Monday, July 1, 2019

Did climate change cause the fall of the Roman Empire? No, but what may have actually happened is amazing.

"Vanity Fair" may not be the best source for reliable scientific information, but this cover is typical of an idea that's becoming popular in the memesphere: that the Roman Empire fell because of climate change. Alas, this means stretching the data more than a bit and surprisingly, the opposite may be true: the climate changed because the Empire fell. Read on! (image source)

We have a problem with history: we often try to frame the past as if it were the same as the present. And that means projecting on the ancient our own troubles and fears. Add to this the difficulties we have in dealing with complex systems, the kind of systems that normally behave the way they damn please, and the results are often a complete mess.

The fall of the Roman Empire is a case in point. Maybe you know that in 1984 the German historian Demandt listed 210 (!!) causes proposed for the fall. It is fun to read how people just transferred to the Roman society whatever they were afraid of, from Communism to Culinary Excess.

In more recent times, we started being worried about things that weren't well known in the 1980s. One is the decline of the energy return on energy invested (EROI), which is a true problem for our fossil-based society. It is much less obvious that it was a problem the ancient Romans and I wasn't impressed by the attempts of Thomas Homer-Dixon to paint the Roman collapse as the result of an EROI decline. No data, no proof, just vague analogies.

Nowadays, our worries have shifted to Climate Change and, as you might have expected, the idea that Climate Change can destroy civilization has been projected to the fall of the Roman Empire. You can read a popularized version of the idea on "Vanity Fair" (see above), but also serious researchers seem to have bought into it. For instance, Kyle Harper, professor at the University of Oklahoma, titles his 2017 article "Climate Change Helped Destroy the Roman Empire."

The kind of climate change that's supposed to have destroyed the Roman Empire is different than the current version: we are affected by global warming, in ancient times the problem seems to have been global cooling. Lower temperatures negatively affected agriculture, that caused famines and pestilences, and that reduced the population. Then, bang! The Empire collapsed. The problem with this idea is that the dates, simply, don't match.

I already discussed this matter in a brief post in 2016, but let me go back to the story with more details. The Western Roman Empire officially disappeared during the 5th century, but the real collapse was much earlier. Here are data on lead and silver pollution in Roman times from a 2017 paper by McConnell et al. Likely, these data are a good proxy for the whole Roman economy.

You see how the decline of the Empire started around 100 CE and the collapse was complete around 250 CE, a true Seneca Collapse, faster than the growth that preceded it. That corresponds to what we know from the historians of the time.

Now, how about climate? Do we see something happening during the economic collapse? Here, the data are much less certain, but a "proxy" of temperature can be obtained from measurements on tree rings. Here is a data set published in 2011. More recent data substantially confirm these results.

First of all, note the uncertainty in the data: variations under ca. 0.5 °C are probably not significant. Also note how two different sets of data, marked with the black line and the red line, do not match exactly. But some "dips" seem to indicate significant temperature drops -- here, too, we may have a kind of "Seneca Collapse" of the temperature. The most intense drop occurred in mid 6th century CE with, it seems, a full 2 °C temperature decline.

Now, compare with the data of the previous figure on the Roman economy. Clearly, there was no significant cooling during the economic crash of the 3rd century AD. Temperatures started falling, badly, after the crash. And when temperatures reached their minimum - around the year 600 CE, the Western Roman Empire was only a memory. Of course, if you want to say that "A" caused "B," at least it should be that A precedes B!

Besides, you can see that other correlations just don't work the way they should if you want to blame climate change for something bad that happened to the Roman Empire. Consider the decrease of temperature in mid 1st century BCE. It is marked in the figure as "Roman Conquest," correctly so because Caesar's military campaign in Gallia was in full swing at that time: the Roman Empire was probably at its peak power. If cold is supposed to be able to cause the fall of an empire, it surely didn't do that at that time!

So, we can conclude that, no, the Roman Empire didn't fall because of climate change. It is one more of those "explanations" that don't explain anything and that will become part of Demandt's list, together with "Tiredness of life" and "Escapism."

But let's consider the data a little more in depth. It looks like they are telling something to us. Could it be that the opposite conclusion holds? That is, it could it be that the climate changed because the Empire fell?

Let's follow this line of thought. We know that the Roman collapse was accompanied by a considerable decline in population. It is extremely difficult to have reliable data on this point, but it may be that the maximum European population in Roman times was of some 35 million people at the peak, then it shrunk to only 18 million inhabitants in 650 CE.

