SAI Microjet: Optimizing Sulfur Ratios for Geoengineering

A Climate Engineering Experiment

Climate change is one of the most pressing challenges of our time. At its core, it is an energy problem - greenhouse gases trap more solar radiation in the Earth's atmosphere, increasing surface temperatures.

If you think about climate change from a physics perspective - Earth is essentially a self contained system (all the energy that arrives on Earth from the sun gets dispersed in some way or form - energy cannot be created or destroyed!) and so a thicker atmosphere with higher PPMs of greenhouse gases that trap energy more effectively would lead to more of that energy being dispersed as heat in the atmosphere instead of staying as radiation being reflected back into space.

Current efforts to mitigate climate change generally fall into two categories: reducing carbon emissions (by transitioning to renewable energy) and removing carbon already in the atmosphere (through carbon capture technologies). However, this ignores the fact that from a physics perspective, if you want to reduce the energy trapped in a system - you could increase permabilility of the system so less energy is trapped (current methods) - but you could also just prevent that energy from reaching the system in the first place.

Geoengineering strategies aim to reduce the amount of solar radiation reaching Earth. Among the various proposals - such as placing giant mirrors in space or brightening marine clouds - one of the most promising and realistic methods (not as pie in the sky as the other ones) is stratospheric aerosol injection (SAI). This technique involves releasing small aerosol particles, such as sulfur compounds, into the stratosphere to reflect sunlight and cool the planet, similar to how volcanic eruptions impact global temperatures. The reason this is the most realistic is this has actually happened naturally before - A historical example of this effect was the 1991 eruption of Mount Pinatubo, which led to a temporary global cooling of 1-2 degrees Celsius.

Even if you reduce the amount of sunlight reaching Earth by 1-2%, you would completely negate the 2-3 centuries of anthropogenic climate change we have experienced up till today. Isn't that crazy? And the effect to plant fauna would be negligible - would you really be thirsty if your glass of water had 1% less water in it?

A Practical Approach: Sulfur Injection via Jet Engine Fuel

Rather than deploying a specialized fleet of aircraft carrying sulfur dioxide tanks for stratospheric aerosol injection (as current research is doing), my project explored a more efficient approach: integrating sulfur directly into jet fuel. This concept allows aircraft to release sulfur precursors passively as they fly, eliminating the need for dedicated spraying equipment.

My research, conducted with Andrew Lockley from the University College London, began with a conceptual analysis of this approach. However, to validate the feasibility of sulfur-infused jet fuel, I moved beyond theory into practical experimentation.

Testing with a Microjet Engine

To test the concept, I acquired a microjet engine - a scaled-down jet engine that operates on the same physical principles as full-scale aircraft turbines. Instead of experimenting on a multimillion-dollar Rolls-Royce engine, a microjet provided a cost-effective, low-risk way to evaluate fuel modifications.

Experiment Design

The experiment involved three key components:

• Engine Modification & Sensor Integration:

The microjet engine had built-in safety mechanisms that prevented it from running when detecting foreign substances in the fuel. I modified the firmware to bypass these restrictions and allow sulfur-infused fuel to be burned.

• Measuring Sulfur Dioxide Emissions:

A sulfur dioxide sensor was placed in a copper tube behind the engine to monitor emissions and quantify how much sulfur was successfully converted from fuel to atmospheric aerosol precursors.

• Thrust Performance Analysis:

The engine was mounted on a force dynamo, which measured changes in thrust output. Since fuel composition can impact engine efficiency, it was important to determine if adding sulfur affected performance.

Findings

The results confirmed the fundamental hypothesis: adding sulfur to the fuel led to increased sulfur dioxide emissions, making this a viable method for stratospheric aerosol injection. However, an interesting tradeoff emerged:

• Small amounts of sulfur improved engine performance slightly while still generating the desired emissions.

• Excessive sulfur content led to a decline in thrust output, which could make it difficult for an aircraft to reach the stratosphere.

This revealed a crucial optimization problem - balancing sulfur levels to maximize climate impact while maintaining efficient aircraft performance.

Implications & Future Research

While SAI remains a high-risk, last-resort solution for climate change, my project demonstrated that passively integrating sulfur into jet fuel is a feasible approach to geoengineering. It eliminates the need for dedicated spraying infrastructure and aligns with existing aviation technology.

Of course, serious risks remain. Any large-scale geoengineering effort requires extensive environmental modeling and international collaboration. History has shown that manipulating natural systems can have unintended consequences. Therefore, SAI should not be seen as an immediate solution, but rather as a potential tool for the future - one that could be deployed if climate conditions become dire enough to warrant it.

Any time humans have messed with nature, we have had unintended consequences. Stories of humans releasing a certain species into the wild, and it becoming an invasive species, are a dime a dozen. And that's in a relatively simple system like an ecosystem. The atmosphere is a much more complex system, and so the potential for unintended consequences is much higher.

However, I still think the climate threat, if left unchecked, is so severe that we should be exploring all options. As a last resort option to governments in the latter half of the century, SAI could be a crucial tool to save humanity.

This project was an exciting step toward understanding practical climate intervention methods. The next phase could involve further optimizing the fuel mixture, testing on larger jet engines, and assessing the long-term atmospheric impacts of such an approach. While geoengineering is not a silver bullet, it may one day provide a crucial buffer as humanity transitions to a sustainable energy future.