such thing as an “anti-greenhouse gas”? Yes – aerosols [3,11]! These are ‘suspensions’ of small particles in the atmosphere – they are not gases . Most aerosols reflect sunlight and send it back into space, cooling the atmosphere (swipe for a diagram!) [13,14]*.
Some scientists have suggested injecting aerosols called sulfates into the atmosphere  by shooting particles with guns or hoses, or emptying particles out of an airplane . Aerosol injection could also be used on a local (rather than global) scale e.g. to cause more cooling over the poles .
CO2 stays in the atmosphere for centuries to millennia . Therefore, if we want to stop temperatures from rising we can’t just reduce emissions – we also need to either remove greenhouse gases or use a method like aerosol injection to cool the planet [9,19]. Aerosol injection could cool the Earth by 0.3°C for $36 billion spent over 15 years . This is much cheaper than removing CO2 from the air [2,9].
However, there are many risks associated with aerosol injection :
We don’t fully understand how it would affect regional climates  but it might cause droughts in India, South Asia, and parts of Africa . The sulfate aerosols could cause acid rain , and aerosol injection would not solve ocean acidification: CO2 would continue to dissolve in the ocean [6,19]. Furthermore, aerosols would have to be continually added to the atmosphere for centuries (until CO2 added by humans has left the atmosphere, e.g. by being taken up by the ocean) [19,22].
Despite this, we could need aerosol injection in addition to radically reducing emissions and adapting to the consequences of climate change [7,8,18,21]. It is a topic deserving further research, so that we know how to do it safely if we have no better option [7,9].
*There are many different types of aerosols – you may have heard of CFCs, which damage the ozone layer (which protects us from damaging radiation from the Sun) [12,20].
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016EF000465%4010.1002/%28ISSN%292328-4277.GEOENGIN1 See: Table 1 and 5
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