There is no law or treaty to prevent a private company from tinkering with geo-engineering—say, releasing sulphur dioxide high in the stratosphere to alter the climate. And so there will be no fines or arrests following recent news that a startup quietly pulled off such a release last year by launching two balloons over Mexico. This sort of manipulation can alter the energy balance between the sun and Earth. In the upper atmosphere, sulphur dioxide forms suspended particles of sulphuric acid that act to scatter sunlight and cool the planet. The US Clean Air Act isn’t set up to deal with this sort of thing—it’s focused on power plants, cars and regional air-quality standards, said UCLA environmental law professor Edward Parson.