Homeric Minimum

The Homeric Minimum is a persistent and deep[1] solar minimum that took place between 2,800 and 2,550 years before present,[2] starting around 830 BC[3] and resembling the Spörer Minimum.[4] This minimum is sometimes considered to be part of a longer "Hallstattzeit" solar minimum between 705–200 BC that also includes a second minimum between 460 and 260 BC.[5] The Homeric Minimum however also coincided with a geomagnetic excursion named "Etrussia-Sterno", which may have altered the climate response to the Homeric Minimum.[6]

Variations in the solar output have effects on climate, less through the usually quite small effects on insolation and more through the relatively large changes of UV radiation and potentially also indirectly through modulation of cosmic ray radiation. The 11-year solar cycle measurably alters the behaviour of weather and atmosphere, but decadal and centennial climate cycles are also attributed to solar variation.[2]

The Homeric Minimum has been linked with a phase of climate change,[7] during which the Western United States,[8] Europe and the North Atlantic became colder and wetter[9] although the eastern parts of Europe appear to have become drier.[10] This climate oscillation has been called the "Homeric Climate Oscillation"[7] or the "2.8 kyr event",[11] and it has been associated with the Iron Age Cold Epoch.[12]

Human cultures at that time underwent changes,[7] which also coincide with the transition from the Bronze Age to the Iron Age.[13] The climate fallout of this prolonged solar minimum may have had substantial impact on human societies at that time.[14]

It has been speculated that some ancient literary references refer to these phenomena. For example, the period saw the growth of a glacier on Mount Olympus, while Greek mythology and Homer refer to ice and storms on the mountain, which may also be reflected in the name "Olympus".[15] Increased activity of the polar lights at the end of the Homeric Minimum may have inspired Ezekiel's vision of God in the Old Testament.[16]

a stormy wind ... out of the north ... with brightness around it, and fire flashing forth ... as it were gleaming metal ... an expanse, shining like awe-inspiring crystal.

— Ezekiel, Ezekiel 1:4–28, Old Testament

A variety of phenomena have been linked to the Homeric Minimum:

• Increasingly cold, wet and windy climate recorded from Meerfelder Maar in Germany,[17] where the Homeric Minimum has been associated with a permanent climate transition.[18] A wetter climate was also recognized in a bog in the Netherlands;[19] the present-day Czech Republic, where it also became colder; and in the British Isles.[11]
• A growth in the size of lakes and downward expansion of conifer forests took place in Western North America at the time of the Homeric Minimum.[8]
• Decreased sea levels are recorded from the Homeric Minimum.[20]
• Increased storminess in Scotland and Sweden.[12]
• Increased precipitation in northern Iberia. Such a precipitation increase took place a few decades after the Homeric Minimum and increased wetness has been noted after other solar minima, as well.[21]
• Cold sea surface temperatures in the Santa Barbara Basin of California and a cold interval in the Campito Mountain tree ring record. The Homeric Minimum in general seems to be associated with a cold climate in California.[5]
• Decreased atmospheric pressure differences between Iceland and the subtropics, that is a decreased North Atlantic Oscillation.[22]
• Cooling is also recorded from Asia and the Southern Hemisphere.[23]
• A wetter climate is recorded for Central Asia.[24]
• Lake levels in the Caspian Sea rose.[24]
• More frequent floods and storms in the Alps.[25]
• A dry period in the Eastern Mediterranean and the Dead Sea appears to coincide with the Homeric Minimum, although the mechanisms for this are not clear.[26]
• Increased incision along the River Soar.[27]
• Increased production of carbon-14 and beryllium-10 by cosmic rays, recorded in Greenland.[2] The carbon-14 excursion is also recorded elsewhere and constitutes the largest such spike since 2000 BC, exceeding the Maunder Minimum.[13]
• The switch from the Subboreal to the Subatlantic climate epoch in the Blytt-Sernander sequence about 2,800 years before present.[2]
• The "Göschenen I" glacier advance in the Alps relates to the 2.8 kiloyear event.[28]