Younger Dryas

From Wikipedia, the free encyclopedia

Three temperature proxies showing the Younger Dryas event at around 12,000 calendar years BP. The NGRIP sequence (red – mislabelled as GRIP) uses the water molecule isotopic composition – δ18O. The Vostok and EPICA Dome C series show delta-deuterium. All 3 proxies use the same vertical axis.

The Younger Dryas, from c. 12,900 to c. 11,700 calendar years ago (BP), was a sharp decline in temperature over most of the northern hemisphere, at the end of the Pleistocene epoch, immediately preceding the current warmer Holocene. It was the most recent and longest of several interruptions to the gradual warming of the Earth's climate since the severe Last Glacial Maximum, c. 27,000 to 24,000 calendar years BP. The change was relatively sudden, taking place in decades, and resulted in a decline of 2 to 6 degrees Celsius, advances of glaciers and drier conditions, over much of the temperate northern hemisphere. It is thought to have been caused by a decline in the strength of the Atlantic meridional overturning circulation, which transports warm water from the equator towards the North Pole, and which in turn is thought to have been caused by an influx of fresh cold water from North America into the Atlantic. The Younger Dryas was a period of climatic change, but the effects were complex and variable. In the southern hemisphere, and some areas of the north such as the south-eastern United States, there was a slight warming.^[1]

The presence of a distinct cold period at the end of the Late Glacial interval has been known for a long time. Paleobotanical and lithostratigraphic studies of Swedish and Danish bog and lake sites, e.g. the Allerød clay pit in Denmark, first recognized and described the Younger Dryas.^[2][3][4] The Younger Dryas is named after an indicator genus, the alpine-tundra wildflower Dryas octopetala. Leaves of Dryas octopetala are occasionally abundant in the Late Glacial, often minerogenic-rich, lake sediments of Scandinavian lakes.

The Younger Dryas is the youngest and longest of three stadials that resulted from typically abrupt climatic changes that took place over the last 16,000 calendar years.^[5] Within the Byltt-Sernander classification of north European climatic phases, the prefix ‘Younger’ refers to the recognition that this original ‘Dryas’ period was preceded by a warmer stage, the Allerød oscillation, which in turn was preceded by the Older Dryas around 14,000 calendar years BP. This is not securely dated, and estimates vary by 400 years, but it is generally accepted that it lasted around 200 years. In northern Scotland the glaciers were thicker and more extensive than during the Younger Dryas.^[6] The Older Dryas, in turn, is preceded by another warmer stage, the Bølling oscillation that separates it from a third and even older stadial. This stadial is often, but not always, known as the Oldest Dryas. The Oldest Dryas, occurred approximately 1,770 calendar years before the Younger Dryas and lasted about 400 calendar years. According to the GISP2 ice core from Greenland, the Oldest Dryas occurred between about 15,070 and 14,670 calendar years BP.^[7]

In Ireland, the Younger Dryas has also been known as the Nahanagan Stadial, while in the United Kingdom it has been called the Loch Lomond Stadial.^[8][9] In the Greenland Summit ice core chronology, the Younger Dryas corresponds to Greenland Stadial 1 (GS-1). The preceding Allerød warm period (interstadial) is subdivided into three events: Greenland Interstadial-1c to 1a (GI-1c to GI-1a).^[10]

Abrupt climate change

Since 1916 and the onset and later refinement of pollen analytical techniques and a steadily growing number of pollen diagrams, palynologists have concluded that the Younger Dryas represented a distinct period of vegetational change in large parts of Europe during which warmer climate vegetation was replaced by generally cold climate – glacial plant succession that often contained Dryas octopetala. This drastic change in vegetation is typically interpreted to be an effect of a sudden decrease in (annual) temperature, unfavorable for the previously rapidly northward-spreading forest vegetation. This cooling not only favored the expansion of cold-tolerant, light-demanding plants and associated steppe fauna, it also led to regional glacial advances in Scandinavia and a lowering of the regional equilibrium line altitude (snow line).^[2]

