The chart is actually not so terrifying once the cause of it was discovered. Basically, new regulations reducing sulfur content in ship fuel that went into effect in 2020 and put an end to an unintentional geoengineering experiment that had been running due to the enhanced cloud cover the sulfur dioxide particles were causing.

So this is a strong data point showing that solar geoengineering could work, if we resume it deliberately under more controlled conditions.