The Subatlantic is the current climatic age of the Holocene epoch. It started about 2,500 years BP and is still ongoing. Its average temperatures are slightly lower than during the preceding Subboreal and Atlantic. During its course, the temperature underwent several oscillations, which had a strong influence on fauna and flora and thus indirectly on the evolution of human civilizations. With intensifying industrialisation, human society started to stress the natural climatic cycles with increased greenhouse gas emissions.
History and stratigraphy The term subatlantic was first introduced in 1889 by Rutger Sernander to differentiate it from Axel Blytt's atlantic. It follows upon the previous subboreal. According to Franz Firbas (1949) and Litt et al. (2001) the subatlantic consists of the pollen zones IX and X. This corresponds in the scheme of Fritz Theodor Overbeck to the pollen zones XI and XII. In climate stratigraphy, the subatlantic is usually subdivided into an older subatlantic and a younger subatlantic. The older subatlantic corresponds to pollen zone IX (or XI in an alternate nomenclature made of more zones) characterized in central and northern Europe by beech or oak-beech forests, the younger subatlantic to pollen zone X (or XII in the alternate nomenclature made of more zones). In eastern Germany, Dietrich Franke subdivides the subatlantic into four stages (from young to old):
youngest subatlantic: 1800 until present: modern history younger subatlantic: 1250 until 1800: High Middle Ages, Late Middle Ages, Early Modern Period middle subatlantic: 500 until 1250: migration period and Slavic migrations older subatlantic: 500 BC until 500 AD: pre-Roman Iron Age, ancient Rome and start of migration period.
Ages The beginning of the subatlantic is usually defined as 2,400 calendar years BP or 450 BC, but this lower limit is not rigid. Some authors prefer to define the start of the subatlantic as 2,500 radiocarbon years, which represents roughly 625 BC. Occasionally, the onset of the subatlantic has been pushed back to 1200 BC. According to Franz Firbas, the changeover from the subboreal (pollen zone VIII) to the older subatlantic (pollen zone IX) is characterized by the recession of hazel and lime and the simultaneous spreading of hornbeam due to anthropogenic influences. This recession was not synchronous. It occurred in the western reaches of the Lower Oder valley between 930 and 830 BC, whereas in southwestern Poland this event had taken place already between 1170 and 1160 BC. The beginning of the younger subatlantic at 1250 AD coincides with the medieval population increase and distinguishes itself by an increase in pine and in indicator plants for human settlements. In Silesia this event can be dated between 1050 and 1270 AD. If one equates the onset of the younger subatlantic with the first maximum of beech occurrence it shifts back to Carolingian times around 700 AD.
Climatic evolution
The summer temperatures of the subatlantic are generally somewhat cooler (by up to 1.0 °C) than during the preceding subboreal, the yearly average temperatures reduced by 0.7 °C. At the same time the winter precipitations augmented by up to 50%. Overall the climate during the subatlantic therefore tends to cooler and wetter conditions. The lower limit of the glaciers in Scandinavia descended during the subatlantic by 100 to 200 meters. The beginning of the subatlantic opened at the middle of the first millennium BC with the so-called Roman Warm Period which lasted to the beginning of the 4th century. This corresponds broadly to classical antiquity. The optimum is marked by a temperature spike centered around 2,500 BP. As a consequence in Europe the winter temperatures were raised by 0.6 °C during this period, yet on average were still by 0.3 °C lower than during the subboreal. Ice cores from Greenland also demonstrate a distinct temperature rise after the younger subboreal. The cooling that followed coincides with the Migration Period. It was not very pronounced and of short duration – an average temperature drop of 0.2 °C and a winter temperature drop of 0.4 °C center around 350 AD (or 1,600 years BP). This climatic deterioration with the establishment of drier and cooler conditions might have forced the Huns to move west thus in turn triggering the migrations of the Germanic tribes. At about the same time the Byzantine Empire reached its first acme and Christianity established itself in Europe as the leading monotheistic religion. After this relatively short cool interlude the climate ameliorated again and between 800 CE and 1200 CE almost reached the values of the Roman Warm Period (used temperature proxies are sediments in the North Atlantic). This warming happened during the High Middle Ages wherefore this event is known as Medieval Global Warming or the Medieval Warm Period. This warmer climate peaked around 850 AD and 1050 AD, and raised the tree line in Scandinavia and in Russia by 100 to 140 meters; it enabled the Vikings to settle in Iceland and Greenland. During this period the Crusades took place and the Byzantine Empire was eventually pushed back by the rise of the Ottoman Empire. The end of the Medieval Warm Period coincides with the early 14th century reaching a temperature minimum around 1350, and by the Crisis of the Late Middle Ages. Many settlements were abandoned and left deserted. As a consequence, the population in Central Europe drastically receded by as much as 50 percent. After a short warming pulse around 1500, the Little Ice Age followed, lasting from c. 1550 until 1860. The Northern Hemisphere snow line descended by 100 to 200 meters. Human history during this time includes the Renaissance and the Age of Enlightenment, and also major rebellious events like the Thirty Years War and the French Revolution. The beginning of the Industrial Revolution also dates back to this period, while Southeast Asia experienced the Post-Angkor Period. From 1860 onwards, the temperatures started to rise again and initiated the modern climatic optimum. This warming was severely amplified by anthropogenic influences (i.e. increasing industrialisation, greenhouse gas emissions and global warming). The modern warming shows a distinct temperature rise from the 1970s onwards. According to NASA, this is not expected to change within the 21st century.
Atmosphere
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