It’s already happened.
Back in 1610, there was a sudden and sharp decline in farming activity in the Amazon rainforest. The civilisations that were cutting down that forest had become infected by smallpox, causing the deaths of an estimated 50 million people.
This is an event that has been recorded in rocks and Antarctic ice, because there was also a corresponding sharp reduction in the CO2 in our atmosphere, courtesy of the increased plant growth there.
And we weren’t even burning thousands of gigatonnes of coal every year.