Depending on who you ask, geoengineering is either a threat to serious climate action, a faraway back-up plan or a necessary part of today’s climate policy. All would likely agree it is contentious.
Geoengineering encompasses a broad spectrum of proposed large-scale, deliberate interventions to mitigate or even reverse temperature rise.
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Many scientists and activists are concerned that climate interventions which rely on these “technological fixes” will divert attention away from emissions cuts and disrupt a complex and poorly understood global climate system even further. “It would really be a dangerous experiment, and it’s impossible to really test before,” says Linda Schneider, a climate policy expert at the Heinrich Böll Foundation.
However, others think it increasingly likely that we’ll need to deploy these measures, alongside rapid emissions reductions, in order to avoid dangerous levels of warming.
Geoengineering tends to be separated into two main strands: techniques which aim to remove carbon dioxide from the atmosphere and the much more controversial proposals to reflect sunlight away from the Earth.
Here we take an in-depth look at the main proposed techniques in each category, their feasibility and concerns about the potential negative impacts they could bring with them.
Carbon dioxide removal
Many researchers argue the use of carbon removal technologies are now all but essential since the world is so off track to cut emissions enough to avoid dangerous warming. “By the point we get to zero emissions, we’re going to have unacceptable amounts of warming,” says Douglas MacMartin, a researcher in mechanical and aerospace engineering at Cornell University who specialises in solar geoengineering. “The long-term solution to that is to pull the CO2 out of the atmosphere.”
But others have warned against a reliance on unproven negative emissions technologies. “If we rely on these and they are not deployed or are unsuccessful at removing CO2 from the atmosphere at the levels assumed, society will be locked into a high-temperature pathway,” climate scientists Kevin Anderson and Glen Peters argued in 2016.
Bioenergy with carbon capture and storage, often known by its shorthand BECCS, has come to be seen as one of the most viable negative emissions technologies.
BECCS involves farming crops or trees, which sequester CO2 from the air as they grow, then burning them to create energy while also capturing the carbon emitted. The carbon would then be stored underground, preventing it from returning to the atmosphere, before the whole process is repeated. Over time, and at a large enough scale, the technique could in theory remove substantial amounts of carbon from the atmosphere.
BECCS has become especially popular with climate modellers, and is now included as a key part of most decarbonisation pathways compatible with the Paris Agreement. However, scientists have also developed deeper decarbonisation pathways, that rely on lifestyle change and the rapid roll out of renewables. These would limit warming to 1.5C and, in theory, eliminate the need for BECCS. Meanwhile, some call for a “post-growth” economics that entails a deeper shift to smaller or steady-state economies, a change not considered by these models.
Despite its widespread use in climate models, BECCS has not yet been proven at scale, with just a handful of plants operating around the world. The upfront costs of building carbon capture plants are also high, while the storage of carbon underground could cause earthquakes or result in CO2 leaking back into the atmosphere.
“Without having done it at even a remote fraction of the scale that you need, I would be worried about being overconfident in our ability to scale anything up by three or four orders of magnitude,” says MacMartin.
Another major concern with BECCS is the vast amounts of land that would be needed to grow the bioenergy in the first place, which could compete with food provision or lead to deforestation.
One study found the bioenergy crops needed to deliver the scale of CO2 removal included in pathways to limit warming to 2C could occupy up to 700 million hectares – equivalent to around half of the world’s current cropland. Expanding bioenergy further to meet the 1.5C limit could cause overall losses in carbon from land, the study said, by replacing forests and other high-carbon ecosystems with crops.
“I think whether or not we should scale BECCS is a bigger question because of land use issues, environmental justice issues,” says Shuchi Talati, a fellow on solar geoengineering governance at the Union of Concerned Scientists (UCS). “I don’t think BECCS deserves the prominence it’s getting right now.”
Direct air capture
This is the other main large-scale negative emissions technology being proposed. Direct air capture (DAC) involves machines removing carbon dioxide directly from the air, rather than from a point source, such as a thermal power station, as with BECCS.
In order to be a negative emissions technology, the CO2 would then need to be stored in a way that stopped it returning to the atmosphere, similar to BECCS.
When compared to BECCS, direct air capture can look like an attractive option because it does not rely on huge changes in land use and avoids potential complications around deforestation. Theoretically, plants could also be more easily built close to storage and utilisations sites, reducing the need for long-distance CO2 transport, such as via pipelines.
But it comes with several caveats. It would likely require vast amounts of energy to run: a paper published last year found it could require more than half of today’s total electricity production by 2100. The paper also warned against the risks of assuming it can be deployed at scale then finding out this is not possible.
There are also concerns about what the captured CO2 may be used for. For example, a joint venture between US start-up Carbon Engineering and fossil-fuel company Occidental Petroleum to develop the world’s first large-scale DAC plant will use the captured CO2 to improve the efficiency of oil recovery.
In fact, enhanced oil recovery is currently the biggest industrial use of CO2. It is the only large-scale carbon sequestration industry that exists, so offers a financial pathway to developing DAC and BECCS on a wider scale. However, using captured CO2 to extract more oil has obvious problems.
Captured CO2 could also be used in other applications, such as to produce fuels or building materials like cement – an industry known as carbon capture and utilisation (CCU). However, to be considered negative emissions, the CO2 must be kept locked up long-term – if it is released into the atmosphere again, such as by burning fuel, the whole process becomes carbon neutral at best. There are currently 15 small DAC plants operating worldwide, but not all the carbon they capture is being stored.
Other CO2 removal technologies
There are a huge number of nature-based solutions which could increase carbon sequestration, such as afforestation, reforestation, and the restoration of peatlands and coastal ecosystems. These are important but rarely considered to be part of the “geoengineering” bracket.
