Climate and the Collapse of the Roman Empire | Part 2: Climate
A few months ago, I visited Iceland. It is a shockingly beautiful piece of basalt that is located just south of the Arctic circle between North America and Europe. In fact — the continental boundary between the American and Euroasian plates runs through the middle — you can walk from one continental plate to the next.
Geologically, Iceland is very weird. And you don’t have to be a geologist to know it. The island is eerily silent, missing the cacophony of birds and insects. Few plants grow there, and as you drive down the one-lane road that circles the island, you’ll see endless fields of green moss growing on black basalt.
Glaciers still exist in Iceland, though like all other land glaciers today they are receding rapidly in the face of climate change. There are glacial lagoons where icebergs queue up before entering the ocean.
Iceland is made of volcanic basalt, which is very unlike the continental granite you are most likely standing/sitting on right now. This restricts the ability for plants to easily grow there — a difficulty compounded by the subarctic climate of the area. The Vikings, when they first arrived, believed they had found an isolated land of plenty. The felled the trees for homes, sowed the fields, and watched helplessly as thousands of years of soil eroded away in decades. The island was left bare and damaged up to the present day — it may never recover in human history.
Iceland has seeded glaciers — such as the majestic Vatnajökull — which contributes to creating a low-pressure zone that creates a highly variable climate — clouds soar overhead and rainfall can come at any time. Far to the south, near the Azores islands almost a thousand miles west of Portugal, a permanent high pressure zone sits, creating a mild climate. These two pressure zones pinch the westerly winds that soar over Europe, and direct their intensity over different areas. They can almost be thought of as two gears which change the direction of the rainstorms which pass over Europe. This movement is called the North Atlantic Oscillation (NAO for short). And it may be more important to the history of Rome than guys like Gaius Julius Caesar.
Let’s take the positive case, when the high pressure zone near the Azores Islands to the south is strong, and the low pressure zone over Iceland is weak.
The westward jet stream stays pretty far north, and delivers lots of precipitation to northern European countries like Sweden, Germany, and Poland. By contrast, the Mediterranean, or as the Romans called it, “Our Sea”, has a cooler and drier climate. For those who live on the Eastern Seaboard of the USA, this movement tends to bring precipitation to that area as well. This, however, is not the only climactic flavor of the NAO.
The negative NAO creates an opposite pattern. A stronger Icelandic low puts a kink in the jet stream, which causes storms to track further south. Under these conditions, the Roman Empire would have had more rain than usual, while droughts would be much more common in northern Europe. And, this would cause colder winters on the Eastern Seaboard of the USA. The movement of the North Atlantic Oscillation is not binary — it actually exists along a continuum. This is something that can be measured each year — the measurement is simply subtracting the strength of the Icelandic Low from the Azores High.
The instrumental record shows that while there are highs and lows, the general trend for the past 150 years has been pretty stable at around 0. We’ve only seen a movement to a +1 index in the past 10 years or so. The question is — what would this tell us about ancient history? Here, we have a big problem — the Romans didn’t know about Iceland or the Azores, much less record their respective air pressures each year. For most of my time thinking about this problem, there was no reliable record for the NAO beyond our instrumental data. That changed in 2012, when Jesper Olsen of (today) Aarhus University and his colleagues published a new reconstruction that went back 2,500 years. Now — a quick note before we continue — we are already spoiled on our expectations, because we have year-to-year data on the NAO. The data Olsen and his colleagues put together is much lower resolution — maybe giving us a 20-year view of the phenomena at best. So, as a result, his data will look more like the stable blue line above. But that is important — that tells us what happens over the course of human generations.
How did Olsen and friends do it? They used sediments deposited in an isolated lake in southern Greenland, Kangerlussuaq (misspellings will be docked 10 points). If the NAO is negative, the ice melts early, and the water that gets to the lake causes more mixing and thus more oxygen, which in turn gets preserved in the form of calcium carbonate (CaCO3). A positive NAO creates a colder year, and ultimately less mixing and more anoxic conditions, with effects on calcium carbonate and both manganese (Mn) and iron (Fe). Because the agreement in these effects was so good in the recent record, they felt confident that they could use it as an indicator to reconstruct what happened in deeper time with the NAO, including the time of the Roman Empire.
When I first saw this data, my eyes jumped to the blue line at about 1,500 BP (before-persent), which translates to roughly 500 A.D. This was the migration period — when Germanic peoples like the Goths, Franks, Lombards, Turkick peoples like the Avars, and the Slavs were moving in large numbers across Europe. Fortunately, Olsen and his colleagues deposited the data with the National Oceanic and Atmospheric Administration. I cannot sing their praises high enough — making data accessible to other researchers means that others can see if it addresses other research.
