Greenhouses in the Netherlands would like a word, which are growing high nutrient crops fast using elevated CO2 concentrations. It's not the rate at which things grow that makes them less nutricious, it's selectively breeding for things that grow faster without bother to also select for "and taste good/have plenty of nutritional value".
This one doesn't get to hide behind global climate change.
The last time carbon dioxide was so plentiful in our planet's atmosphere was in the Pliocene era, around 3 million years ago. Life on Earth was dominated by giant mammals; humans and chimps had shared their last common ancestor. Although the sun's force was about the same, the sea levels were 15 metres higher and Arctic summer temperatures were 14 degrees higher than the present day.
No, this is not yet another 'climate change' thing no matter how much you want to pull that concept into the discussion. It is a large-scale 'industrial' agriculture thing with crops which have been bred for fast growth on relatively mineral-poor soil using simple nitrogen fertiliser instead of manure from pasture-raised livestock.
> How then did the Plioocene megafauna grow so large if the plants they consumed were 'faster, bigger [...] with less nutrient density'?
Herbivores are adapted to extracting nutrients from a food source that isn't nutrient or calorie dense. Compared to carnivores, they eat a massive amount of food relative to their body weight. If a food source is less nutritious, they just eat more of it.
Lower nutrient density isn't an issue when there's plenty of food to eat. There weren't 8 billion people to feed 3 million years ago, and back then they lived right alongside their food.