The change to glacial conditions at the onset of the younger Dryas in the higher latitudes of the Northern Hemisphere between 12,900–11,500 BP in calendar years has been argued to have been quite abrupt.^[11] in sharp contrast to the warming of the preceding Older Dryas interstadial. It has been inferred that its end occurred over a period of a decade or so,^[12] but the onset may have been faster.^[13] Thermally fractionated nitrogen and argon isotope data from Greenland ice core GISP2 indicate that the summit of Greenland was approximately 15 °C (27 °F) colder during the Younger Dryas^[12] than today. In the UK, coleopteran (beetle) fossil evidence suggests that mean annual temperature dropped to −5 °C (23 °F),^[14] and periglacial conditions prevailed in lowland areas, while icefields and glaciers formed in upland areas.^[15] Nothing of the size, extent, or rapidity of this period of abrupt climate change has been experienced since its end.^[11]

In addition to the Younger, Older, and Oldest dryases, a century-long period of colder climate similar to the Younger Dryas in abruptness has occurred within both the Bølling oscillation and the Allerød oscillation interstadials. The cold period, which occurred the Bølling oscillation, is known as the intra-Bølling cold period (ICBP) and the cold period, which occurred in the Allerød oscillation, is known as the intra-Allerød cold period (ICAP ). Both of these cold periods are comparable in duration and intensity with the Older Dyras and began and ended quite abruptly. These cold periods have been recognized in sequence and relative magnitude in paleoclimatic records from Greenland ice cores, European lacustrine sediments, Atlantic Ocean sediments, and Cariaco Basin, Venezuela.^[16]

Examples of older Younger Dryas-like events have been reported from the ends, which are called terminations,^[17] of older glacial periods. Temperature-sensitive lipids, long chain alkenones, found in lake and marine sediments are well-regarded as a powerful paleothermometer for the quantitative reconstruction of past continental climates.^[18] The application of alkenone paleothermometers to high-resolution paleotemperature reconstructions of older glacial terminations have found that very similar Younger Dryas-like paleoclimatic oscillations occurred during Terminations II and IV. If so, then the Younger Dryas is not the unique paleoclimatic event, in terms of size, extent, and rapidity, as it has been often regarded to be.^[18][19] Furthermore, paleoclimatologists and Quaternary geologists reported finding what they characterized as well-expressed Younger Dryas events in the Chinese (δ18O records of Termination III in stalagmites from high-altitude caves in Shennongjia area, Hubei Province, China.^[20] Various paleoclimatic records from ice cores, deep sea sediments, speleothems, continental paleobotanical data, and loesses show similar abrupt climate events, which are consistent with Younger Dryas events, during the terminations of the last four glacial periods. They argue that Younger Dryas events might be an intrinsic feature of deglaciations that occur at the end of glacial periods.^[20][21][22]

Timing of the Younger Dryas

During the early 1990s, it became obvious with an increasing number of high-resolution radiocarbon dates associated with the Younger Dryas that this distinct paleoclimatic period is very difficult to date in detail using radiocarbon dating. Radiocarbon dating of samples on either side of the Allerød–Younger Dryas Boundary yielded dates that exhibited a very rapid age shift. Typically, they jump from 11,000 radiocarbon years BP to 10,700–10,600 radiocarbon years BP at the boundary. The 11,000 radiocarbon years BP dates clearly pre-dates the boundary. The first (oldest) overlying Younger Dryas radiocarbon samples often yielded ages of 10,700–10,600 radiocarbon years BP without any evidence of either an intervening unconformity or other evidence of erosion or nondeposition. Radiocarbon dates of terrestrial macrofossils and tree rings in Europe show that this decline in apparent radiocarbon age occurred over about a 50-year period. For the Younger Dryas–Preboreal transition, high-resolution dating of identified plant macrofossils yielded dates that fall within so-called radiocarbon plateau, i.e., a 200–250 year long period when radiocarbon dating yields dates that are all roughly the same. As a result, the end of Younger Dryas can at best can only be estimated to have occurred sometime about 10,000–10,050 radiocarbon years BP.^[2][23]

The analyses of stable isotopes from Greenland ice cores provide a more precise estimate for the onset and end of the Younger Dryas period. The analysis of Greenland Summit ice cores as a part of the Greenland Ice Sheet Project-2 (GISP-2) and Greenland Icecore Project (GRIP) estimated that the Younger Dryas started about 12,800 ice (calendar) years BP. Depending on the specific ice core analysis consulted, the Younger Dryas is estimated to have lasted 1,150–1,300 years.^[2][23] Measurements of oxygen isotopes from the GISP2 ice core suggest the ending of the Younger Dryas took place over just 40–50 years in three discrete steps, each lasting five years. Other proxy data, such as dust concentration, and snow accumulation, suggest an even more rapid transition, requiring about a 7 °C (13 °F) warming in just a few years.^{[11][12][24][25]} Total warming in Greenland was 10 ± 4 °C (18 ± 7 °F).^[26]

