It is a known fact that the climate of the earth varies from era to era and small differences in average temperatures and rainfall can devastate crops and populations. They can even trace the movements of prehistoric peoples into and out of Europe as a result of those temperature changes.
Only in recent years has there been some degree of ability to adapt to major changes and even that is limited.
Look at U.S. In the middle of a great drought this summer. And cities that are built in the desert (such as Las Vegas) are ultimately unsustainable.