Global Warming Trends and Emissions Overview
Recent data indicates that 2024 marked the first full year with global temperatures exceeding the 19th-century average by more than 1.5 degrees Celsius. Concurrently, global emissions continue to rise, with projections indicating that fossil fuel consumption is expected to reach a new high in 2025. Current permanent carbon removal technologies extract tens of thousands of tonnes of carbon annually, which constitutes a small fraction compared to the estimated 5-10 billion tonnes required to mitigate climate change.
Stratospheric Aerosol Injection (SAI) as a Research Focus
The concept of reflecting a portion of incoming solar radiation to reduce global warming, an idea initially proposed by Lyndon B. Johnson's science advisers in 1965, is under renewed investigation. Earth naturally reflects approximately 30% of sunlight. Increasing this fraction slightly could potentially enhance the planet's heat reflection capacity.
Historical events, such as the 1991 eruption of Mount Pinatubo, which injected an estimated 15 million tonnes of sulfur dioxide into the stratosphere, resulted in a temporary global cooling of approximately 0.5 degrees Celsius. This natural event has served as a basis for the concept of Stratospheric Aerosol Injection (SAI).
Models suggest that SAI could potentially offset 1 degree Celsius of warming by introducing approximately 12 million tonnes of sulfur dioxide (SOâ‚‚) into the stratosphere annually. This quantity is significantly less than current industrial SOâ‚‚ emissions, yet is modeled to achieve a greater cooling effect.
Potential Considerations and Risks of SAI
SAI is not presented as a replacement for emissions reduction efforts. Projections indicate that if an SAI deployment were initiated and then abruptly discontinued, the planet could experience rapid rebound warming. Furthermore, uncoordinated or inadequately designed interventions could alter global precipitation patterns.
Phased Research Framework for SAI
Research is being conducted to evaluate whether SAI could be implemented safely and effectively. A structured, phased research program, analogous to clinical trials in medicine, has been proposed to gather necessary evidence.
Phase Zero: Pre-Clinical Studies
Currently, research is in a "pre-clinical" phase, involving laboratory experiments and computer modeling. These methods are used to predict risks associated with rising emissions. For SAI, this phase aims to build confidence in predictions by verifying that models accurately represent key stratospheric processes, such as aerosol formation, evolution, and dispersion, and their environmental interactions.
Phase One: Small-Scale Aerosol Release
A potential Phase One would involve the release of a small quantity of SOâ‚‚, approximately 10 tonnes, at stratospheric altitudes. This amount is considerably less than daily emissions from many coal power plants and is too small to influence the climate. Researchers would then use instruments on aircraft, ground-based systems, and satellites to measure the aerosols' evolution. This phase would study aerosol formation and behavior, which are current areas of scientific uncertainty. Comparing observations to model predictions would test assumptions and identify areas for model refinement.
Phase Two: Larger-Scale Distribution Studies
Following successful Phase One outcomes, a potential Phase Two experiment could involve releasing 10 to 100 times more SOâ‚‚. This scale would remain significantly smaller than a minor volcanic eruption, such as Mount Ruang's 2024 eruption (approximately 300,000 tonnes with no measurable global climate impact). This phase would focus on studying aerosol mixing and distribution, particle spread rates, and interactions with stratospheric circulation to refine model accuracy. The observational capabilities developed for these tests could also aid in detecting unauthorized deployments.
Phase Three: Deliberate Cooling with Oversight
If global research data supports further progression and governments decide to move forward, a Phase Three, comparable to a post-licensure trial in medicine, could involve small, deliberate cooling, potentially around 0.1 degrees Celsius over five years, under continuous observation and strict oversight.
Current Research Initiatives
The UK's Advanced Research and Invention Agency (Aria) is funding projects in this domain. For example, one Aria-funded project is developing theoretical foundations to determine the minimum scale for outdoor experiments that could reduce key scientific uncertainties safely and transparently. The organization Reflective also supports open science, coordination, and public accountability within the field of solar radiation modification research.
Conclusion on Research Approach
Generating real-world evidence transparently is considered essential for future informed decision-making regarding SAI. This includes developing necessary tools, rules, and oversight mechanisms in advance. Small-scale experiments are viewed as a method to reduce both scientific uncertainties and political risks, providing a fact-based approach to assessing SAI's potential and implications.