The end of the Younger Dryas has been dated to around 11.55 ka BP, occurring at 10 ka bp (uncalibrated radiocarbon year), a "radiocarbon plateau" by a variety of methods, with mostly consistent results:

11.50 ± 0.05	ka BP:	GRIP ice core, Greenland[27]
11.53 + 0.04 − 0.06	ka BP:	Krakenes Lake, western Norway[28]
11.57	ka BP:	Cariaco Basin core, Venezuela[29]
11.57	ka BP:	German oak/pine dendrochronology[30]
11.64 ± 0.28	ka BP:	GISP2 ice core, Greenland[24]

Although the start of the Younger Dryas is regarded to be synchronous across the North Atlantic region, recent research concluded that the start of the Younger Dryas might be time-trangressive even within this region. After an examination of laminated varve sequences, Muschitiello and Wohlfarth found that the environmental changes that define the beginning of the Younger Dryas are diachronous in their time of occurrence according to latitude. According to these changes, the Younger Dryas occurred as early as c. 12,900–13,100 calendar years ago along latitude 56–54°N. Further north, they found that these changes occurred at c. 12,600–12,750 calendar years ago.^[31]

According to the analyses of varved sediments from Lake Suigetsu, Japan and other paleonenvironmental records from Asia, a substantial delay occurred in onset and end of the Younger Dryas between Asia and the North Atlantic. For example, paleoenvironmental analysis of sediment cores from Lake Suigetsu in Japan found the Younger Dryas temperature decline of 2–4 °C between 12,300 and 11,250 varve (calendar) years BP instead of about 12,900 calendar years BP in the North Atlantic region. In contrast, the abrupt shift in the radiocarbon signal from apparent radiocarbon dates of 11,000 radiocarbon years to radiocarbon dates of 10,700–10,600 radiocarbon years BP in terrestrial macrofossils and tree rings in Europe over a fifty-year period occurred at the same time in the varved sediments of Lake Suigetsu. However, this same shift in the radiocarbon signal predates the start of Younger Dryas at Lake Suigetsu by a few hundred years. Interpretations of data from Chinese also confirms the observation that the Younger Dryas dry and cold event in East Asia lags the North Atlantic Younger Dryas cooling with at least 200–300 years. Although, the interpretation of the data is more murky and ambiguous, it is likely that end of the Younger Dryas and start of Holocene warming was similarly delayed in time in Japan, and in other parts of East Asia.^[32]

Similarly, an analysis of a stalagmite growing from a cave in Puerto Princesa Subterranean River National Park, Palawan, Philippines, found that the onset of the Younger Dryas was also delayed in that region. Proxy data recorded in the stalagmite indicated that it took more than 550 calendar years for Younger Dryas drought conditions to reach their full extent in the region, and about 450 calendar years to return to pre-Younger Dryas levels after it ended.^[33]

Global effects

In western Europe and Greenland, the Younger Dryas is a well-defined synchronous cool period.^[34] But cooling in the tropical North Atlantic may have preceded this by a few hundred years; South America shows a less well defined initiation but a sharp termination. The Antarctic Cold Reversal appears to have started a thousand years before the Younger Dryas, and has no clearly defined start or end; Peter Huybers has argued that there is fair confidence in the absence of the Younger Dryas in Antarctica, New Zealand and parts of Oceania.^[35] Timing of the tropical counterpart to the Younger Dryas – the Deglaciation Climate Reversal (DCR) – is difficult to establish as low latitude ice core records generally lack independent dating over this interval. An example of this is the Sajama ice core (Bolivia), for which the timing of the DCR has been pinned to that of the GISP2 ice core record (central Greenland). Climatic change in the central Andes during the DCR, however, was significant and characterized by a shift to much wetter, and likely colder, conditions.^[36] The magnitude and abruptness of these changes would suggest that low latitude climate did not respond passively during the YD/DCR.

