Humans have been a powerful geological and biological force on this planet for quite a while. The first inhabitants of any land typically triggered a wave of extinctions, and deforestation. While preindustrial climate change might suggest that a natural process at play, you might also conclude that the impact of humans is substantially bigger than we might expect. The little ice age is an interesting example here. We don’t know the exact causes, however some have suggested that the population decline of the black plague and later measles/smallpox in the new world were the culprits. This would of course make the situation for geoengineering a solution that much harder.