20.3 Geoengineering and Earth system interventions

Climate Intervention Strategies

Geoengineering refers to large-scale, deliberate interventions in Earth's climate system designed to counteract the effects of climate change. These strategies fall into two broad categories: reducing how much solar energy Earth absorbs, and pulling $CO_2$ out of the atmosphere. Both approaches are actively researched, but neither is a proven fix, and both raise serious questions about risks, governance, and ethics.

Solar Radiation Management (SRM) Techniques

SRM works by reducing the amount of solar energy that reaches and warms Earth's surface. It doesn't address the root cause of climate change (greenhouse gas concentrations), but it could lower global temperatures relatively quickly.

• Stratospheric aerosol injection is the most widely discussed SRM technique. The idea is to inject reflective particles, typically sulfate aerosols, into the stratosphere. These particles scatter incoming sunlight back into space, mimicking the cooling effect observed after large volcanic eruptions. The 1991 eruption of Mount Pinatubo, for example, temporarily cooled global temperatures by about 0.5°C.
• Marine cloud brightening involves spraying fine sea salt particles into low-lying clouds over the ocean to make them more reflective, bouncing more sunlight away from the surface.
• Cloud seeding is a weather modification technique that introduces particles (like silver iodide) into clouds to encourage precipitation. While sometimes grouped with SRM, its goal is altering local weather patterns rather than global temperature, and its effectiveness remains poorly understood.

A critical point about all SRM approaches: they treat the symptom (warming) without treating the cause (excess $CO_2$). If SRM were ever deployed and then suddenly stopped, temperatures would rebound rapidly. This is called the termination effect, and it's one of the biggest risks of relying on SRM.

Carbon Dioxide Removal (CDR) Methods

CDR takes the opposite approach: instead of blocking sunlight, it targets the greenhouse gases themselves by removing $CO_2$ from the atmosphere and locking it away in long-term reservoirs.

• Afforestation means planting trees in areas that were not previously forested. Trees absorb $CO_2$ through photosynthesis and store carbon in their biomass and in soils. This is one of the most straightforward CDR methods, though it requires vast land areas to make a significant dent in atmospheric $CO_2$.
• Ocean fertilization involves adding nutrients (usually iron) to ocean surface waters to stimulate phytoplankton blooms. Phytoplankton absorb $CO_2$ as they grow, and when they die and sink, some of that carbon gets transported to the deep ocean. However, this method carries ecological risks (more on that below).
• Carbon capture and storage (CCS) captures $CO_2$ emissions at their source, such as power plants or cement factories, and injects the compressed gas into deep geological formations for permanent storage. CCS is one of the few methods that can directly reduce emissions from heavy industry.
• Direct air capture (DAC) uses chemical processes to pull $CO_2$ directly from ambient air, rather than from a point source. It's technologically promising but currently very energy-intensive and expensive.

Carbon Sequestration Methods

Carbon sequestration is the long-term storage of carbon in reservoirs that keep it out of the atmosphere. The two main categories are terrestrial (land-based) and geological (underground).

Terrestrial Carbon Sequestration

Land ecosystems already absorb a significant share of human $CO_2$ emissions. These methods aim to enhance that natural capacity.

• Afforestation and reforestation increase the total area of forest cover. Afforestation plants trees where none existed recently; reforestation restores degraded or cleared forests. Both enhance carbon uptake in biomass and soils.
• Improved land management practices like no-till agriculture and cover cropping increase soil carbon storage. No-till farming reduces soil disturbance, which slows the breakdown and release of organic carbon. Cover crops add plant residue that builds soil organic matter over time.
• Wetland restoration protects some of the most carbon-dense ecosystems on Earth. Peatlands, for instance, cover only about 3% of Earth's land surface but store roughly twice as much carbon as all the world's forests combined. Mangrove forests are similarly effective carbon sinks. When these ecosystems are drained or destroyed, they release stored carbon rapidly.

Geological Carbon Sequestration

Geological methods store carbon deep underground, where it can remain isolated from the atmosphere for thousands to millions of years.

• Carbon capture and storage (CCS) captures $CO_2$ from industrial emissions, compresses it, and injects it into deep geological formations such as depleted oil and gas reservoirs or saline aquifers. The Sleipner project in Norway, operating since 1996, stores roughly 1 million tonnes of $CO_2$ per year beneath the North Sea.
• Mineral carbonation reacts captured $CO_2$ with calcium- or magnesium-rich rocks (like basalt) to form stable carbonate minerals such as calcite or magnesite. This is essentially speeding up a natural weathering process. The result is permanent storage, since the carbon is locked into solid rock. Iceland's CarbFix project has demonstrated this approach successfully.
• Enhanced oil recovery (EOR) injects $CO_2$ into aging oil reservoirs to push out additional oil while trapping the $CO_2$ underground. This does sequester carbon, but the net climate benefit is debatable: the extra oil that's extracted and burned releases $CO_2$ of its own. Whether EOR is a net positive for the climate depends entirely on lifecycle emissions accounting.

Geoengineering Challenges

Unintended Consequences and Risks

Earth's climate is a tightly coupled system, and intervening in one part can trigger cascading effects elsewhere.

• Disrupted precipitation patterns are a major concern with SRM. Models suggest that stratospheric aerosol injection could reduce global average precipitation and shift monsoon patterns, potentially causing droughts in tropical regions that billions of people depend on for agriculture and water.
• Marine ecosystem disruption is a risk of ocean fertilization. Artificially stimulating phytoplankton blooms can alter species composition, intensify ocean acidification in deeper waters, and create hypoxic (low-oxygen) zones, sometimes called dead zones, where most marine life cannot survive.
• The termination effect deserves emphasis because it's uniquely dangerous. If an SRM program were deployed for years or decades and then abruptly stopped (due to war, economic collapse, or political disagreement), the accumulated warming that had been masked would hit all at once. The rate of warming could be far faster than what ecosystems and societies could adapt to.
• Irreversibility is a concern for several interventions. Once aerosols are injected or ocean chemistry is altered, you can't simply undo the change. Some consequences may not become apparent for years or decades.

Governance and Ethical Considerations

Even if geoengineering technologies work as intended, deciding who gets to deploy them and under what rules is an enormous challenge.

• No international governance framework currently exists for geoengineering deployment. A single country could theoretically deploy stratospheric aerosols unilaterally, with consequences felt worldwide. This makes international cooperation and agreed-upon rules essential, yet difficult to achieve.
• Unequal distribution of impacts raises justice concerns. SRM might cool the planet on average but worsen conditions in specific regions. Developing countries, which have contributed least to climate change, could bear disproportionate risks from altered rainfall or agricultural disruption.
• Moral hazard is the concern that the mere possibility of a geoengineering "fix" could reduce the urgency to cut emissions. If policymakers believe they can engineer their way out of the problem later, they may delay the difficult transition away from fossil fuels. Most climate scientists stress that geoengineering should only be considered as a supplement to emissions reductions, never a replacement.
• Public engagement and consent are necessary for any responsible path forward. Decisions about technologies that affect the entire planet shouldn't be made behind closed doors. Transparency in research, open debate about risks and tradeoffs, and meaningful input from affected communities are all essential to legitimate governance.