solar radiation management, a form of geoengineering, would aim to reflect away some of the heat from the sun


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.

1 – Innovations result in lower energy demand up to 2050 while living standards rise, especially in the global south. A downsized energy system enables rapid decarbonisation of energy supply. Afforestation is the only CDR option considered; neither fossil fuels with CCS nor BECCS are used.

2 – A broad focus on sustainability including energy intensity, human development, economic convergence and international cooperation, as well as shifts towards sustainable and healthy consumption patterns, low-carbon technology innovation.

3 – A middle-of-the-road scenario in which societal as well as technological development follows historical patterns. Emissions reductions are mainly achieved by changing the way in which energy and products are produced, and to a lesser degree by reductions in demand.

4 – A resource- and energy-intensive scenario in which economic growth and globalisation lead to widespread adoption of greenhouse-gas-intensive lifestyles, including high demand for transportation fuels and livestock products.

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.

Read about Carbon dioxide removal:

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.

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.

Read about Solar radiation management:

Geoengineering glossary

To some the term geoengineering only includes CDR technologies, which have the potential to permanently lower global temperatures. To others, it includes SRM too, which means reflecting some of the sun’s rays to cool the planet in the short term. Because of the lack of consensus on its meaning and the strong reactions it can produce, some experts avoid the term geoengineering in favour of discussing CDR, SRM or individual technologies themselves.

Any of the feasible paths to limit global warming to 1.5C above pre-industrial levels will require some climate intervention as well as massive reductions in greenhouse gas emissions. The technical challenges are huge, as are the questions for society. Who pays and how? Who decides which tools are deployed and when? Will they work?

To help track the debate – one of humanity’s most important in coming years – here’s a guide to the language and common terms used to discuss geoengineering, listed in tentative order of importance rather than alphabetically.

Solar radiation management (SRM)

Geoengineering techniques that aim to cool the planet by reducing the amount of sunshine reaching its surface, or increases the amount of infrared radiation escaping. They include:

Stratospheric aerosol injection
The use of aircraft, rockets or balloons to spread clouds of tiny particles (aerosols) such as sulphur dioxide into the high atmosphere to reflect some of the sun rays. This could temporarily cool the planet as very large volcanic eruptions have in the past.

Marine cloud brightening
Spraying seawater into the air to make clouds whiter and so more reflective of sunlight

Cirrus cloud thinning
High-altitude cirrus clouds act like greenhouse gases and trap heat. Injections of dust could thin them and allow more heat to pass back into space

Chemicals added to seas or to the wake of ship to encourage foaming. This could make the sea surface brighter and more reflectiv

White roofs
Lighter roofs reflect more sunlight, keeping towns and cities cool.

Reflective sheets
Massive plastic covers could bounce back sunlight from deserts

Space mirrors
Structures placed between the earth and the sun to block solar radiation

Carbon dioxide removal (CDR)

Techniques to reduce atmospheric concentrations of the main greenhouse gas, including:

Tree planting
To capture carbon, in formerly tree-covered areas (known as reforesting) or in other areas (afforestation)

Bioenergy with carbon capture and storage (BECCS)
Farming fast-growing crops, which extract carbon dioxide from the air as they grow, then burning them for energy, trapping the carbon emissions produced and storing them underground

Blue carbon
Protecting and restoring saltmarshes, seagrass meadows, mangrove forests, kelp forests, and freshwater tidal ecosystems so they absorb and store more carbon

Ocean fertilisation
Adding iron to nutrient-poor waters to encourage the growth of photosynthetic algae, which soak up carbon dioxide from the air as they grow

Artificial upwelling
Using pipes to bring cold, nutrient-rich seawater from the depths to the surface, where it could stimulate algae growth to draw down carbon dioxide, and even cool the air above

Ocean alkalinity
Alkaline (as opposed to acid) seawater can hold more carbon as dissolved ions. Therefore making the surface of the sea more alkaline – perhaps by adding lime – could keep more CO2 from the air locked away

Direct air capture
Also known as artificial trees, chemical sponges soak CO2 from the air. When saturated, the sponges are recharged and can go again.

Other terms

Negative emissions
A collective term for the effect of successful carbon dioxide removal measures.

By definition, geoengineering would have a global impact. So which country or countries should be allowed to try it and under what circumstances? International agreements could be needed to decide fairly.

London Convention
An international treaty that governs marine geoengineering techniques and tests, such as ocean fertilisation

The Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection, a UN advisory body that reviews geoengineering ideas and makes recommendations

Organic, usually plant, materials used to generate heat when burned, often to make electricity

Also known as charcoal. Heat biomass without oxygen (a chemical process called pyrolysis) and the thick, tarry solid that emerges can be mixed into the soil to sequester carbon

How much light a surface reflects back from a source, measured on a scale of 0-1 where 0 is none and 1 is all of it. Geoengineering frequently tries to increase albedo.