Scientists and policymakers are divided over whether to intervene in the climate to deal with global warming and how to go about it

Geoengineering, also known as climate intervention, refers to a group of largely-untested techniques that could reduce or counteract the temperature rise caused by climate change.

This group is generally separated into two very different approaches: techniques to withdraw carbon dioxide from the atmosphere, and techniques to reflect incoming sunlight before it hits the Earth.

The various geoengineering technologies being considered are at different stages of development and none has yet been deployed at scale.

The idea of deliberate large-scale intervention in the Earth’s climate systems has been discussed for decades but it is rising in prominence again as scientists and other groups become increasingly concerned about the world’s failure to cut greenhouse gas emissions rapidly enough. Many now argue other solutions will be needed to keep global temperature rise below dangerous levels. 

The chart below from Climate Action Tracker, which shows warming projections based on existing pledges and policies to reduce emissions, helps to explain why.

In particular, notice the gap between the trajectory of current policies and the required trajectory for staying below 2C and 1.5C of warming. The 2015 Paris Agreement included an aspirational goal to keep warming near to 1.5C. As the chart shows, the world is currently very far from this trajectory, even with recent pledges and targets.

The ambition of pledges to reduce emissions is expected to increase over time. But many climate scientists argue these emissions cuts alone are not sufficient to put the world on track to limit warming to 1.5C, meaning negative emissions technologies, that take greenhouse gases out of the atmosphere, will also be needed.

Carbon dioxide removal

Often called CDR, carbon dioxide removal is a broad term referring to any means of removing CO2 from the atmosphere.

Some of these approaches are based entirely on natural processes, such as planting forests, restoring mangroves or increasing soil carbon. These are not generally considered to be geoengineering.

Other techniques mix natural processes and technology. Bioenergy with carbon capture and storage (BECCS), the most prominent geoengineering technique, falls under this umbrella. BECCS involves removing carbon dioxide by growing plants for fuel and energy, capturing the CO2 emitted from their combustion and storing it underground.

Some proposed techniques are purely technological, such as direct air carbon capture and storage (DAC), which involves chemically removing carbon dioxide from the air and concentrating it. To be a negative emissions technology, this carbon would then need to be stored to prevent it returning to the atmosphere.

In its 2018 report on limiting warming to 1.5C, the International Panel on Climate Change (IPCC) modelled four “illustrative” pathways to stabilise global temperatures at 1.5C by 2100, shown below.

The pathways show that delaying action to cut emissions means a steeper emissions reduction curve in the long run. However, all four pathways also involve the need for achieving negative emissions by the latter half of the century. 

In the first pathway, this consists only of “natural” emissions removal, such as changes in agriculture, forestry and other land uses, which are not typically considered as geoengineering. In the remaining three pathways, which require higher amounts of negative emissions, BECCS is used alongside these natural removals.

The IPCC also warns that deploying carbon removal technologies at scale is “unproven”, and reliance on them “a major risk” in attempts to limit warming to 1.5C.

Also, some people argue that none of these CO2 removal techniques should be labelled geoengineering because that conflates them with solar geoengineering, which is potentially much riskier. But others do group them together because the strategies and politics around their deployment are so interrelated.

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Solar radiation management

Solar geoengineering, also known as solar radiation management (SDR), describes techniques that aim to reflect sunlight away from the Earth to reduce warming.

The most prominent of these is stratospheric aerosol injection – the idea of adding aerosols such as sulphates into the stratosphere to mimic the cooling effect of large volcanic eruptions. Marine cloud brightening, where sea water is sprayed into low-lying clouds to brighten them, is another relatively prominent solar geoengineering idea. 

A range of other solar geoengineering techniques have been proposed, including the idea of introducing a fleet of mirrors into space to stop as much sunlight arriving on Earth.

The IPCC has not modelled any solar geoengineering measures into its pathways, noting their potential for adverse side-effects and that the uncertainties surrounding these measures “constrain their potential deployment”. They are unlikely to be modelled in its next report either.

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All proposed geoengineering techniques are contentious in one way or another. CO2 removal on the scale that would be needed to reduce global temperature rise would likely be extremely expensive as well as energy and resource intensive. Scaling up these techniques would present huge logistical challenges, even if research proved they were viable. BECCS would also require vast amounts of land to provide the needed bioenergy, leading to concerns about food security and human rights.

Solar radiation techniques do not address the root cause of global warming – excess greenhouse gases in the atmosphere – but could deliver quick results. However, they could have significant side effects such as acid rain, ozone depletion or changes in weather patterns.

There are concerns they could be deployed by one country but impact others, creating serious geopolitical tensions. They also fail to address ocean acidification, a major impact of climate change, and pose the risk of “termination shock” if their use is suddenly stopped, where the world would see a rapid rise in temperature.

Finally, many argue focussing on the promise of future geoengineering could lead to complacency on cutting emissions in the nearer term – a concern known as the “moral hazard” problem.