On Climate Change and the Fall of the Roman Empire

Map of the "barbarian" invasions of ...
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T’other day we were criticizing the Guardian for its credulity in buying into a claim about the purported discovery of Caligula’s tomb. While that story was breaking, simmering on the backburner was a story that’s still making the rounds claiming some sort of correlation between the rise and fall of empires and climate change. To give you a sense of how varied the coverage on this one is, all you have to do is see the range in tone and the-sky-is-falling-ness of the headlines … ecce:

The original article (which is behind a pay wall) can be accessed via:

… where one can also read the abstract (the article itself is a rather difficult read):

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.

We’ll reproduce the BBC coverage as it sort of falls in the middle of the ‘tonality’:

A team of researchers based their findings on data from 9,000 wooden artifacts from the past 2,500 years.

They found that periods of warm, wet summers coincided with prosperity, while political turmoil occurred during times of climate instability.

The findings have been published online by the journal Science.

“Looking back on 2,500 years, there are examples where climate change impacted human history,” co-author Ulf Buntgen, a paleoclimatologist at the Swiss Federal Research Institute for Forest, Snow and Landscape, told the Science website.

Ring record

The team capitalised on a system used to date material unearthed during excavations.

“Archaeologists have developed oak ring width chronologies from Central Europe that cover nearly the entire Holocene and have used them for the purpose of dating artefacts, historical buildings, antique artwork and furniture,” they wrote.

“Chronologies of living and relict oaks may reflect distinct patterns of summer precipitation and drought.”

The team looked at how weather over the past couple of centuries affected living trees’ growth rings.

During good growing seasons, when water and nutrients are in plentiful supply, trees form broad rings, with their boundaries relatively far apart.

But in unfavourable conditions, such as drought, the rings grow in much tighter formation.

The researchers then used this data to reconstruct annual weather patterns from the growth rings preserved in the artefacts.

Once they had developed a chronology stretching back over the past 2,500 years, they identified a link with prosperity levels in past societies, such as the Roman Empire.

“Wet and warm summers occurred during periods of Roman and medieval prosperity. Increased climate variability from 250-600 AD coincided with the demise of the western Roman empire and the turmoil of the migration period,” the team reported.

“Distinct drying in the 3rd Century paralleled a period of serious crisis in the western Roman empire marked by barbarian invasion, political turmoil and economic dislocation in several provinces of Gaul.”

Dr Buntgen explained: “We were aware of these super-big data sets, and we brought them together and analyzed them in a new way to get the climate signal.

“If you have enough wood, the dating is secure. You just need a lot of material and a lot of rings.”

As can be seen in the penultimate quotation from Dr Buntgen, the study is somewhat careful to avoid making a cause-and-effect relationship out of this. Indeed, in the original article (which was kindly forwarded to me), the authors of the study say specifically (p. 3):

Comparison of climate variability and human history, however, prohibits any simple causal determination and other contributing factors, such as socio-cultural stressors must be considered in this complex interplay.

Despite that, we are seeing a pile of folks in newspapers and various social media venues who seem to be inferring from the study that climate change actually caused the fall of Rome to a greater or lesser degree. While the traditional date for the “fall” is outside of the purview of this blog, some of the material of this study does come from the mid- to late third century, so we feel a need comment on it.

The obvious starting point is that the study is based on evidence from some 9000 samples of wood (7200 or so which are oak), which certainly sounds like an impressive database. What isn’t made clear in much of the press coverage is that all these samples come mostly from central Germany and northeastern France, which seems somewhat ‘localized’ for the sweeping continent-wide claims being attached to it. The tree rings themselves are used to infer precipitation during April-May-June, which seems oddly specific and seems to ignore (doesn’t it?) that there might be precipitation in other forms at other times of the year. There are lots of impressive graphs which show variations in precipitation of roughly 100-150 mm in various semi-regular cycles as well as parallel temperature variations which are mostly +1 or -1 degrees ‘off’. I’m not really in a position to say how ‘serious’ such variations might be.

That said, we should note is that the article does make claims about anthropogenic climate change based on ‘historical tree harvest’. It isn’t clear what they mean by this, but I’m assuming there are fewer samples available in certain time periods and the authors assume this means there was less wood to harvest. That might be accurate, but it is also extremely possible that this is just another example of the fickleness of archaeological finds (cf. problems with studies which try to establish ancient life expectancy based on Latin inscriptions). Statistical studies based solely on what has been found in this or that dig are always hazy and need to be supported by other evidence. I’m not sure whether the claims in this study are.

The other thing which bothers me about both the study and the way this is being spun by various folks is that the links between ‘barbarian migrations’ and climate change don’t quite make sense. The idea seems to be that climate change meant the folks had to hop on their horses and invade some other place because they had problems adapting to changes in climate. We’ll ignore the implication that samples from Central Europe can be used to determine climate over a much larger area, which may or may not be valid. However, in regards to the period of our purview, the late third century falls into the people-having-problems-adapting category. For the most part, this is when assorted Goth and Germanic groups are moving down from the North into areas controlled by Rome. The question must be asked: did the climate variability — in areas where samples do not come from — cause the migration? Or were these folks just doing the migrating thing and decided it was good to settle in an area where climate variation and deforestation were apparently happening? Whatever the case, the purported links being made do not quite make sense in the period of our purview and possibly beyond it as well …

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