Effects in North America

In western North America it is likely that the effects of the Younger Dryas were less intense than in Europe; however, evidence of glacial re-advance^[37] indicates Younger Dryas cooling occurred in the Pacific Northwest.

Other effects

Other features include:

• Replacement of forest in Scandinavia with glacial tundra (which is the habitat of the plant Dryas octopetala)
• Glaciation or increased snow in mountain ranges around the world
• Formation of solifluction layers and loess deposits in Northern Europe
• More dust in the atmosphere, originating from deserts in Asia
• Drought in the Levant, perhaps motivating the Natufian culture to develop agriculture
• The Huelmo/Mascardi Cold Reversal in the Southern Hemisphere ended at the same time
• Decline of the Clovis Culture and extinction of animal species in North America

Effects on agriculture

The Younger Dryas is often linked to the adoption of agriculture in the Levant.^[38][39] It is argued that the cold and dry Younger Dryas lowered the carrying capacity of the area and forced the sedentary Early Natufian population into a more mobile subsistence pattern. Further climatic deterioration is thought to have brought about cereal cultivation. While there exists relative consensus regarding the role of the Younger Dryas in the changing subsistence patterns during the Natufian, its connection to the beginning of agriculture at the end of the period is still being debated.^[40][41]

Sea level

Based upon solid geological evidence, consisting largely of the analysis of numerous deep cores from coral reefs, variations in rates of sea level rise have been reconstructed for the postglacial period. For the early part of deglacial sea level rise, three major periods of accelerated sea level rise, called meltwater pulses, occurred. They are commonly called meltwater pulse 1A0 between 19,000 and 19,500 calendar years ago; meltwater pulse 1A between circa 14,600 and 14,300 calendar years ago; and meltwater pulse 1B between circa 11,400 and 11,100 calendar years ago. The Younger Dryas occurred after meltwater pulse 1A, which was a 13.5 m rise over about 290 years centered at about 14,200 calendar years ago and before meltwater pulse 1B, which was a 7.5 m rise over about 160 years centered at about 11,000 calendar years ago.^[42][43][44] Between meltwater pulse 1A and meltwater pulse 1B, the Younger Dryas was an interval of a significantly reduced rate of sea level rise relative to the periods of time before and after it. For example, the analysis of cores from Tahiti coral reefs found that sea level rose at a rate of about 7.5 ± 1.1  mm/yr during the Younger Dryas. Just after the end of the Younger Dryas, the rate of sea level rise accelerated to 17.4 ± 0.4  mm/yr and just before its start, it was 12.1 ± 0.6  mm/yr. This reduction in the rate of sea level rise directly reflected a substantial reduction of the global inflow of meltwater into the world's oceans during the Younger Dryas.^[42][45]

Possible evidence of short-term sea level changes have been reported for the beginning of the Younger Dryas. First, the plotting of data by Bard and others suggested a small step, less than 6 m, in sea level near the onset of the Younger Dryas. There is also possibly a corresponding change in the rate of change of sea level rise that they saw in the data from both Barbados and Tahiti. Given that this change is …within the overall uncertainty of the approach…, it was concluded that a relatively smooth sea-level rise, with no significant accelerations occurred at this time.^[45] Finally, research by Lohe and others in western Norway reported a sea-level low-stand at 13,640 calendar years ago and a subsequent Younger Dryas transgression starting at 13,080 calendar years ago. They concluded that the timing of the Allerød low-stand and the subsequent transgression were the result of increased regional loading of the crust and geoid changes caused by an expanding ice sheet that started growing and advancing in early Allerød about 13,600 calendar years ago and well before the start of the Younger Dryas.^[46]

Causes

The prevailing theory is that the Younger Dryas was caused by significant reduction or shutdown of the North Atlantic "Conveyor", which circulates warm tropical waters northward, in response to a sudden influx of fresh water from Lake Agassiz and deglaciation in North America. Geological evidence for such an event is thus far lacking.^[47] The global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the north Atlantic Ocean. An alternative theory suggests instead that the jet stream shifted northward in response to the changing topographic forcing of the melting North American ice sheet, bringing more rain to the North Atlantic which freshened the ocean surface enough to slow the thermohaline circulation.^[48] There is also some evidence that a solar flare may have been responsible for the megafaunal extinction, though it cannot explain the apparent variability in the extinction across all continents.^[49]

