ES 331/767 Lecture 20
SUNSPOTS, AURORAS, AND
HOLOCENE CLIMATE

James S. Aber

Table of Contents
Sunspot minima Auroras
Radiocarbon Volcanic activity
Human influence Conclusions
Related sites References

Sunspot Minima

The
Sun is the source of all energy that drives the Earth's climatic system. Solar energy is the ultimate control of climate. The 11-year cycle of sunspots is so well known to astronomers and the public alike that it is usually considered to be a permanent aspect of solar activity. But the sunspot cycle itself is a fairly recent phenomenon that was not always so prominent or even discernible.

Sun-spot cycle, 1750 to 2000. Although relatively constant in duration, the cycle shows marked changes in the number of sun spots. Note low intervals during the early 1800s and again during the late 1800s and early 1900s. The current sun-spot cycle reached a peak late in 2000 and since declined. Image from NASA Jet Propulsion Laboratory.

In 1893, the English astronomer E.W. Maunder reported that the modern sunspot cycle apparently did not exist before A.D. 1700. He found that from 1645 to 1716 sunspots were scarce to absent. In fact the total number of sunspots reported by competent astronomers with high-quality telescopes during this period was less than the number of sunspots seen in a single year nowadays—see Fig. 20-1. Maunder's discovery went largely unnoticed until the existence of this deficient period of sunspots was again verified and named the Maunder Minimum (Eddy 1977). Three episodes of sunspot minima are now recognized during the past millennium (see table).

Sunspot minima of the past millennium.
Name Date
Maunder Minimum 1645-1715
Spörer Minimum 1416-1534
Wolf Minimum 1280-1350

Based on Nesje and Dahl (2000).

Auroras

The existence of the Maunder Minimum is confirmed by historical observations of the aurora borealis, or northern lights. The aurora borealis (aurora australis in southern hemisphere) is the spectacular display of shimmering waves and rays of light often seen in the night sky at high latitudes. The aurora is commonly whitish green in color, and more rarely a deep red color is seen. Although normally confined to high latitudes (>60°), the aurora is occasionally seen at much lower latitudes. Dramatic auroras are seen in Kansas, for example, about once a decade.

During Medieval time, auroral displays were considered supernatural events, which usually instilled terror in the populations of Europe—see Fig. 20-2. The most common forms of auroras are wavy or arching curtains or loops with a fine rayed structure—see Fig. 20-3. Auroras are actually formed in the ionosphere, some 120-480 km above the Earth—see Fig. 20-4. The color is generated by interaction of energetic electrons with atomic oxygen. Electrons strike and excite atomic oxygen; as the oxygen atom returns to ground state, visible light is emitted in two wavelengths: 0.557 µm (green) and 0.630 µm (red).

Auroras are the visible results of electrical discharges in the magnetosphere that are powered by solar wind. These discharges are visible in the night sky in a pattern that is roughly semicircular around the pole. Auroras are often accompanied by strong geomagnetic disturbances. A close connection between solar activity and geomagnetism was noted early in the twentieth century by Maunder himself. The occurrence of auroras follows a cycle similar to the sunspot cycle, and great auroral displays in mid-latitudes are usually associated with intense flares in the vicinity of sunspots—see Fig. 20-5. Auroras are thus an earthly manifestation of solar activity.

Auroras can be seen almost nightly at high latitudes today. Yet during the Maunder Minimum, auroras were extremely rare, so rare in fact that the English Royal Astronomer, Edmund Halley, did not see his first aurora until the end of the Maunder Minimum in 1716. Halley was at that time 60 years old and had always searched the night sky for an aurora. During one 37-year period of the Maunder Minimum, auroras were not recorded anywhere.

According to Eddy (1977, p. 83), "... in the long view the present-day frequency of sunspots and auroras is probably unusual, and since the 17th century the activity of the sun has risen steadily to a very high level--a level perhaps unequaled in this millennium."

Radiocarbon and tree rings

The production of 14C is controlled by the amount of cosmic radiation reaching the upper atmosphere. The solar magnetic field partially shields the Earth from cosmic radiation, so any long-term change in solar activity should result in changing 14C content in the atmosphere. During prolonged periods of reduced solar activity, more cosmic radiation would reach the Earth, and more 14C would accumulate in the atmosphere. Greater solar activity would produce the opposite effect on 14C abundance.

Ancient tree rings contain a year-by-year record of past 14C abundance. During the period AD 1650-1750, 14C content is anomalously high in tree rings. This is called the DeVries effect, and it is exactly what would be expected during a long sunspot minimum. The DeVries effect provides strong confirmation for the existence of the Maunder Minimum. A complete tree-ring record for bristlecone pine (Pinus aristata) of eastern California extends back over 8000 years. During this interval, several fluctuations in 14C content, both positive and negative, are present, and some of these events are greater in magnitude than the DeVries effect—see Fig. 20-6.

