I think the author fails to adequately discuss the impact of climate change on the economy. We know from modern history that recessions lead to political change. It's not implausible that an agrarian economy like that of Rome would be greatly impacted by climate change reducing crop yields, causing political upheaval. Those actors in the political system didn't just decide one day to depart from 200 years of stability to overthrow the government, there had to be some motivating factor.