I plotted the data over the course of the existence of the Roman Republic and Empire, with a little bit of buffer before and after.
There is a fair bit of activity going on here over the course of the Roman Empire. The big dip at 500 A.D. was what first caught my attention, but it is hardly the only one. Next, I took a look at major conflicts between the Roman Republic/Empire with Germanic or proto-Germanic tribes. For extra credit, I added trendlines to show the frequency of historical descriptions of famine and drought in the historical records.
There is a lot to parse here, but let me start by saying that good science is not description, but rather prediction. We shouldn’t be able to tell just-so stories, but rather predict things. Obviously, it is pretty hard for an archaeologist to predict. But in this case, it kinda happened. When I started this analysis, I expected it to match the Migration Period of the 500 A.D.’s, and the Gothic Migration in 376 A.D. I did not forsee the Cimbrian War at all, and after researching it I realized that it did in fact match the much later Germanic migrations. This will be discussed in Part 3, but for now, suffice it to say that the NAO reconstruction here matched other Germanic migration effects. So this isn’t just a description, the NAO hypothesis would have predicted these movements even if the Romans had not recorded them. Of note, the Migration Period (500–600 A.D.) doesn’t just have a significant movement of people, but also increased records of both famine and drought. To a lesser extent, this is also true of the Gothic migrations starting in 376 A.D. One other point to make before continuing — the ‘normal’ NAO conditions for the Roman Republic and Empire at the peak of its power was a positive NAO of around 1. The migration events occurred when the NAO dipped to a range between 0 and 1. Remember, this graph is an average of conditions, so a drop from 0–1 means many more severe NAO negative events, with a changed middle point. Still, this is a relatively modest change, though not necessarily an unimportant one.
Let’s cover each of these very briefly, but a more detailed history will follow in Part 3.
Cimbrian War (113–101 B.C.): Seemingly out of nowhere, two large proto-Germanic confederations descend on to Italy, allegedly having migrated from present-day Denmark. They defeat a series of Roman legions, and evolve into an existential threat to the Republic. The Romans, under the leadership of Gaius Marius, reform the military and defeat them in two significant battles at the end of 101 B.C. This turns out to be one of the last unifying events for the Roman Republic before it descends into a decades-long civil war until it’s collapse into an Empire.
Marcomannic Wars (166–180 A.D.): Following two centuries of (relative) peace and prosperity, Germanic tribes attack all along the northern borders of the Empire. Then-emperor Marcus Aurelius, the so-called last good Emperor, spends the remainder of his reign moving from battle to battle to secure the Empire’s defenses.
Gothic Migration (376–410 A.D.): In 376 A.D., tens of thousands of Goths show up on the Danube, begging for entrance into the Roman Empire as refugees. Emperor Valens ascents, but lack of food and plenty of corruption drive the Goths to war, where they defeat and kill Valens at Adrianople in 378 A.D. They become a permanent presence, and sack Rome in 410 A.D. — to some a date which marks the end of the Empire.
The Migration Period (500–600 A.D.): Few primary historical sources cover this period, but it is evident that lots of people were moving around. Germans consolidate their migration into North Africa, Spain, France, and Italy while the Slavs begin migration into the Balkans. The movement of peoples lays the framework for the modern languages and nations of Europe.
As you can see, there are a lot of moving parts. So lets pause and think about this in space. Let’s set aside climate for the moment, and look at the distribution of both Romans and the Germanic tribes before their migration.
The Roman Empire — the shaded black area surrounding the Mediterranean Sea, covered a wide variety of climatic zones, from the deserts of the Sahara to the rich farmlands of Gaul. The earliest migration a) was of the Cimbri and Teutones, proto-Germanic peoples who came south. The second b) was the Marcomanni, Quadi, Iazyges, and other tribes which were pushed against the Roman frontier. The third c), and arguably most significant, was the movement of the Goths past the Danube and subsequent battle of Adrianople. The Migration Period was a more complicated affair, but involved the Huns d) and Slavs e).
The question is — what happens to these areas when the NAO changes? Here, we’ll use the data in a pretty straightforward way — what are drought conditions like when the NAO is around +1 (when the Roman Empire was at its height), and what were they like when they dipped into a range from 0–1? Here, we can use modern data to assess what drought conditions look like. In particular, we can use the modern measurements of the self-calibrated Palmer Drought Sensitivity Index (PDSI). This measures the intensity of a drought (or, of wetter conditions). For example, the Dust Bowl, a series of droughts and dust storms that plagued the midwest in the 1930’s during the Great Depression, had a PDSI of around -3. Before the Dust Bowl, it was around 3 — so this was a movement of -6 PDSI units toward drought. And one with very severe consequences for the people who did (and didn’t) survive it.