Two periods of glacial expansion are generally recognized in the Alps, Scandinavia, and western Canada during the late Holocene. These periods correspond closely with solar minima known from historical and tree-ring records (based on Eddy 1977; Osborn and Karlstrom 1988).

Solar output during the Maunder Minimum was reduced approximately 0.25%, which is equivalent to a global temperature reduction of about ½°C; however, the actual temperature decline during the Little Ice Age was ½ to 1½°C (Nesje and Dahl 2000). This suggests that other factors, namely volcanism, were also at work to create cooler climatic conditions.

Volcanic activity

The Earth experiences more-or-less continuous volcanic eruptions along mid-ocean ridges throughout the ocean basins and along subduction zones of the Pacific Ring of Fire. The activity of hot-spot volcanoes seems to be somewhat less regular, however. Iceland is without question the most active hot spot in the world today. It possesses more than 30 volcanoes that have erupted during historical times. Icelandic volcanoes were relatively quiet during the Medieval climatic optimum, when the island had a prosperous environment. Beginning with Hekla in 1104, devastating volcanic eruptions became frequent; some of these were undoubtedly large enough to have influenced global climate.

A series of major volcanic eruptions took place in tropical regions during the 13th century (1258, 1268, 1275, 1285). These may have triggered the start of the Little Ice Age (Zorin 2012). Another large tropical eruption in 1450 coincided with the Spörer Minimum. These tropical volcanoes combined with elevated Icelandic eruptions contributed to climatic severity of the Little Ice Age.

Ice cores provide high-resolution records of global volcanic eruptions, as noted by geochemistry and conductivity variations in the ice. Antarctic and Greenland ice cores demonstrate volcanic eruptions during the past few millennia capable of influencing climate on decadal timespans. This evidence has been combined with composite tree-ring series from the northern hemisphere, which reflect summer temperature variations. On this basis, it appears that volcanic eruptions played a major role in cooling events in the northern hemisphere during the past 600 years (Nesje and Dahl 2000; Morton 2012).

More recently, the eruption of Mt. Pinatubo and other volcanoes in the early 1990s led to a pronounced global cooling (see volcanic dust). Many lesser eruptions have taken place in the early twenty-first century, and the cumulative effect of these is to restrain climate change (Biello 2014). For example, Mount Kelud's 2014 eruption in Indonesia injected sulfur dioxide up to 28 km high into the stratosphere. The sulfur dioxide then turns to sulfuric acid that reflects incoming sunlight. Numerous volcanic eruptions, reduced solar output, and massive air pollution from burning coal (mainly in China) have combined to lessen the impact of global warming from increased greenhouse gases.

Eruption from the Tompaluan crater of the dual volcano Lokon-Empung in 2013. Seen from ~5 km (3 miles) away, Tomohon, Indonesia. Adapted from original photo by H. Soosalu; obtained from Wikimedia Commons.


Human influence on climate
Recent or ancient?

According to popular reports, human impact on climate is a consequence of the industrial revolution, particularly with massive burning of fossil fuels (coal and petroleum) that liberates vast quantities of greenhouse gases into the atmosphere. However, the eminent paleoclimatologist, W.F. Ruddiman (2005), has argued that significant human impact on climate began with the advent of agriculture about 8000 years ago—see Fig. 20-7. Agriculture has a profound effect on natural vegetation--forest, prairie, and wetland are replaced in favor of various annual crops. The net result is transfer of carbon previously stored in the biomass into the atmosphere. Carbon dioxide is derived mainly from forest clearing; methane is a byproduct of rice and cattle. Increasing levels of atmospheric carbon cannot be absorbed into the oceans at rates as fast as they were created by agricultural expansion. Although this subject is exceedingly complex and much scientific uncertainty remains, there can be no doubt that human impact on climate is ancient.

The natural trend for climatic cooling in the late Holocene may well have been arrested or even reversed by increasing levels of greenhouse gases derived mainly from agriculture—see Fig. 20-8. The role of industrial greenhouse gases has yet to manifest its climatic impact fully. The rate of these recent increases far exceeds the rate of climatic change, given the huge thermal masses and lag times involved with ocean and ice-sheet systems. Further climatic warming seems likely, at least until such time as most fossil fuels have been depleted within this millennium.

Conclusions

Based on our present geological and historical understanding of late Holocene events, major climatic changes of decade- and century-long duration are closely tied to solar and volcanic activity. All other short-term climatic factors seem to exert secondary or transient influences during the late Holocene. Slow, long-term warming may well reflect agriculture beginning in ancient times; however, rapid climatic warming and glacier retreat of the late 19th and early 20th centuries were probably not caused by man-made emissions of greenhouse gases (Nesje and Dahl 2000). Nonetheless, the future role of industrial gas emissions seems likely to spur further warming (Ruddiman 2005). It is currently not possible to understand, let alone predict, changes in volcanic and solar activity. While it appears likely the Earth's climate will continue to warm during the 21st century, any attempt to model future global climatic change is an illusory goal.

Related sites

References

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ES 331/767 © J.S. Aber (2014).