Depopulation is both cause and effect of the decline of agriculture and, with less agricultural land, forests can regrow. And, of course, forests tend to absorb CO2 from the atmosphere, that affects climate by lowering temperatures. According to a hypothesis put forward by Victor Gorshkov and Anastassia Makarieva, the effects of forests on climate may be even larger because of the biotic pump mechanism. It is a complicated story: it mainly affects rainfall, but it also cools the land.

If we look at the figure above with forest extents in mind, we see that some things start clicking together. Consider the temperature drop when Caesar conquered Gallia. We have no data on how many people his troops killed, but you can read a chronicle of the war written by Caesar himself, the De Bello Gallico, and you can see that it was no gentleman's war. Caesar's troop not only devastated Gallia but surely also brought back to Rome large numbers of Gauls as slaves. A depopulated Gallia may well have seen its forests regrow. Something similar may have taken place earlier on, during the 3rd century BCE, when the Celts expanded all over Europe.

So, things seem to make sense: depopulation and reforestation may really cool the Earth. But, also, a lot of caution is necessary: the matter is complicated and the data are scant and uncertain. Things become even more complicated with the "Little Ice Age" which also appears in the graph above, starting approximately with the 15th century CE. In this case, in contrast with the previous cases, the cooling occurs in correspondence with strong growth of the European population, although punctuated by various disasters in the form of plagues and famines. Maybe it was the depopulation of the North American continent that caused extensive reforestation and hence cooling. But the data are uncertain at best, with some interpretations explicitly denying this effect and proposing that the cooling may have been related largely to volcanic activity.

As you see, this story both uncertain and fascinating. It will take a lot more work before we'll be able to disentangle the various factors that affected climate in historical times. I don't claim here to have said anything new on these matters, but I was impressed to find so much work pointing at the strong interaction between human beings and climate -- even before fossil hydrocarbons started to be called "fuels".

The Earth's ecosystem is a typical complex system. It reacts to perturbations, even minor ones, sometimes very strongly. Don't expect it to remain stable just because it has been stable up to a certain moment. Remember that a pebble can cause an avalanche and don't forget the straw that broke the camel's back. Then, think of how large is the forcing generated in our times by the combustion of fossil fuels, maybe orders of magnitude larger than anything our ancestors could do. We are moving toward interesting times (as in the ancient Chinese malediction).

(h/t Steve Kurtz, Franco Miglietta, Stefano Caserini, Paolo Gabrielli)


  1. >
    > Dear Dr. Bardi,
    I tried to look at collapse from a different paradigm. The scientific paradigm of quantum physics; that reality is mental and spiritual instead of the conventional paradigm of science that reality is material.
    > Shakespeare: We are such stuff as dreams are made on.
    > Prof.Richard Conn Henry: The universe is immaterial - mental and spiritual. Live and enjoy.
    > Saturdays’ Extinction Rebellion demonstration in Paris ended in tear gas. Search on YouTube: Extinction Rebellion protesters sprayed with tear gas in Paris [28.06.2019]

    > According to science we are spiritual expressions of an infinite Mental Universe. Then why do we still believe we are material bodies and fear collapse and extinction?

    1. You sound a little like Lucius Seneca when he says that the virtuous man never feels any pain. Maybe it is true, but I wonder what he would have done if he had needed a dentist -- they didn't exist in his times!

    2. Great response to the general / specific situational ethic conundrum, Professor Bardi. As the inimitable Mike Tyson (American boxer) used to say 'Everybody's got a plan, until they get punched in the face'.

    3. Dear Dr. Bardi, Please check that one out with Mr. Bruce Lipton. Go to YouTube and search: Bruce Lipton The biology of belief full lecture. He has some very interesting things to say on the subject.

    4. It has to be hugely interesting but, ouch.... 2h 30 minutes.... maybe one of these days.

    5. Dear Dr. Bardi, No need to stay in the present paradigm out of fear of a toothache or a punch in the face. Google: Richard Conn Henry The Mental Universe. It is a one page essay.
      If you conceptualize your body as a thing you are the owner of the thing you conceptualize as a toothache. If you do not CONCEPTUALIZE your observations as things. What are your experiences like? Pay close attention and be aware of the Seneca cliff.

