So-called ‘climate proxies’ (indicators) are preserved physical characteristics of the past that stand in for direct meteorological measurements and enable scientists to reconstruct the climatic conditions. They provide the only means for scientists to determine climatic patterns before record-keeping began (around 1880) (Wikipedia). The most common climate proxies are gas bubbles, pollens, dinocysts, isotopes, the quantities of which tell us something about past climate conditions. Proxies are found in lake sediment, marine sediment, peat bogs, ice, speleothems, tree rings and coral skeleton rings. Coring is often used to extract the proxies.
Let’s look at the temperature variations of surface ice in Greenland (deduced from ice cores) which is assimilated with temperatures in the European area and possibly in the whole Mediterranean area, and let’s compare it with the initiation of the major civilisations.
The so-called ‘Holocene Climatic Optimum’ (7000-4000 BC) is clearly visible with a thermal maximum around 6000 BC. It can be noted that the drop of temperature between this maximum and the 20th c. temperature is around 3°C. The temperature variations between warm and cold peaks are in the order of 1°C, except for the ‘8200 BP event’ where it is around 2°C. A quick look at the intervals between the warm peaks shows that they are fairly equidistant with an average of ca. 400 years. This is perhaps showing some astronomical influence?
The main Holocene warm and cold periods are listed very schematically as follows:
- Around 6200 BC: cold peak: ‘8200 BP Cold Period’
- Around 6000 BC: warm peak (‘Holocene Thermal Maximum’)
- Around 4900 BC: warm peak
- Around 4500 BC: warm peak
- Around 3800 BC: warm peak
- Around 3300 BC: warm peak (initiation of Harappa-Indus Valley civilisation)
- Around 3000 BC: warm peak (initiation of Egyptian and Sumerian civilisations)
- Around 2900 BC: cold peak (‘Piora Cold Period’)
- Around 2300 BC: cold peak
- Around 2200 BC: warm peak with severe drought (initiation of Minoan civilisation, start of First Intermediate Period in Egypt)
- Around 1900 BC: cold peak (‘Early Neoglacial Anomaly’, ENA) (migration of the Harappa-Indus Valley civilisation)
- Around 1800 BC: warm peak (start of ‘Second Intermediate Period’ in Egypt)
- Around 1600 BC: warm peak (initiation of Mycenaean and Hittite civilisations, and of New Kingdom in Egypt)
- Around 1200 BC: warm peak (Sea Peoples raiding the eastern Med, end of Bronze Age)
- Around 1000 BC: warm peak (start of ‘Third Intermediate Period’ in Egypt)
- Around 700 BC: ‘Iron Age Cold Period’
- Around 500 BC: warm peak (initiation of Greek civilisation during a period of rising temperatures starting in 700 BC)
- Around 200 BC: cold peak
- Around 0 AD: Roman Warm Period (initiation of Roman civilisation during a period of rising temperatures starting in 200 BC)
- 100-200 AD: cold period: decline of Roman Empire
- Around 400 AD: warm peak: Byzantine civilisation
- 400-900 AD: ‘Late Antique Little Ice Age’ or ‘Late Neoglacial Anomaly’, LNA (Migration Period, Arab Conquest, European Dark Age)
- 900 -1350 AD: ‘Medieval Warm Period’ (initiation of European Renaissance)
- 1350-1850 AD: ‘Little Ice Age’.
Eventhough such a comparison between temperatures and the initiation of civilisations leaves some room for wishful thinking, it is quite striking that the initiation of civilisations occurred around the warm peaks. It might perhaps be suggested that civilisations were initiated during periods with rising temperature and collapsed with prolonged droughts of several decades due to falling temperatures. Clearly, cooling down of the atmosphere induces less evaporation, less humidity and less rain, leading to droughts and famine (but excessive warmth may also lead to drought!). This makes sense from a farming point of view, but obviously, exceptions exist, and endless discussion may arise around this analogy which is called ‘simplistic’ by many scientists …
Let’s stress again that the climate is not the only factor involved: to explain the end of the Bronze Age, Cline (2014) adds earthquakes/volcanic activity, droughts/famines, internal mismanagement/rebellion/civil war, outside migrants/pandemics/invaders with new technologies, disruption of international trade with domino-effect on inter-dependent states. All of these factors may have co-operated in some way to generate the ‘Perfect Storm’ that put an end to the Bronze Age and to the Roman Empire …
 VINTHER, B., et al., 2009, “Holocene thinning of the Greenland ice sheet”, Nature, volume 461, (p 385-388). See: https://www.carbonbrief.org/factcheck-what-greenland-ice-cores-say-about-past-and-present-climate-change. According to Richard B ALLEY (2010), “ice cores are remarkably faithful recorders of past climate”. The temperature reconstruction produced using 18O isotope data from six ice cores is shown in the figure, and spans the period from 9690 BC to 1970 AD. It has a resolution of around 20 years, meaning that each data point represents the average temperature of the surrounding 20 years. So, the end of the record (1970) shows the average temperature between 1960 and 1980. The present author added a 200-year triangular filtering in order to smooth the signal without altering the main peaks.
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