But some geoengineering proposals aim to bolster natural processes. Enhanced mineral weathering, for example, aims to speed the natural absorption of CO2 by rocks. Ocean fertilisation, another example of this, would add vast amounts of iron or other nutrients to the ocean to stimulate the growth of algal blooms that absorb CO2. However, according to Talati, ocean fertilisation is rarely now considered viable at scale, describing it as a research success story. “We did research into it and found that it really wasn’t a viable option, and people really stopped talking about it in the geoengineering space.”
Biochar has also been suggested as a potential large-scale carbon removal solution. Here organic material such as biomass would be burnt to create charcoal then buried in soil to store the carbon. While several oil majors have pushed biochar as a win-win climate solution it has yet to be proven at scale.
Solar radiation management
Also known as solar geoengineering, this group of proposed technologies would, in theory, reflect sunlight away from the Earth’s surface before it has a chance to warm the atmosphere.
Proponents of these controversial techniques argue they should be researched and understood in case the world overshoots the carbon budget for 1.5C or 2C of warming and carbon removal techniques have not yet reached an adequate scale to reduce emissions to safer levels.
But solar geoengineering has serious limitations. All techniques would fail to address ocean acidification, since they would not directly reduce the amount of CO2 in the atmosphere. They also run the risk of “termination shock” – a rapid rise in warming should the method fail for some reason – and of changing local rainfall patterns or temperatures in unexpected or undesirable ways.
Stratospheric aerosol injection
Currently the most discussed approach to solar radiation management consists of sulphate particles or other aerosols being injected into the stratosphere from planes or high-altitude balloons.
Stratospheric aerosols are the only solar geoengineering techniques “we know work today”, says Macmartin of Cornell University. “We see what happens after a large volcanic eruption like Mount Pinatubo: put sulphate aerosols into the stratosphere and the planet cooled by about a half a degree.” However, current aircraft could not bring enough to the stratosphere, he says – vehicles with this payload are “at least five years” away from development.
Building a solar geoengineering system that is beneficial to most of the world is likely much further away than that, adds Talati. “There are just so many unanswered questions,” she says. “We would need a robust governance system, we would need to know how successful solar geoengineering at a large scale really was, we would need a large-scale monitoring system in place. All of that is really costly. And so I think we’re pretty far away.”
A range of potential problems are already causing concern. Injected sulphate aerosols could deplete the ozone layer. Sulphates would also eventually come down as acid rain, says MacMartin, a particular worry for the relatively pristine parts of the planet which have not previously experienced this.
It could also pose a risk to geopolitical systems, says Talati. “If a country decides to deploy solar geoengineering unilaterally, that could completely disrupt weather systems for a particular country, which would then disrupt its agricultural systems, and its GDP, resulting in massive tensions.”
Several research programmes continue to develop stratospheric aerosol injection today in a bid to better understand its risks and benefits, despite calls from some quarters for no testing or deployment to go forward before a credible global governance mechanism is in place.
Another, less-researched solar radiation management proposal would see ships spray seawater into low-lying stratocumulus clouds. This would add salt particles around which more water vapour could condense and so, in theory, increase the reflectivity of the clouds.
However, scientists don’t yet know where and when it does and doesn’t work, says MacMartin. “Until we can answer that question that makes it a little bit hard to make much in the way of global predictions,” he adds.
Australia is already experimenting with the technique on a local scale in the hope it could be used to provide protection for the Great Barrier Reef. But attempting to scale this up to cool the entire planet would introduce other problems, says MacMartin. The right type of clouds probably sit above about 10% of Earth’s surface and “the effects don’t stay right where you’re putting in that forcing,” he says. “You are going to probably get larger perturbations in precipitation patterns and things like that from marine cloud brightening than you would from stratospheric aerosols.”
Other solar geoengineering technologies
There are a host of other hypothetical techniques, largely with far less research interest than the above two methods.
One of the most outlandish is the space mirrors proposal, whereby a fleet of mirrors would be sent into orbit to deflect light away from Earth. While this could avoid concerns of chemical intervention in the Earth’s atmosphere, it could still have negative consequences, such as drought.
It is also generally considered to be prohibitively expensive. “I think one of the reasons solar geoengineering has the prominence it does is because it could be a cheaper way to limit harm while we scale up things like carbon removal and mitigation and adaptation,” says Talati. “In that way, I feel like [space mirrors] wouldn’t offer a lot of the benefits that make stratospheric aerosol injection attractive.”
Another lesser-known proposal is cloud thinning. While not technically reflecting sunlight, the aim here would be to disperse high altitude cirrus clouds by adding additional nuclei, allowing more heat to escape from the Earth. However, research has found that seeding the clouds could accidentally lead to more warming, or impact other aspects of the climate system in unexpected ways.
More localised changes could also be made on the Earth’s surface. From installing white roofs to genetically modifying crops to reflect more sunlight, changing the reflectivity of the ground could help to counteract warming – particularly for hot extremes in densely populated and important agricultural regions. However, the smaller scale of these various proposals typically exclude them from being considered as solar geoengineering.
Large-scale changes to the Earth’s surface albedo have also been proposed, from covering deserts in plastic sheeting to protecting Arctic ice using hollow reflective glass beads – a scheme that is now being tested. Other suggestions would try to brighten the surface of oceans, such as by generating millions of tiny air bubbles or spreading microbeads across the water.
Many argue geoengineering methods need more attention and research, at the very least to better understand their potential harmful effects and develop systems for how they should be governed. Meanwhile, scientists agree that to have a chance of keeping global warming within safe levels the focus should remain on rapidly reducing greenhouse gas emissions.