The 90-Year Solar Quiet Period That Probably Helped Killed the Norse Greenland Colony

The Spörer Minimum is a roughly 90-year interval of reduced solar activity that ran from approximately 1460 to approximately 1550 — about 130 years before the better-known Maunder Minimum of 1645–1715. During the Spörer interval the Sun produced approximately one-fifth its normal number of sunspots, the 11-year solar cycle was either suppressed or significantly reduced in amplitude, and the total solar irradiance reaching Earth was reduced by approximately 0.2–0.3% relative to the modern average. The associated climate effect was a roughly 0.5–1.0 °C cooling of northern hemisphere mean surface temperature for about a century.

It is the older of the three named modern grand solar minima — Spörer (1460–1550), Maunder (1645–1715), and Dalton (1790–1830) — and the longest of them. It is also the most-likely candidate single cause for the deepest portion of the Little Ice Age, the multi-century European cool interval that ran approximately from 1300 to 1850.

How it was identified

The Spörer Minimum is named for the German astronomer Gustav Spörer (1822–1895), who in 1887 published a careful re-analysis of the surviving European astronomical sunspot observations of the 15th and 16th centuries. Spörer’s analysis was the first systematic study of pre-telescopic sunspot records. He compiled the surviving naked-eye sunspot observations from European, Chinese, and Korean records and noticed that the 15th- and 16th-century European observers had recorded almost no sunspots — a sharply different pattern from the abundant 13th-century pre-Renaissance and the substantial 17th-century post-Galilean observation periods.

Spörer published the finding in a short paper in the Vierteljahrsschrift der Astronomischen Gesellschaft in 1887 and proposed the existence of an earlier Minimum-Periode — a multi-decade interval of reduced solar activity — preceding the well-known Maunder Minimum of the late 17th century that Maunder had recently been documenting.

The Spörer paper was not immediately influential. The early-20th-century solar-physics community was substantially focused on the contemporary observation of the 11-year solar cycle and the recent post-1850 sunspot record; the pre-telescopic minima were treated as a curious historical sidelight rather than as a fundamental phenomenon of solar variability.

The decisive recovery of both the Spörer and Maunder minima as fundamental solar-physics phenomena was the work of the American solar physicist John A. Eddy (1931–2009) in the mid-1970s. Eddy’s 1976 paper in Science — The Maunder Minimum — was a comprehensive review of the historical, observational, and proxy evidence for both minima. Eddy substantively rehabilitated the Spörer and Maunder records by combining the surviving direct sunspot observations with two indirect proxies: the aurora-frequency records (auroras are produced by solar-wind interaction with the Earth’s magnetic field; a quiet Sun produces fewer auroras) and the carbon-14 tree-ring records (the cosmogenic isotope carbon-14 is produced by cosmic rays striking the upper atmosphere; a quiet Sun has a weaker heliospheric magnetic field, which lets more cosmic rays through, which produces more carbon-14, which is incorporated into growing plant tissue and preserved in tree-ring records).

The three independent lines of evidence agreed. Both the Spörer and Maunder minima were substantively real solar phenomena, both involved reductions in solar activity by approximately a factor of five relative to the modern average, and both correlated with documented cold periods in the northern hemisphere climate record.

The climate impact

The Spörer Minimum’s climate impact was substantively larger than the Maunder Minimum’s, partly because the Spörer interval was longer (about 90 years vs about 70) and partly because the Spörer interval coincided with three large volcanic eruptions (Samalas in 1257 — which preceded the minimum but contributed to its long-tail cooling; Kuwae in 1453 in the South Pacific; and an unidentified event in approximately 1465 traced from polar ice-core sulphate signals) that further reduced the available solar input through atmospheric stratospheric aerosol effects.

The combined effect was an unusually-cold northern-hemisphere climate through the late 15th and early 16th centuries. Documented climate-history effects of the Spörer interval include:

The advance of European mountain glaciers to their late-medieval maximum positions, particularly in the Alps and in Norway. Several Alpine villages were progressively destroyed by glacial advance between 1480 and 1550; the abandonment of high-altitude European farmland that had been viable during the medieval warm period (approximately 950–1300) extended through the Spörer interval to a fully-established lower agricultural altitude limit by the early 16th century.

The collapse of the Norse Greenland colonies. The Western Settlement was abandoned around 1360 (predating the Spörer minimum but during the broader Little Ice Age cooling); the Eastern Settlement persisted through the early 15th century but was experiencing severe agricultural and trade difficulties by the 1410s; the last documented contact was a wedding at Hvalsey Church in 1408; the colony was substantively extinct by approximately 1450, just before the Spörer minimum’s deepest phase. The combined climatic-economic-cultural pressure of the 14th-century Little Ice Age and the deepening 15th-century Spörer cooling is the substantively-accepted explanation for the Norse Greenland collapse.

The severe European harvest failures of 1490–1510. Multiple European regions experienced repeated catastrophic harvest failures during the deepest Spörer phase, with associated peasant uprisings, urban food riots, and substantial population dislocation. The German Peasants’ War of 1524–1525 had multiple substantive causes but was substantively rooted in the cumulative agricultural distress of the preceding three decades.

The expansion of sea ice around Iceland and northern Scotland. The Icelandic fimbulvetr of 1490–1500 (the “great winter” of late-medieval Norse literary tradition) reflected substantively colder Atlantic-northwestern conditions than had been typical in the preceding two centuries. The English Domesday Sea (the unusually-cold North Sea conditions of the 1500s and 1510s) was substantively the same phenomenon.

Why it matters for modern climate science

The Spörer Minimum is the principal modern observational anchor for the substantive question of how much of the long-term climate variability of the past two millennia can be attributed to solar-activity variations rather than to other natural drivers (volcanic activity, ocean-circulation oscillations) or to anthropogenic effects (deforestation, agricultural land-use change, post-1850 carbon emissions). The Eddy 1976 paper substantively established that solar variations on the timescale of multi-decade minima could produce observable surface-temperature signals of approximately 0.5–1.0 °C in the northern hemisphere — a finding that substantively shaped subsequent modelling of the relative contributions of natural and anthropogenic climate drivers.

The substantive modern consensus is that solar-activity variations have contributed approximately 10–20% of the observed climate variability of the past two millennia, with volcanic-aerosol forcing contributing another 30–40%, internal ocean-atmosphere variability contributing 20–30%, and the remainder (substantively the dominant post-1900 contribution) attributable to anthropogenic effects. The Spörer Minimum sits at the heart of the empirical evidence for the solar-contribution estimate.

The most-recent observational data from the modern solar cycle (since approximately 2008) suggests that the Sun is currently entering a substantively quieter phase than the late-20th-century peak — solar cycle 24 (2008–2019) was approximately 30% weaker than the average of the preceding seven cycles, and the early data on solar cycle 25 (2019–present) suggests a similar reduced amplitude. Whether the modern solar cycle is heading into a substantive grand minimum comparable to Spörer or Maunder is substantively debated; the available proxy evidence and the modern observational data are consistent with several different scenarios.

If the modern Sun is heading into a Spörer-magnitude grand minimum, the climate impact would be approximately a 0.3 °C cooling effect over approximately a century — substantively smaller than the projected anthropogenic warming over the same period, but substantively non-negligible as a moderating factor. The Spörer interval is therefore not only of historical interest but is substantively the closest pre-modern observational analogue for the conditions the modern climate system may face over the next several decades.