I took this same type of data, and plotted it against the same map of Europe with the ancient political boundaries. I took years of the scPDSI when the NAO index was 0–1 (migrations), and then subtracted from them the years when the scPDSI was 1–2 (stable). This created a measurement of the movement towards (or away from) drought conditions.
The first thing that jumps out is that during those drops in the intensity of the NAO, the Roman Empire would have been a good place to be. Conditions tended to shift towards more rainfall and less evaporation — great conditions for an agrarian society. So why was Rome facing existential war when this happened? Because this NAO shift was much less pleasant for its northern neighbors. The areas of origination for the proto-Germanic and Germanic confederations which attacked Rome in 4 different episodes appear to have come from areas with more drought. And remember — we are not talking drought years, we are talking drought decades. Or, in the case of the Migration Period itself, a century of more drought-like conditions. They had a strong incentive to go down to the greener pastures of the Roman Empire — and die trying if necessary.
There are two important points to consider here. First, unlike the Roman Empire — the tribal confederations were not sedentary. At least not in the same way. They tended to pick up and move when conditions got rough. And, if necessary, push their neighbors out of the way. When the neighbor is the Roman Empire, that job is much tougher. Second, it wasn’t just one tribe or confederation that was affected by a higher frequency of droughts. Their neighbors were too. For two of the historical migrations, the ancient sources indicate that the migration was like a chain of dominoes. As we mentioned earlier, the Goths sought refuge in Rome because a new people, the Huns, had moved and challenged them for their territory. An earlier period of warfare — the Marcomannic Wars — also appear to have been in response to the movements of people from present-day Scandinavian countries into the Baltic states. When the NAO index dropped, it appears to have strongly increased pressure for peoples to move toward the Mediterranean. Unfortunately, unlike the Romans, these tribal confederations did not maintain written records. While in some cases we have some oral historical accounts (Part 3), we don’t have anything approaching their side of the story.
My take on this is that the North Atlantic Oscillation (NAO) is an important driver of the stability of complex societies in Europe. But it is not the only one. There are other climatic records we can look at to see what was happening in general to the environments within and surrounding the Roman Empire.
What catches my eye first is the δ13C record of Lake Holzemaar in Germany. This ostensibly records lake productivity, but has a weird deviation around the time of the Marcomannic wars at 180 A.D., and then has a long decline after 200 A.D. that persists until the end of the Migration Period (600 A.D.). The historical record of Nile Floods looks like it tracks the record of Lake Holzemaar as well. Both records suggest what I’ve called (controversially) in the past a “climatic gear shift”. In other words, one of the drivers of climatic variation fundamentally changes, and alters the prospects for agrarian societies at a centennial scale (e.g. hundreds of years). Interestingly, this lines up with the Crises of the Third Century (235–284 A.D.), and stabilizes following it to a new normal. It is possible that against the backdrop of the North Atlantic Oscillation, other important climate drivers were changing as well, contributing to the changing viability of the Roman Empire and its neighbors.
Lastly, Europe has excellent tree ring records. These are at a yearly scale. The good news about that is that we can see, year by year, the changes in climate. But, it doesn’t show systematic changes in climate like Lake Holzemaar or the Nile. Instead, it tends to show single-year events like droughts. Ulf Büntgen of the University of Cambridge and his colleagues (2016) have found that a particularly significant cold, arid episode occurred from 536 to 660 A.D., an event that also shows up in a separate tree ring sequence from Finland. This is a separate, external control suggesting that the Migration Period was a particularly unusual time for Europe. Numerous pollen records also suggest that the environment was becoming more forested, with less evidence of farming. The transition from Antiquity (Greece and Rome) to the Medieval era was not just a social event, but also a climatic one.
This data, along with the data we have from the NAO thanks to Olsen and colleagues, suggests that climate played a role in the history of Europe in general and the Roman Empire in particular. In Part 3, we are going to take a much deeper look at what was happening, and how people responded to these changing conditions. Climate is not a determinant to history or the future— it is instead a major push factor. Normally, we don’t have historical records from ancient societies to see how people responded to changing conditions. The Roman Empire is different — we have a relative wealth of information of how these climatic events and migrations changed the course of history.