But creating enough greenhouse capacity to feed 8 billion people would be incredibly difficult. Bottom line would be, if something causes sunlight to be largely blocked around the world for a sustained period of time, the number of people who would starve to death would likely number in the billions.

LED lighting could also be used for outdoor crops. From my perspective green houses are most beneficial for pest management and water conservation. They might actually prevent light from reaching plants to the same degree as an outdoor grow due to reflection and opacity.

Greenhouses cool down rapidly after sunset though, they still freeze if it's many deg below freezing outside. (Also mass crops like wheat and soy would have to solve harvesting machinery and resource cost vs area size)