I also saw that documentary years ago and recalled it recently in light of the global shutdown that is happening. It was a BBC Horizon documentary aptly named 'Global Dimming.' You can watch it here: https://vimeo.com/138779240 It was also re-packaged for the American audience later the same year in a Nova episode: https://www.pbs.org/wgbh/nova/sun/ If the next few months see continued drastic reductions in air and automobile traffic and other sources of particulate pollution, it will present an opportunity to see if the hypothesis in the documentary holds up. If it does, this might be a hotter and drier summer than it would have been otherwise.