That is the new and very important book by Kyle Harper, with the subtitle Climate, Disease, & the End of an Empire. I am just reading through this now, but it appears to be an significant revision of our views on the decline of Rome. p.21 offers a capsule summary, which I will summarize in turn:
1. During the reign of Marcus Aurelius, a pandemic “interrupted the economic and demographic expansion” of the empire.
2. In the middle of the third century, a mix of drought, pestilence, and political challenge “led to the sudden disintegration of the empire.” The empire however was willfully rebuilt, with a new emperor, new system of government, and in due time a new religion.
3. The coherence of this new empire was broken in the late fourth and early fifth centuries. “The entire weight of the Eurasian steppe seemed to lean, in new and unsustainable ways, against the edifice of Roman power…and…the western half of the empire buckled.”
4. In the east there was a resurgent Roman Empire, but this was “violently halted by one of the worst environmental catastrophes in recorded history — the double blow of bubonic plague and a little ice age.”
Here is a key passage from the book:
The centuries of later Roman history might be considered the age of pandemic disease. Three times the empire was rocked by mortality events with stunning geographical reach. In AD 165 an event known as the Antonine Plague, probably caused by smallpox, erupted. In AD 249, an uncertain pathogen swept the territories of Roman rule. And in AD 541, the first great pandemic of Yersinia pestis, the agent that causes bubonic plague, arrived and lingered for over two hundreds years. the magnitude of these biological catastrophes is almost incomprehensible.
I do not feel I can assess the veracity of this thesis, but it does seem to be intelligently and reasonably argued.