The accelerating challenge of global warming has prompted the investigation of climate intervention, a set of deliberate, large-scale modifications to the Earth’s systems aimed at counteracting rising temperatures. These measures, collectively known as geoengineering, go beyond the traditional approach of reducing greenhouse gas emissions. Geoengineering is broadly divided into two major strategies: removing existing carbon dioxide from the atmosphere and managing the amount of solar energy that reaches the Earth’s surface. Consideration of these interventions reflects the gravity of the climate situation, suggesting that traditional mitigation efforts alone may not be sufficient to avoid dangerous levels of warming.
Large-Scale Carbon Removal Technologies
One category of climate intervention, known as Carbon Dioxide Removal (CDR), focuses on processes designed to extract carbon dioxide directly from the air and store it permanently. Direct Air Capture (DAC) is a prominent example, functioning much like a giant air filter. Large fans pull ambient air into a contact chamber where it passes over chemical media, such as liquid solvents or solid sorbents, which selectively bind to the carbon dioxide molecules.
Once saturated, the capture material is heated or subjected to a pressure change to release the concentrated stream of carbon dioxide. This gas is then compressed and injected deep underground into stable geological formations, such as saline aquifers, where it can be locked away permanently. The primary hurdles for DAC technologies are their high energy demands and current cost, which can exceed $1,000 per ton of \(\text{CO}_2\) removed at smaller scales. Projections suggest that with sufficient scaling, the cost could fall significantly, potentially reaching a range between $94 and $232 per ton for large-scale plants.
Another technological CDR method is Enhanced Weathering (EW), which accelerates a natural geological process that regulates atmospheric carbon. This involves mining, crushing, and spreading vast quantities of silicate rock, such as basalt, onto agricultural fields or coastlines. When these finely ground alkaline minerals react with atmospheric \(\text{CO}_2\) dissolved in rainwater or soil moisture, they form stable bicarbonate ions. These ions are eventually washed into the ocean, permanently storing the carbon.
The natural weathering process currently absorbs about \(1.1\) gigatons of \(\text{CO}_2\) per year; enhancing it could potentially increase this removal capacity to up to \(4\) gigatons annually. EW offers the benefit of potentially counteracting ocean acidification and improving soil health in agricultural applications. Challenges involve the high energy expenditure required for mining and grinding the rock, as well as the need for monitoring systems to verify the amount of carbon removed.
Methods for Reflecting Solar Energy
Solar Radiation Management (SRM) techniques represent the other major strategy, designed to reflect incoming sunlight back into space to reduce global temperatures rapidly. These methods do not remove greenhouse gases but instead act as a temporary global “dimmer switch.” Stratospheric Aerosol Injection (SAI) is one of the most-researched SRM concepts, drawing inspiration from the cooling effect observed after large volcanic eruptions.
SAI proposes to inject reflective particles, such as sulfur dioxide or calcite, into the stratosphere, an upper layer of the atmosphere. These particles would scatter sunlight, increasing the Earth’s albedo, or reflectivity, to counteract warming. Modeling suggests that SAI could rapidly cool the planet and is relatively inexpensive compared to technological CDR methods, though the effectiveness and impacts are uncertain. Deployment requires sustained, regular injections to maintain the cooling effect, as the aerosols naturally fall out of the stratosphere.
An alternative SRM approach is Marine Cloud Brightening (MCB), which aims to increase the reflectivity of low-lying marine clouds over the ocean. This technique involves spraying a fine mist of microscopic seawater particles into the air using specialized vessels. These salt particles act as cloud condensation nuclei, causing the clouds to form more, smaller droplets, which makes the clouds appear brighter.
If successful, brighter clouds would reflect more solar energy away from the Earth’s surface. MCB is expected to have a rapid and reversible cooling effect, but the complex nature of clouds makes the technique difficult to model and predict with certainty. Both SAI and MCB are potential short-term measures to mitigate warming while global emissions are reduced, but they do not address the root cause: the concentration of \(\text{CO}_2\).
Enhancing Natural Carbon Sinks
Nature-based solutions offer methods to enhance the Earth’s capacity to absorb and store carbon biologically. Large-scale Afforestation and Reforestation (A/R) involve planting new forests or restoring historical forest lands, using trees to pull \(\text{CO}_2\) from the atmosphere through photosynthesis. Forests currently function as terrestrial carbon sinks, sequestering approximately \(25\%\) of annual human carbon emissions.
Reforestation efforts provide long-term carbon storage in biomass, roots, and soil, offering co-benefits like biodiversity restoration and soil stabilization. However, the sequestration benefit takes decades to realize, and forests are vulnerable to disturbances like fires and insect infestations, which can release stored carbon back into the atmosphere. The total potential of A/R is also limited by the availability of suitable land that does not conflict with food production.
Soil Carbon Sequestration, promoted through regenerative agriculture practices, focuses on increasing the organic carbon content in agricultural soils. Techniques such as reduced tillage, cover cropping, and holistic grazing management can slow the loss of existing soil carbon and potentially store more. Healthy soils can store several times the amount of carbon currently in the atmosphere, making this a large reservoir.
The practical potential for long-term climate mitigation is modest due to several constraints. The amount of carbon that soil can hold eventually reaches a saturation limit, which some experts estimate could be reached within a few decades. Carbon stored in soil is also less permanent than geological storage, as a change in farming practice, such as a return to deep tilling, can quickly release the sequestered carbon.
Coastal and marine ecosystems, known as Blue Carbon, offer another powerful natural sink, specifically mangroves, salt marshes, and seagrasses. These environments are effective at carbon burial, sequestering carbon in their waterlogged sediments at rates estimated to be two to four times higher than terrestrial forests. A single hectare of healthy mangrove forest can sequester up to \(10\) tons of carbon annually, locking it away for centuries. Conservation and restoration of these threatened ecosystems is crucial, as degradation can turn them into sources of greenhouse gas emissions.
Global Risks and Governance
The deployment of large-scale geoengineering methods introduces substantial environmental and political risks that must be carefully considered. SRM techniques, which only mask warming, are particularly susceptible to the risk of “Termination Shock,” a rapid and damaging temperature rebound. If SAI, for example, were suddenly stopped for any reason—political, economic, or technical—the unmasked warming would occur at a much faster rate than the initial climate change, severely impacting ecosystems and human societies.
Both SAI and MCB carry the risk of uneven global effects, potentially altering atmospheric chemistry and disrupting regional weather patterns. Modeling suggests that while the global average temperature might be lowered, some regions could experience significant changes in precipitation, potentially leading to droughts in critical areas. These uneven impacts raise profound ethical questions about the distribution of risks and benefits among nations, as a deployment decision by one country could negatively affect others.
The question of who decides when, where, and how much to deploy is a major obstacle, as current international legal and governance structures are inadequate for such globally impactful actions. The low direct cost of deploying SRM creates a “free-driver” problem, where a single nation or small coalition could unilaterally initiate a program without international consensus. The lack of a global agreement on deployment standards and liability means these high-leverage technologies remain subjects of intense debate and political tension.