What is geoengineering, why is it on the agenda, and why is it controversial?

Geoengineering, also known as climate engineering, describes a range of ways to intervene on a large scale in the Earth’s natural systems – the oceans, soils and atmosphere – to directly combat climate change. They mostly fall into two categories: those designed to remove carbon dioxide (CO2) from the air and those that try to limit the amount of sunlight that reaches the Earth’s surface.

The significant increase in greenhouse gases in the atmosphere due to human activity since the Industrial Revolution is enhancing the natural greenhouse effect and causing climate change. Many scientists now say that we need to consider geoengineering as part of a solution to avoid dangerous levels of climate change: without some form of geoengineering it will be hard to achieve the goal of the Paris Agreement on climate change of keeping the amount of atmospheric warming well below 2°C, let alone meeting the more ambitious targets of a 1.5°C limit.

However, geoengineering methods are not yet proven to work on a large scale, can have unintended negative impacts on the environment, and some argue that they may discourage efforts to reduce greenhouse gas emissions. Public consent for research or deployment may create barriers to its use and there are questions over how to govern technologies that have application and impacts across national boundaries.

What is the aim of Carbon Dioxide Removal and how would it work?

Carbon Dioxide Removal (CDR), or more generally Greenhouse Gas Removal (GGR), is sometimes referred to as ‘negative emissions’ technology. Some scientists argue that to keep the amount of atmospheric warming well below 2°C, ‘trapping’ and removing greenhouse gases may have to be part of the solution, with others arguing for the acceleration of these methods. Most of the Intergovernmental Panel on Climate Change’s models assume that carbon will be taken out of the atmosphere if the world is to have a good chance of meeting the Paris Agreement target.

Some CDR methods are ‘low-tech’, such as large-scale tree-planting. Others involve technologies that as yet have not been proven on any significant scale although some are at pilot project stage. Approaches under consideration include: Bio-Energy with Carbon Capture and Storage (BECCS), which works by collecting the CO2 released from burning biomass (organic materials such as wood or crops) for power and pumping and storing it deep underground, while the biomass absorbs CO2 from the atmosphere as it grows; removing carbon from the atmosphere and storing it deep in the ocean; biochar, where carbon waste from agriculture is ‘charred’ and buried, to lock up its carbon in the soil; creating artificial trees that suck CO2 from the air and store it underground; and ocean fertilisation, adding nutrients to the ocean to encourage growth of CO2-eating plankton.

However, achieving CDR on a scale that would substantially contribute to mitigating climate change is a massive undertaking and the methods have multiple negative side effects. For example, ocean fertilisation may have unwanted side effects on the ecology of the oceans, while growing biomass for BECCS, and large-scale tree-planting, would cause competition for land space for food crops, nature conservation and other uses.

What is the aim of Solar Radiation Management and how would it work?

Solar Radiation Management (SRM) aims to bring down temperatures by reflecting a small amount of sunlight back into space, to limit the amount that reaches the Earth’s surface.

One proposed technique is to inject minute particles of sulphur dioxide into the stratosphere (the upper atmosphere). These particles, known as aerosols, would reflect some sunlight back into space, a process known as ‘global dimming’. This technique copies the natural cooling effect caused by major volcanic eruptions.

Another proposal is ‘marine cloud brightening’, where the clouds that cover oceans could be made lighter in colour and more reflective by spraying tiny droplets of seawater into them, causing them to reflect more sunlight back out into space. This is a way of enhancing the albedo effect, the way that light surfaces reflect more heat than dark surfaces. Another way to enhance albedo is to increase the reflectiveness of the land surface, for example by increasing the amount of white rooftops and light-coloured pavements in urban areas, or enhancing albedo on a larger scale by covering large areas of desert in reflective sheets, or planting crops, shrubs and grasses that are light in colour. However, these larger-scale interventions could cause extreme regional cooling and interfere with local weather.

The possible unintended consequences of stratospheric engineering include changes to precipitation patterns and damage to the ozone layer. The Intergovernmental Panel on Climate Change indicates that SRM methods could substantially reduce the global temperature rise, but that these techniques are unable to address the harmful effects of increased CO2 levels unrelated to temperature, such as ocean acidification. Furthermore, should any of these methods be abruptly stopped, rapid warming of the atmosphere would occur.


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