Aerosol Masking Effect. It's been known about for ages and it amazes me that they banned it from ship fuel. It seems incredibly short sighed to me.
Before reducing SO~2~ we need to stop and reduce CO~2~, otherwise we cook. Look at how things have escalated this year. I mean we also went straight from La Niña to El Niño too but the lack of aerosol masking has very likely had an impact.