TheSubboreal is aclimatic period, immediately before the present one, thesubatlantic period. Both are part of theHolocene epoch. It lasted from 3710 to 450BCE. During this time, conditions on earth were somewhat warmer and drier than the present with lower levels ofcarbon dioxide. There were important changes to vegetation, sea level, and the expansion of human civilizations, and the first instances of humanrecorded history.

The composite scientific termSubboreal, meaning "below the Boreal," is derived from theLatinsub (below, under) and theGreekΒορέας, fromBoreas, the god of the North Wind. The word was first introduced in 1889 byRutger Sernander^[1] to distinguish it fromAxel Blytt'sBoreal, which had been established in 1876.^[2]

The Subboreal followed theAtlantic and was followed by theSubatlantic. The Subboreal is equivalent to W. H. Zagwijn'spollen zones IVa and IVb^[3] and T. Litt's pollen zone VIII.^[4] In the pollen scheme ofFritz Theodor Overbeck, it occupies pollen zone X.

Inpaleoclimatology, it is divided into anOlder Subboreal and aYounger Subboreal. Historically, the Subboreal is equivalent to most of theEneolithic and the entireBronze Age, which started 4200 to 3800 years ago, as well as the start of theIron Age.

This period was also whenhumans first beganrecording history, at about 2900BCE, with the rise of theSumer,Indus Valley Civilization,Ancient Egypt and various other civilizations in theAncient Near East.

The Subboreal is usually defined as 5660 to 3710 years before the present (BP). The lower limit is flexible, as some authors prefer to use 4400 BCE, or 6350 BP^[5] even 4830 BC, or 6780 BP.^[6] Others use 5000 calendar years, or 3050 BCE. The upper limit of the Subboreal and, therefore the beginning of the Subatlantic, is also flexible and can be attributed to 1170 to 830 BCE,^[7] but it is usually fixed at 450 BCE. Invarve years, the Subboreal corresponds to 5660 to 2750 years BP.^[8]

The boundary between the older and the younger Subboreal is considered to be 1350 BCE.

The climate was generally drier and slightly cooler (by about 0.1 °C) than in the preceding Atlantic but still warmer than during the 20th century. The temperatures were 0.7 °C higher than during the following Subatlantic. Consequently, in Scandinavia the lower limit of glaciers was 100 to 200 m higher than during the Subatlantic.^[9] On the whole, the oscillating temperatures slightly receded in the course of the Subboreal by about 0.3 °C.^{[citation needed]}

In theAegean, the beginning of the Subboreal was marked by a pronounceddrought, centered around 5600 years BP.^[10] Of far greater importance was the coming to an end of theAfrican Humid Period, reflected in thelakes of subtropicalAfrica (such asLake Chad) experiencing a rapid fall in their water levels.^[11] During the interval 5000 to 4000 years BP, drier conditions in southernMesopotamia caused great demographic changes and the abandonment of settlings due to extreme aridity.^[12]

InGermany, a drastic climatic cooling can be observed around 5000varve years BP in themaars of theEifel. In the preceding interval lasting from 8200 till 5000 varve years (Holocene Climatic Optimum), the July temperatures were on average still 1 °C higher. At the same time, the January temperatures were rising and the yearlyprecipitation increased.^[8]

InNorthern Africa and in theNear East, the interval from 4700 to 4100 years BP had renewed and lasting dry conditions, as is indicated by lake level minima. Between 4500 and 4100 years BP,monsoonal precipitations weakened,^[13] a possible cause for the upheavals that led to the end of theOld Kingdom of Egypt.^[14]

The Levant shows a similar climatic evolution.^[15] The dry conditions prevailing in Mesopotamia around 4200 years BP probably resulted in the downfall of theAkkadian Empire.^[16]

Levels ofcarbon dioxide had reached at the beginning of the Subboreal its Holocene minimal value of 260 ppm. During the Subboreal, it started rising and reached 293 ppm at the end of the period.^[17] As a comparison, today's value is over 400 ppm.^[18]

InScandinavia, the Atlantic/Subboreal boundary shows a distinct vegetational change. That is less pronounced inWestern Europe, but its typical mixedoak forest shows quite a fast decline inelm andlinden. The decline in linden is not fully understood and might be due to cooling or human interference. The decline in elm is most likely due to elm disease, caused by theascomyceteCeratocystis ulmi, but climatic changes and anthropogenic pressure on the forests certainly must be considered as well.^[19] The decline in elm, with a recession from 20 to 4%, as observed in Eifel maar pollen, has been dated in Central and Northern Europe as from 4000 years BC,^[20] but it more likely was diachronous over the interval 4350 to 3780 BC.^[21]

Another important event was the immigration ofEuropean beech (Fagus sylvatica) andhornbeam (Carpinus betulus) from their retreats on theBalkan and south of theApennines. That happened also diachronously: beech pollen are found for the first time in the interval 4340 to 3540 BC, hornbeam pollen somewhat later between 3400 and 2900 BC. With the start of the Younger Subboreal is the massive spreading of beech. The establishment of beech and hornbeam was accompanied by indicator plants for human settlements and agriculture likecereals and plantain (Plantago lanceolata), andhazel was receding.

The relatively-dry climate during the subboreal furthered the spreading of heath plants (Ericaceae).

Like in the Atlantic period, the globalsea level kept on rising during the Subboreal but at a much slower rate. The increase amounted to about 1 m, which corresponds to a rate of 0.3 mm per year. At the end of the Subboreal, the sea level was about 1 m below the current value.

In theBaltic theLitorina Sea had already established itself before the onset of the Subboreal. During the Older Subboreal thesecond Litorina transgression raised the sea level to 1 m below the actual value. After an intermediatePost-litorine Regression thethird Litorina transgression reached 60 cm below present and during the beginning Subatlantic, it reached today's value.

In theNorth Sea region, theFlandrian transgression of the Atlantic was followed by a slight regression or standstill at the beginning of the Subboreal.

• Holocene
• Geological ages
• Paleoclimatology

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