I had a class back when I was young and pretty that had as one of its texts, "The Invasion of America", a predictably anti-colonialist screed. One of the claims was that the population of North America (as was the case in Central America) had hit a height before 1492-ish that wouldn't be reached again until the 19th century or early 20th century. Obviously the culprit (if there was one) would have been inadvertently introduced diseases.
Interestingly enough, in a completely different class we learned that the Mayans had I think four different population peaks, with the final one in the early 16th century, just as Cortez arrived. Unless one attributes the earlier ones to a similar introduction of diseases by ocean-crossers -- which could help clarify where some of the ancient pandemics in Eurasia came from -- one is stuck with the 100 percent natural climate changes that continue to this day.
I think it may be both.
There is convincing proof of temporary settlements of Vikings in Canada who could have brought European diseases to the Americas. Who know where else they may have landed and interacted with American natives for even a day or two.
Transpacific travelers are also likely to have brought disease to the Americas.
Even homegrown pandemics are also inevitable that could drastically reduce population just as they did with the medieval plagues in Europe and Asia.
Changes in weather patterns certainly do cause crop failures and famines. The Dust Bowl crop failures of the 1930s would certainly have caused a famine without the presence of the modern transportation.