You can measure temp by what grew. One study did involve Utah. Scientists studied packrat middens and found pollen/seeds and other artifacts from trees and plants that now grow mostly in the alpine zone. So it was colder then. Now plants from warmer climates seem to be migrating north. We will either have to bioengineer our food crops or find others. Corn and wheat require a certain growing season. Should we only be able to grow them in the far north, the land area is not there. You can see what too thick a cloud layer can do. The probes sent to Venus showed that. Too light an atmosphere and you have Mars. One article I saw said that climate change would cause humans to evolve differently.