Backup plan: Mimicking volcanoes
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One solar geo-engineering proposal is to put tiny particles called aerosols into the stratosphere to reflect sunlight. This would reduce the amount of sunlight reaching the Earth’s surface, cooling the planet down, but could have unintended consequences and might damage the ozone layer.
Cooling the Earth by reflecting sunlight with aerosols
Without significant cuts in greenhouse gas emissions, atmospheric carbon dioxide (CO2) could soon be at double preindustrial levels. Some solar geo-engineering proposals aim to cancel out the warming effect of this extra CO2 by reducing the sunlight reaching the Earth’s surface. Some sunlight is reflected back to space by aerosols in the atmosphere. To compensate for doubled CO2 levels, we’d need to add enough aerosols to reflect about 2% more sunlight. Considering the Earth gets more energy from the Sun each hour than humans use in a year, 2% is a huge amount. If we increase CO2 even more, we’d need to add even more aerosols to keep global temperature stable.
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Image: SOHO (ESA & NASA)
The cooling effect of volcanic aerosols
Volcanic eruptions pump out clouds of dust, ash and other particles. Among the debris are gases including sulphur dioxide. Once in the atmosphere, sulphur dioxide molecules combine with water vapour and other chemicals to form sulphate aerosols, which reflect sunlight away from the Earth’s surface. The resulting cooling effect often outweighs the warming caused by volcanic greenhouse gases – the explosive eruption of Mount Pinatubo in 1991 caused a 0.5 °C drop in global temperature. The global cooling following even very large eruptions usually only lasts a few years, because the aerosols eventually ‘rain’ back to the ground.
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Image: USGS/Cascades Volcano Observatory
Pumping aerosols into the atmosphere
To counteract the warming effect of extra carbon dioxide using aerosols, the number of these tiny particles in the atmosphere would need to be permanently increased. Aerosols pumped into the stratosphere only stay aloft for a few years, so we’d have to keep replacing them. We could build large ‘chimneys’ on the ground to throw aerosols up into the sky, or spray them from aeroplane exhausts. Scientists have calculated that the amount of particles needed would be less than 10% of the annual weight currently carried by aircraft. Many commercial aeroplanes already fly up to the lower stratosphere, so experts think this solar geo-engineering proposal is both realistic and affordable.
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Image: Flickr / Axwel
Choosing the right height for extra aerosols
Not all volcanic eruptions cause noticeable cooling. In weak eruptions – or sideways eruptions such as Mount St Helens in 1980 – the aerosols only reach the lower atmosphere, where they get caught up in clouds and soon fall in raindrops. But if a volcano erupts with great force, especially if it erupts vertically like Mount Pinatubo in 1991, the aerosols reach the stratosphere, where there are no rain clouds and they can stay aloft, reflecting sunlight for several years. Stratospheric aerosols are also preferable for air quality. In the lower atmosphere these particles act as pollutants and can cause breathing difficulties such as asthma. In the stratosphere no-one has to breathe them in.
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Image: NASA
The effect of increased aerosols on the ozone layer
The stratosphere contains the ozone layer, which absorbs the most energetic of the Sun’s ultraviolet rays. But scientists think that adding aerosols to the stratosphere might reduce ozone levels, as happened after the increase in stratospheric aerosols caused by Mount Pinatubo’s eruption in 1991. The interactions between aerosols and ozone are not yet fully understood, but scientists think they trigger chemical reactions that convert ozone into other gases. This reduces the amount of ozone in the stratosphere, weakening its ability to block out harmful ultraviolet rays. Any significant damage to the ozone layer could have negative impacts on human health.
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Image: NASA
The effect of increased aerosols on climate patterns
Using aerosols to reflect more sunlight could cool the planet and counteract global warming, but it wouldn’t simply return the climate to its preindustrial state. Scientists think that adding more aerosols to the atmosphere could affect climate patterns – particularly rainfall. When Mount Pinatubo erupted in 1991, the aerosols it released blocked out some sunlight and temporarily cooled the planet. There was also a substantial decrease in rainfall over land, which scientists think was caused by the reduction in sunlight. If we mimic this effect in an effort to stop global warming, it could also reduce rainfall and even alter the Asian and African summer monsoons.
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Image: abcdz2000 / SXC
Profile: Dr Alan Robock
Some scientists have suggested cooling the planet by injecting aerosols into the stratosphere, blocking out sunlight. But would this work? Would there be unwanted consequences? Alan Robock uses climate models and observations of volcanic eruptions to explore these questions. Dr Robock’s work shows aerosols won’t provide a quick fix. ‘While blocking out sunlight could cool the planet, it could also produce drought in Asia and Africa by reducing the strength of the summer monsoon, threatening food supplies for billions of people,’ he says. ‘Furthermore, there would be ozone depletion and it wouldn’t stop ocean acidification. Geo-engineering isn’t a solution to global warming. The solutions are mitigation and adaptation.’
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