O colapso do Império Romano do Ocidente

O gráfico acima, mapeando a dimensão do exército romano ao longo dos séculos, foi retirado de  File:Roman-military-size-plot.png - Wikipedia, the free encyclopedia

A questão de saber a razão, ou as razões que explicam a queda do Império Romano do Ocidente (as consequências são referidas aqui), remontam pelo menos, pelo que sei, a Gibbon: a ascensão do cristianismo. H.G. Wells referia, em The Outline of History (1920), entre outras razões, o facto do Império Romano não ter conseguido reduzir a extensão da sua fronteira, do "limes" romano, levando-a a posicionar-se na distância mais curta entre o Báltico e o Mar Negro (numa das referências abaixo é referido os custos da extensão da fronteira norte e, é um facto, houve a tentativa de levar a fronteira romana até ao rio Elba, com Augusto). Num livro lido há poucos anos, era referido que o Império Romano do Oriente não tinha caído devido à linha de defesa natural que é o conjunto de estreitos (e o mar de Mármara) ligando o Mediterrâneo ao Mar Negro. Outra prende-se com a dimensão crescente das despesas militares romanas: li que o peso dessas despesas foi aumentando à medida que aumentava o período de vida do Império (o gráfico acima, substantiva a tese, mas o que se diz aqui, não, ao contrário do que se diz, numa leitura rápida, aqui). Mas as razões conjecturadas, multiplicam-se, como seria de esperar, face à realidade global e complexa da sociedade romana. As referências abaixo, obtidas via The Oil Drum | Drumbeat: February 12, 2010, apontam para outras possibilidades, discutindo-as como é feito no primeiro caso (início não muito bom, mas vale a pena ler até ao fim) onde se afirma que as sociedades complexas colapsam de modo complexo (sobre  isso ler aqui, também)

• The Oil Drum: Europe | "Peak Civilization": The Fall of the Roman Empire: "A complex entity should fall in a complex manner, and I think it is correct. In Tainter's view, societies always face crisis and challenges of various kinds. The answer to these crisis and challenges is to build up structures - say, bureaucratic or military - in response. Each time a crisis is faced and solved, society finds itself with an extra layer of complexity. Now, Tainter says, as complexity increases, the benefit of this extra complexity starts going down - he calls it 'the marginal benefit of complexity'. That is because complexity has a cost - it costs energy to maintain complex systems. As you keep increasing complexity, this benefit become negative. The cost of complexity overtakes its benefit. At some moment, the burden of these complex structures is so great that the whole society crashes down - it is collapse. I think that Tainter has understood a fundamental point, here. Societies adapt to changes. Indeed, one characteristic of complex systems is of adapting to changing external conditions. It is called 'homeostasis' and I tend to see it as the defining characteristic of a complex system (as opposed to simply complicated). So, in general, when you deal with complex systems, you should not think in terms of 'cause and effect' but, rather, in terms of 'forcing and feedback'. A forcing is something that comes from outside the system. A feedback is how the system reacts to a forcing, usually attaining some kind of homeostasis. Homeostasis, is a fundamental concept in system dynamics. Something acts on something else, but also that something else reacts. It is feedback. It may be positive (reinforcing) or negative (damping) and we speak of 'feedback loops' which normally stabilize systems - within limits, of course. Homeostasis has to be understood for what it is. It is not at all the same thing as 'equilibrium' as it is defined in thermodynamics. For example, a human being is a complex system. When you are alive, you are in homeostasis. If you are in equilibrium, it means that you are dead. Homeostasis is a dynamical equilibrium of forces."

• The Oil Drum: Europe | Peak soil: "Was the Roman Empire doomed by the loss of fertile soil to erosion? This is a much discussed point that I also examined in a study of mine on the fall of the Roman Empire. Fertile soil generates food that, in turn, causes population to increase and that is what makes an empire able to expand, as all empires do. But fertile soil is also subject to overexploitation. It is fragile; is easily washed to the sea by rain. And, when it is gone, it takes centuries, at least, to reform. So, did the Roman Empire collapse because of soil loss? Historians are still debating this point but, in this book, 'Dirt', David Montgomery makes a forceful case that soil erosion was a major cause of the decline of the Roman economy and that, in general, it strongly affected ancient civilizations. Montgomery connects the dots of what we know and shows - among other things - that the Romans clearly understood the importance of agriculture in their economy. Yet, they never were able to understand the role of soil erosion. Of course, there are alternatives to the simple linear chain of positive feedbacks that goes as more people -> more land cultivated -> more erosion. The sources tell us that many fields went uncultivated at the time of the Roman Empire and that suggests the possibility of a problem of underpopulation. The military needs of the late Empire were so strong thet there were not enough people left to cultivate the land. There is also evidence of droughts at the time of the decline of the Empire which would have affected agriculture, too. None of these explanations excludes the others. In a complex system, there is no simple cause and effect relation. Everything affects everything else and you need good quantitative data to understand the weight of all the factors involved. Unfortunately, good quantitative data is exactly what we are missing for the Roman Empire. But, on the whole, it is clear that soil erosion is a major element at play in the decline of civilizations. The Romans, as many other civilizations before and after them, were destroying their resource base, soil, and they never were able to replace it."

Obviamente, que a questão é importante, a mais de um título, para o que se viverá no século XXI. Leitura aconselhada. Sobre o tópico do colapso ver neste blogue, via "motor de busca" [canto superior direito] sob as palavras: "Jared Diamond".