Geopolitical Risks of Climate Engineering: Marine Cloud Brightening Could Unexpectedly Weaken the El Niño Cycle

Introduction: The Dual Nature of Climate Engineering

As global carbon emissions continue to rise, interest in geoengineering is growing worldwide. From the Intergovernmental Panel on Climate Change (IPCC) to national research institutions, large-scale technical approaches to intervene in Earth’s radiation balance are being explored. However, a study recently published in *Earth’s Future* sounds an alarm: not all cooling strategies are equally safe. Scientists at the University of California, Santa Barbara, have found that one widely discussed approach—Marine Cloud Brightening (MCB)—if deployed in specific regions, could lead to a significant weakening of the El Niño-Southern Oscillation (ENSO), triggering a chain reaction in global weather systems.

This research is not only relevant to climate science but also has profound implications for global development. As one of the most important natural climate rhythms on Earth, ENSO directly affects monsoons, agriculture, and water resources in developing countries across Southeast Asia, South Asia, Africa, and Latin America. When a climate intervention technique could potentially weaken this rhythm, the adaptive capacity and climate resilience of developing nations become core issues.

Marine Cloud Brightening: The Global Cost of Regional Cooling

The basic principle of Marine Cloud Brightening involves spraying sea salt particles into the lower atmosphere (less than 2 km above the sea surface) to make cloud droplets smaller and more numerous, thereby increasing cloud reflectivity and reflecting more sunlight back into space. Because it is relatively low-cost and technically feasible, this approach is considered one of the potential "fast cooling" methods.

However, the study’s lead author, doctoral student Chen Xing, and collaborators found through climate simulations that if MCB is deployed in the southeastern Pacific (i.e., off the coasts of Peru and Chile), ENSO amplitude would drop sharply by about 61%. The driving mechanism behind this change is: brightened clouds cool the sea surface while suppressing precipitation; cooler, drier air propagates toward the central Pacific, weakening evaporation and atmospheric circulation and strengthening trade winds. Stronger trade winds intensify cold water upwelling, further lowering sea surface temperatures, forming a self-reinforcing cycle that eventually causes El Niño and La Niña events to nearly disappear.

Co-author Associate Professor Samantha Stevenson noted that such drastic changes are almost impossible in nature—even under climate change, ENSO would not decline by 60% within a decade. This means that human interference with ENSO would be unprecedented in its speed and systemic nature.

Risks from a Development Perspective: Agriculture, Food, and Water ResourcesThe impact of ENSO weakening on global development cannot be underestimated. El Niño and La Niña events drive precipitation patterns in many regions of the world: for example, El Niño often brings wet winters to California in the United States, while La Niña strengthens monsoon rainfall over South and Southeast Asia, supporting the agricultural livelihoods of billions of people. If ENSO amplitude were to decrease significantly, the predictability of monsoon systems could be disrupted, leading to crop failures, water resource instability, and consequently exacerbating food security risks.

For countries in the Global South, the agricultural sector often accounts for 10–30% of GDP and employs a large share of the workforce. Nations such as India, Indonesia, the Philippines, and Vietnam rely on ENSO signals for irrigation and planting planning. When these signals disappear, farmers lose a key forecasting tool, reducing their ability to cope with extreme weather events. In addition, ENSO influences tropical cyclone formation, marine ecosystems (such as Peru's anchovy fishery), and public health (such as dengue fever transmission).

From an ESG perspective, the non-temperature impacts of MCB may create new environmental and social inequalities. Developed countries may find it easier to deploy climate engineering, while developing countries would bear the negative consequences of regional climate changes—even if global average temperatures decline, the adaptation costs for these countries could far outweigh the benefits.

Lessons from Another Approach: Stratospheric Aerosol Injection

The same study compared MCB with another leading approach—Stratospheric Aerosol Injection (SAI). SAI releases sulfate particles into the stratosphere, creating a reflective layer that is broader and more uniform. Simulations show that SAI has almost no significant effect on ENSO.

The key difference lies in spatial scale: MCB is concentrated in specific near-surface regions, making it highly prone to disturbing local air-sea coupling processes; in contrast, SAI particles disperse globally, producing a more uniform cooling effect and reducing regional climate system distortions. However, this does not mean SAI is risk-free—it could affect the ozone layer, reduce photosynthesis, and raise other ethical concerns.

The research team emphasized that both intervention approaches can achieve similar global cooling targets, but their regional climate effects are markedly different. This highlights a core governance principle: climate engineering cannot be reduced to a single "cooling efficiency" metric; it must involve trade-offs among multiple climate systems, ecosystems, and human societies.

Lessons for Climate Governance: Don't Let Technological Shortcuts Replace Systematic Adaptation

This study provides an important reminder for global climate governance. As global temperatures continue to rise, the desire for fast-acting solutions may drive policymakers to overlook long-term risks. Currently, there is a lack of international consensus and regulatory frameworks for the governance of climate engineering. Organizations such as the United Nations Environment Programme and the World Meteorological Organization are promoting assessments, but actual progress is slow.From the perspective of the Sustainable Development Goals (SDGs), climate engineering must be advanced in synergy with mitigation, adaptation, and resilience building. Any approach that does not take "leaving no one behind" as its bottom line may exacerbate existing development gaps. For example, the deployment of MCB in the Southeast Pacific not only affects local fisheries (Peru, Chile) but also impacts the global food system through changes in ENSO. These cross-sectoral and cross-regional chain reactions call for more prudent ex-ante assessment and early warning mechanisms.

Researchers point out that not taking any intervention also carries risks—climate warming itself can alter ENSO. However, the rate of natural change is far slower than the abrupt changes induced by MCB. Stevenson stated: "A 60% attenuation of ENSO within a decade is impossible in the natural world." This warns us that the risks of human intervention may far exceed expectations.

Conclusion: Toward Responsible Research and Governance

The study from the University of California is not opposed to all climate engineering, but calls for restraint until a full understanding of Earth system complexity is achieved. In the future, more research is needed on the impacts of different geoengineering strategies on marine ecosystems (e.g., phytoplankton productivity) and terrestrial carbon sinks.

For international development agencies, ESG investors, and policymakers, this paper sends a clear signal: climate solutions must pass the test of development resilience. Before advancing any large-scale intervention, the adaptive capacity of Global South countries, regional climate dependence, and intergenerational equity should be incorporated into the decision-making framework. True climate resilience lies not in finding a perfect "thermostat," but in building diverse, inclusive, and self-regulating social-ecological systems.