    6. You are perfectly attuned with Seneca's Stoic view of the world!

  2. Interesting stuffs, but I suggest to focus on the Easter island collapsing on 1400DC

    because Easter island collapsing is a straordinary metaphore of Earth in the XXI century

    1. Personally, as metaphors go for Collapse from times gone by, I like the Babylonian Collapse, as set forth in Revelation 18. It can either be interpreted as a prediction of the future, or as retelling of a time long past when the author actually wrote it down, as retold from oral history (KJV, the best rendition in the English Language. My Latin is a little worse than my modern Italian, which is pathetic, so not sure how it comes out there.

      " 10 Standing afar off for the fear of her torment, saying, Alas, alas, that great city Babylon, that mighty city! for in one hour is thy judgment come.
      11 And the merchants of the earth shall weep and mourn over her; for no man buyeth their merchandise any more:
      12 The merchandise of gold, and silver, and precious stones, and of pearls, and fine linen, and purple, and silk, and scarlet, and all thyine wood, and all manner vessels of ivory, and all manner vessels of most precious wood, and of brass, and iron, and marble,
      13 And cinnamon, and odours, and ointments, and frankincense, and wine, and oil, and fine flour, and wheat, and beasts, and sheep, and horses, and chariots, and slaves, and souls of men...

      ... 17 For in one hour so great riches is come to nought. And every shipmaster, and all the company in ships, and sailors, and as many as trade by sea, stood afar off,
      18 And cried when they saw the smoke of her burning, saying, What city is like unto this great city!
      19 And they cast dust on their heads, and cried, weeping and wailing, saying, Alas, alas, that great city, wherein were made rich all that had ships in the sea by reason of her costliness! for in one hour is she made desolate.
      20 Rejoice over her, thou heaven, and ye holy apostles and prophets; for God hath avenged* you on her.
      21 And a mighty angel took up a stone like a great millstone, and cast it into the sea, saying, Thus with violence shall that great city Babylon be thrown down, and shall be found no more at all.

      Forget the PMs & the Crypto. In a full on Biblical-style Collapse, even the Souls of Men go worthless.


    2. @ Reverse Engineer
      Your nickname sounds like a paleoufologist man ;-)

      So, I tell you a couple of things, there are no signs of ancient astronauts in Babylonia, because:

      1-Sitchin's stories are pure science fiction

      2-Babylonian's gods had human head on animal body, this pattern has no evidence of alien contact, rather than animal head on human body (Egytian's gods).

      3-there are no signs of alien stop time monument in Babylonia's lands, rather than in Egypt with the Gizah plateau and its pyramids are stuffs with a clear alien productivity outside of the historical human context. This fact is quite strange, because Egyptian society was a pre-industrial culture, this means no industrial revolution society, so in the long run the productivity of all factos must be a falling trend, because a non industrial society did not have science to produce innovations (as an industrial society usually does)

  3. Hi Prof. Bardi,

    Interesting topic and charts, though I am not sure what is represented by the black lines.

    I am interested in the period from 1500 - 1600 CE, since that is when many native North Americans died from European diseases, and there was massive forest regrowth in their place. I think I see something like the resulting temperature drop in your graph.


    Eric in Kansas

    1. The black line shows a different set of data. It gives you a good feeling of the uncertainty in these measurements

    2. Please note a similar event occurring during Genghis Khan's rise to power in the 12th century. I'll assume any genocide/mass murder of any substantial size would have an impact on the climate.

  4. QUOTE: ***Remember that a pebble can cause an avalanche and don't forget the straw that broke the camel's back.***

    My favorite analogy is the hurricane whipped up by a butterfly's wings.

  5. According to Nietzsche Christianity is behind the destruction of the Classical World. Behind an empire fall there´s never a sigle cause, but I find the Christian factor one of the key socio-political factors.

    1. That was Gibbon's claim as well. His description of Rome prior to its collapse sounded a lot like the Catholic version of the Taliban. Far more Christians were put to death by these fundamentalist Catholics than by all the pagan Romans combined.

      And of course, Gibbon predated Nietzsche.

  6. I seem to recall James Burke -- during one of his 'Connections' TV shows -- saying Rome's invasion by 'barbarians' had something to do with a warming spell. His claim (in what was a throwaway line at the end of one of his last TV specials) was that the snows that blocked the Alpine passes melted, which in turn allowed the invasion of the Italian peninsula.



Ugo Bardi is a member of the Club of Rome and the author of "Extracted: how the quest for mineral resources is plundering the Planet" (Chelsea Green 2014). His most recent book is "The Seneca Effect" (Springer 2017)