Why blocking sunlight to cool Earth is becoming a global policy concern

Solar radiation modification could temporarily cool the planet by reflecting sunlight, but uncertain effects on rainfall, ecosystems and geopolitics are raising governance concerns

The European Union (EU) has launched a €35 million (US$39.4 million) tender to fund research and coordination on detecting solar radiation modification (SRM), a technology that proposes to cool the Earth’s atmosphere by blocking the Sun.

 

According to the tender, the concern is not only that this technology can already change the global climate, but also that governments or private companies might eventually deploy it without international agreement. The project will look at how governments could spot an intervention, establish where it came from and assess its effects on the climate and ecosystems.

 

But what exactly is solar radiation modification, and what could happen if someone tries to alter the amount of sunlight reaching Earth?

 

 

What is solar radiation modification?

 

Also called solar geoengineering, SRM refers to proposed methods of cooling the Earth by reflecting some incoming sunlight back into space. It does not remove carbon dioxide from the atmosphere, which is the fundamental cause of human-driven climate change.

 

The best-known approach is stratospheric aerosol injection (SAI). It involves releasing material into the stratosphere, where particles could reflect some sunlight and reduce the amount of solar energy reaching Earth.

 

The idea is based partly on what happens after major volcanic eruptions. When volcanoes inject sulphur dioxide high into the atmosphere, it can form sulphate aerosols that reflect sunlight and temporarily lower global temperatures.

 

Other proposed approaches include marine cloud brightening, which would make some low clouds more reflective, and cirrus cloud thinning, intended to reduce the heat-trapping effect of high-altitude clouds. The EU's call specifically refers to research on SAI, marine cloud brightening and cirrus or mixed-phase cloud modification.

 

Is SRM a viable option to fight climate change?

 

A UN Environment Programme (UNEP) report titled Limiting Overshoot, published in September, has warned that the world is set to exceed the 1.5 degree Celsius (2.7 degrees Fahrenheit) global warming threshold within the next few years. Cutting greenhouse gas emissions is essential but takes time. SRM is being examined because, in theory, deliberately reflecting sunlight could lower global temperatures much faster than reducing atmospheric carbon dioxide.

 

However, the UNEP's latest assessment says current scientific evidence does not support SRM as a viable response to climate change. It says the technology remains speculative, with major knowledge gaps, uncertain environmental and social consequences and no global governance framework.

 

Is SRM being implemented currently?

 

There is currently no large-scale deployment of solar radiation modification capable of cooling the planet. The technology remains largely at the research and experimental stage. But research and small-scale activity have moved beyond computer models.

 

According to the US Environmental Protection Agency, one prominent private example is US startup Make Sunsets, which has launched balloons carrying sulphur dioxide and marketed “cooling credits”. The USEPA said in April 2025 that the company had reported releasing about 0.1 tonnes of sulphur dioxide into the stratosphere by May that year. The EPA called the amount extremely small but said it was examining its authority to respond if the activity were to scale up.

 

Harvard's SCoPEx project, which explored the science behind stratospheric aerosol injection, was ultimately cancelled in March 2024 after years of debate over the governance and ethics of outdoor experiments.

 

UK's Advanced Research and Invention Agency has committed £56.8 million to its Exploring Climate Cooling programme, involving 22 research teams working across atmospheric science, modelling, ecology, governance and ethics. The programme allows small, controlled outdoor experiments where necessary and subject to oversight.

 

Why the EU wants to monitor SRM

 

Any changes in atmospheric chemistry, rainfall, temperature or ecosystems could affect countries that had no role in the decision to conduct the solar radiation modification. The EU Council said in April that large-scale climate interventions, particularly SRM, could pose significant risks to the climate, environment, security and geopolitics. It called for the precautionary principle, close monitoring and a moratorium on deployment.