More generally, recent research pointed to climate change as a key player in the decline and fall of historical societies in China, the Middle East, Europe, and the Americas. In fact, paleoclimatogical temperature reconstruction suggests that historical periods of social unrest, societal collapse, and population crash and significant climate change often occurred simultaneously. A team of researchers from mainland China and Hong Kong were able to establish a causal connection between climate change and large-scale human crises in pre-industrial times. Short-term crises may be due to social problems, but climate change was the ultimate cause of major crises, starting with economic depressions. Moreover, since agriculture is highly dependent on climate, any changes to the regional climate from the optimum can induce crop failures.