A team of researchers based their findings on data from 9,000 wooden artifacts from the past 2,500 years.There are theories a-plenty about the Western Roman Empire's demise, and prima facie this seems as good as any. It has a desirable property that, unlike more "historical process" explanations, there is no indeterminedness of whether something that occurred was a symptom of the decline, or its cause: tree rings are presumably oblivious to human societal organization.
They found that periods of warm, wet summers coincided with prosperity, while political turmoil occurred during times of climate instability.
I was recently reading a book on the subject, I believe it was the Grand Strategy of the Byzantine Empire but correct me if I'm wrong, which made the point that the eastern Empire should have been a better candidate for failure for several reasons, including the fact that it was partitioned across three continents, and faced more formidable foes. Did climate do the Western Romans in?
A good test of the theory would be to identify other instances (in space or time) where climate can be linked to social organization; obviously there are not that many candidates with a continuously recorded history that long, and the fact that climate may have unstabilized the Western Empire does not mean that every "collapse" can be traced to climate.
Science DOI: 10.1126/science.1197175
2500 Years of European Climate Variability and Human Susceptibility
Ulf Büntgen et al.
Climate variations have influenced the agricultural productivity, health risk, and conflict level of preindustrial societies. Discrimination between environmental and anthropogenic impacts on past civilizations, however, remains difficult because of the paucity of high-resolution palaeoclimatic evidence. Here, we present tree ring–based reconstructions of Central European summer precipitation and temperature variability over the past 2500 years. Recent warming is unprecedented, but modern hydroclimatic variations may have at times been exceeded in magnitude and duration. Wet and warm summers occurred during periods of Roman and medieval prosperity. Increased climate variability from ~AD 250 to 600 coincided with the demise of the Western Roman Empire and the turmoil of the Migration Period. Historical circumstances may challenge recent political and fiscal reluctance to mitigate projected climate change.