Canvi climàtic i l’imperi Romà de l’oest

L’imperi Romà de l’oest o occidental és el nom que va rebre la part occidental del imperi romà després de la divisió administrativa iniciada amb la tetrarquia de l’emperador Dioclecià (284-305) i consolidada per l’emperador Teodosi I (379-395). Aquest darrer va dividir l’imperi entre els seus fills Arcadi, que és va quedar la part oriental i Honori que és va quedar la part occidental. Paral·lelament a aquesta divisió administrativa l’exercit també va patir una sèrie de modificacions importants, que comportaven una disminució de recursos ja que l’imperi no tenia gaire reserves d’or.

Entre els principals factors que es citen com que van contribuir a la desaparició de l’imperi romà es citen: constants revoltes socials i rebel·lions internes, les invasions Hunes d’Atila, la constant devaluació de la moneda romana, les plagues i la fam que patia la població (i que va provocar l’abandonament de les ciutats), la barbarització de l’exercit romà, els dos saquejos que va patir roma a mans de d’Alaric i Genseric i les conseqüències d’un brutal refredament succeït al llarg del segle VI (període de refredament anomenat LALIA (veure

Doncs bé ara en un article publicat per Ugo Bardi (, podem llegir que les dades obtingudes en el darrer estudi de Buentgen et al. ( permeten afirmar que no va ser el refredament durant el període de LALIA la causa del col·lapse de l’imperi romà. Sembla que la principal causa del col·lapse va estar lligada a lo que James Tainter anomena “rendiments decreixents de la complexitat” ( Aquests rendiments decreixents tenen a veure amb el progressiu declivi de retorns que una societat obté dels recursos que explota.

Per Bardi el col·lapse fou sistèmic, lligat a la manca de recursos minerals per part de l’imperi romà i que va provocar el col·lapse del sistema financer.

Però en l’article també s’apunta a la possibilitat de que el canvi de LALIA podria haver estat produït per la reforestació produïda per la davallada en l’explotació dels boscos com a conseqüència en la davallada de població lligada a la caiguda de l’imperi.

En tot cas per reflexionar sobre la fragilitat del nostre clima. Tant sols 3 erupcions volcàniques semblen les responsables de LALIA (conjuntament amb un descens poblacional i un augment en la quantitat d’arbres en l’imperi romà). Què pot passar ara que les temperatures han començat a canviar al fer-ho la quantitat de gasos d’efecte hivernacle?

Sunday, February 14, 2016

The collapse of the Western Roman Empire: was it caused by climate change?

Image from the recent paper by Buentgen et al., published on “Nature Geoscience” on February 8, 2016. The red curves are temperature changes reconstructed from tree rings, respectively, in the Russian Altai (upper curve) and the European Alps (lower curve). Note the remarkable dip in temperatures that took place starting with the 6th century AD. But these data clearly show that climate change was NOT the cause of the collapse of the Western Roman Empire. 

The relationship of climate and civilization collapse is a much debated subject. From the recent collapse of the Syrian state to the much older one of the Bronze Age civilization, climate changes have been seen as the culprit of various disasters befalling on human societies. On the other side, instead, the “systemic” view of the evolution of complex societies sees collapse as the natural result of the declining returns that a society obtains from the resources it exploits. It is the concept termed “diminishing returns of complexity” by James Tainter.

On this point, we may say that there may well exist several causes for societal collap. Either climate change or resource depletion may sufficiently weaken the control structures of any civilization to cause it to fold over and disappear. In the case of the Western Roman Empire, however, the data published by Buentgen et al. completely vindicate Tainter’s interpretation of the collapse of the Roman Empire: it was a systemic collapse, it was NOT caused by climate change. 

So, we can see that there was a cooling episode that probably affected the whole of Eurasia and that started with the beginning of the 6th century AD.  This period is called LALIA (Late Antiquity Little Ice Age) and it seems to have been stronger than the better known LIA (Little Ice Age) that took place during the 18th and 19th centuries. Overall, temperatures went down by a couple of degrees in comparison to the time that we call the “Roman Warm Period.”

A truly brutal cooling, yes, and it surely had effects on human life, as discussed at length in the paper by Buentgen et al. But it had nothing to do with the fall of the Western Roman Empire. The Empire started its decline in the 3rd century and its final disintegration phase with the beginning of the 5th century; when it ceased to be able to garrison the fortifications at the borders. Then, Rome was sacked one first time in 410 AD; and finally destroyed by the Vandals in 455 AD. That was the true end of the Western Empire, even though for some decades there were still individuals who claimed the title of Emperors. But all that took place in a period of relatively stable climate, at least from what we can say about the available data. So, the collapse was systemic, related to factors other than climate and, in my opinion, mainly related to the collapse of the Roman financial system, in turn caused by mineral depletion.

But could it be that, after all, there is a correlation between the Roman collapse and climate change? Just it would be the reverse of what it had been sometimes proposed: could the Roman collapse have caused the LALIA cooling (or, at least, contributed to it)? The idea is not farfetched: the population collapse that took place with the fall of the Empire could have led to a considerable level of reforestation of Western Europe, and that would have absorbed CO2 from the atmosphere. It is an idea already expressed some time ago by William Ruddiman. It seems to be out of fashion, nowadays, but I think that it should be explored more.

In the end, this story can teach us a lot: first of all, how fragile climate is. In the interpretation by Buentgen et al., just three volcanic eruptions – relatively large ones, but not catastrophic – were sufficient to cause a two-degree cooling extending all over Eurasia. Think of what could be the effect if something similar were to happen in our times! Then, it shows also how the situation, today, has completely changed. Temperatures have taken a completely different trend with the start of large scale emissions of greenhouse gases in the atmosphere. Incidentally, these data also completely vindicate the “Hockey Stick” studies by Michael Mann and others. Global warming is real, the earth’s climate is fragile, and we are in big troubles.