In this continuation of his exchange with Brooke about Brooke’s big-picture model of climate change and human response, Harper argues for careful articulation of what kind of Malthusianism Brooke claims, or does not claim, for or against his model. Harper also challenges Brooke’s description of the paleoclimate known as the Roman Climate Optimum as a period dominated by a persistently positive mode of the North Atlantic Oscillation.

John Brooke’s Climate Change and the Course of Global History is a significant contribution to deep history and an impressive elucidation of the intersection between environmental studies and historical sociology. Brooke’s primary argument is that climate change is a “joker,” a wild card incompatible with theories of deep history that are based on Malthusian theory. However, because of his limited presentation of Malthusianism and his ambiguous treatment of “endogenous” and “exogenous” causal factors, he misses an opportunity to integrate social-science models of long-range historical development with the powerful forces of environmental change.

Growing scientific evidence from modern climate science is loaded with implications for the environmental history of the Roman Empire and its successor societies. The written and archaeological evidence, although richer than commonly realized, is unevenly distributed over time and space. A first synthesis of what the written records and multiple natural archives (multi-proxy data) indicate about climate change and variability across western Eurasia from c. 100 b.c. to 800 a.d. confirms that the Roman Empire rose during a period of stable and favorable climatic conditions, which deteriorated during the Empire's third-century crisis. A second, briefer period of favorable conditions coincided with the Empire's recovery in the fourth century; regional differences in climate conditions parallel the diverging fates of the eastern and western Empires in subsequent centuries. Climate conditions beyond the Empire's boundaries also played an important role by affecting food production in the Nile valley, and by encouraging two major migrations and invasions of pastoral peoples from Central Asia.