Reflecting a small fraction of incoming sunlight to reduce global heating is not a new idea. It is time to safely experimentThe world is warming fast – and our options to avoid catastrophic harm are narrowing. 2024 was the first full year more than 1.5C hotter than the 19th-century average. Emissions are still rising, with fossil fuel use expected to hit a new high in 2025. Permanent carbon removal technologies – often cited as a fix – are removing just tens of thousands of tonnes annually, almost nothing relative to the 5-10bn tonnes needed. Cutting emissions and scaling carbon removal remain essential. But they may not be enough.As suffering grows and ecosystems unravel, more people will ask: is there anything we can do to prevent these harms? The idea of reflecting a small fraction of incoming sunlight to reduce warming is not a new idea. In 1965, Lyndon B Johnson’s science advisers proposed it as the only way to cool the planet. Earth already reflects about 30% of incoming sunli...