Understanding Climate Tipping Points and the Role of Solar Geoengineering

A Talk by Wake Smith, Research Fellow, the Mossavar-Rahmani Center for Business and Government, Harvard Kennedy School

Climate change is rapidly approaching dangerous thresholds, also known as tipping points, where the effects become irreversible and uncontrollable. 
What Are Climate Tipping Points?
Tipping points are thresholds in the climate system that, once crossed, lead to significant and often irreversible changes. These changes are nonlinear, meaning they accelerate rapidly and can spiral beyond the control of traditional climate mitigation methods. While the science surrounding tipping points is still evolving, emerging evidence suggests that certain critical thresholds, particularly in polar regions, may be crossed as early as mid-century. The Arctic and Antarctic are warming far faster than other parts of the globe, increasing the risk of catastrophic climate shifts.
Global Warming Trends:
Data from July 2024 shows that CO2 concentrations have reached 425.5 ppm, a 35% increase from pre-industrial levels (~280 ppm).
There was a brief pause in emissions during the 1950s and 2010s, but global greenhouse gas emissions continue to rise, with a slight decline noted in 2023. Yet, the Paris Agreement’s goal of limiting warming to 1.5°C is in doubt, with projections showing temperature anomalies of +2.7°C by 2090.
Carbon Removal Techniques:
The presentation outlines the importance of carbon dioxide removal (CDR) alongside solar geoengineering. Technologies like direct air capture, flue gas capture, and geologic sequestration are essential but currently operate at a minimal scale. Scaling these technologies is crucial to achieving net-zero emissions, but financial barriers remain significant.
Solar Geoengineering: A Controversial Potential Solution
Solar geoengineering has emerged as a potential tool to prevent these tipping points. The technique involves reflecting a portion of sunlight back into space, cooling the planet in the process. One of the most discussed methods is Stratospheric Aerosol Injection (SAI), which mimics the cooling effect seen after volcanic eruptions, where ash particles reflect solar radiation away from Earth.
Wake Smith advocates for more field research into solar geoengineering, despite its contentious nature. He argues that, given the increasing threat of near-term climate crises, this technology may provide humanity with a critical stopgap while longer-term solutions like decarbonization take effect.
The Science Behind Earth's Energy Imbalance
The fundamental driver of climate change is Earth's energy imbalance, which occurs when the amount of absorbed solar radiation exceeds the outgoing longwave radiation. This imbalance results in a net increase in global temperatures. Historical data (1860-2020) show a significant rise in global mean temperatures, and current projections indicate that temperatures will continue to increase unless global CO2 emissions reach net-zero by mid-century, followed by negative emissions later.
However, solar geoengineering is seen as a "peak-shaving" mechanism that could temporarily decouple rising emissions from rising temperatures, buying time for more permanent emission reduction strategies.
Challenges of Solar Geoengineering
While the potential of solar geoengineering is promising, its implementation faces significant technical and geopolitical hurdles. For example:

• Technical Feasibility: Current aircraft are not capable of deploying aerosols at the altitudes required for effective geoengineering. Research is ongoing into platforms like the SAI Lofter, which could theoretically carry 15 tons of materials to 20 kilometers above the Earth’s surface.
• Geopolitical Issues: The complexities of deploying solar geoengineering globally are immense. Only two countries—the U.S. and China—are considered capable of undertaking such a large-scale intervention, both in terms of technology and geopolitical influence. Coordinating a global response or agreement would be incredibly challenging.

Risks and Uncertainties
As with any climate intervention, solar geoengineering carries risks. Manipulating Earth’s climate systems could lead to unforeseen consequences, exacerbating the very issues it seeks to solve. Critics warn of hubris—attempting to "fix" the planet by further altering natural systems could create new, potentially worse problems. Moreover, political conflicts and diverging preferences between nations could intensify as the effects of geoengineering would not be evenly distributed across the globe.
The Case of the Atlantic Meridional Overturning Circulation (AMOC)
One of the most concerning tipping points is the potential collapse of the Atlantic Meridional Overturning Circulation (AMOC), a system of ocean currents that helps regulate the global climate. Wake discusses a study by Ditlevsen, P., & Ditlevsen, S. (2023), sharing key findings including:

1. AMOC Tipping Point: Recent observations and models indicate early warning signals (EWSs) of a potential AMOC collapse, with increased variance and autocorrelation in sea surface temperature data. This suggests that AMOC is losing resilience and approaching a critical transition.
2. Timing of Collapse: The paper estimates, with a 95% confidence interval, that the AMOC may collapse between 2025 and 2095, most likely around 2057. This prediction is based on statistical analysis and data-driven estimations using the AMOC's historical "fingerprint" from sea surface temperatures.
3. Risks of Collapse: A collapse of AMOC would drastically affect global climate patterns, particularly leading to more extreme weather events in the Northern Hemisphere and disrupting ocean circulation and heat distribution.
4. Model Uncertainties: While models provide robust predictions, uncertainties remain due to the complexity of AMOC's dynamics and the influence of factors such as freshwater influx and greenhouse gas emissions. The paper calls for more precise monitoring and data collection to refine predictions.

The Path Forward
Although solar geoengineering presents risks, it may offer a critical tool for preventing the worst effects of climate tipping points. Early deployment in the Arctic, where warming is most rapid, could provide valuable insights into the feasibility and consequences of this technology. Ongoing small-scale geoengineering tests, like those funded by the UK, aim to explore these risks in a controlled manner.
As climate tipping points draw closer, the need for innovative, even controversial, solutions like solar geoengineering becomes more urgent. Wake Smith's call for accelerated research into solar geoengineering reflects the growing recognition that traditional mitigation efforts may not be enough to stave off the most dangerous effects of climate change. However, the technical, political, and ethical challenges of geoengineering are immense, and caution is key as humanity considers interventions that could reshape our planet.

Reference: 
Ditlevsen, P., & Ditlevsen, S. (2023). Warning of a forthcoming collapse of the Atlantic meridional overturning circulation. Nature Communications, 14, 4254. https://doi.org/10.1038/s41467-023-39810-w
OpenAI, ChatGPT. (2024, September). Used for conversational assistance, providing summaries and enhancing the style of writing.
Smith, W. (2024, September 26). Climate Tipping Points and Solar Geoengineering. Presentation at the Mossavar-Rahmani Center for Business and Government, Harvard Kennedy School, Cambridge, MA.