Impact hypothesis

A hypothesized Younger Dryas impact event, presumed to have occurred in North America around 12,900 calendar years ago, has been proposed as the mechanism to have initiated the Younger Dryas cooling. Amongst other things findings of melt-glass material in sediments in Pennsylvania, South Carolina, and Syria have been reported. These researchers argue that this material, which dates back nearly 13,000 calendar years ago, was formed at temperatures of 1,700 to 2,200 °C (3,100 to 4,000 °F) as the result of a bolide impact. They argue that these findings support the controversial Younger Dryas Boundary (YDB) hypothesis, that the bolide impact occurred at the onset of the Younger Dryas.^[50] The hypothesis has been questioned by research that stated that most of the conclusions cannot be repeated by other scientists, misinterpretation of data, and the lack of confirmatory evidence.^[51][52][53] After a review of the sediments that are found at the sites, new research found that sediments claimed, by the hypothesis proponents, to be deposits resulting from a bolide impact were, in fact, dated from much later or much earlier time periods than the proposed date of the cosmic impact. The researchers examined 29 sites that are commonly referenced to support the impact theory to determine if they can be geologically dated to around 13,000 calendar years ago. Crucially, only 3 of the sites actually date from that time.^[54]
In a study in the Journal of Geology (August 2014), Prof. Kennett (et al.) looked at the distribution of nanodiamonds produced during extraterrestrial collisions; 50 million square kilometers of Northern Hemisphere at YDB was found to have these nanodiamonds. Only two layers exist showing such nanodiamonds: the YDB 12,800 calendar years ago and the Cretaceous-Tertiary boundary 65 million years ago, which is also marked by the mass extinctions.^[55]

“The evidence we present settles the debate about the existence of abundant YDB nanodiamonds,” Kennett said. “Our hypothesis challenges some existing paradigms within several disciplines, including impact dynamics, archaeology, paleontology, paleoceanography, and paleoclimatology, all affected by this relatively recent cosmic impact.”

Laacher See eruption hypothesis

Finally, it has been hypothesized that the eruption of Laacher See might have initiated the Younger Dryas stadial. Laacher See is a maar lake, a lake that lies within a broad, low-relief volcanic crater, that is about 2 km (1.2 mi) in diameter. It lies in Rhineland-Palatinate, Germany, about 24 km (15 mi) northwest of Koblenz and 37 km (23 mi) south of Bonn. This maar lake lies within the Eifel mountain range, and is part of the East Eifel volcanic field within the larger Vulkaneifel. The East Eifel volcanic field has been active since about 400-11 calendar years ago.^[56][57] The maar, which this lake occupies is the result of phreatic and phreatomagmatic eruptions as well as caldera collapse processes. Phreato-plinian eruption sent plumes of phonolitic to mafic phonolitic tephra over the region and pyroclastic flows and surges over the local countryside in predominantly in northeastern and southern directions. This eruption was of sufficient size, VEI 6, with over 10 km³ (2.4 cu mi) tephra ejected, to have caused significant temperature changes in the northern hemisphere.^[58][59]

However, there are problems with the hypothesis that the eruption of Laacher see volcano initiated the Younger Dryas. First, detailed dating of varved and other lake deposits in other maar lakes determined an age of at least 11,230 ± 40 radiocarbon years BP for the Laacher See eruption. This radiocarbon age would make the Laacher see eruption about 200 radiocarbon years older than the start of the Younger Dryas.^[60] More recently, the radiocarbon dating of trees killed by the Laacher See eruption and an examination of Swiss dendrochronology and volcanic sulphur in the Greenland ice cores concluded that the Laacher See eruption predated the onset of the Younger Dryas by some 203 calendar years on average.^[61] Second, Late-Glacial sediment cores from Switzerland demonstrate that a significant period of time separate the Laacher See eruption and the start of the Younger Dryas.^[60][62][63] Finally, Even if the Laacher See eruption was contemporaneous with the start of the Younger Dryas, as big as it was, the size of the Laacher see eruption was likely not large enough to initiate a new stadial. The paleoclimatic proxies found in Greenland ice cores show only a short-term climatic deterioration associated with the Laacher See eruption.^[61][64]

Cultural references

The failure of North Atlantic thermohaline circulation is used to explain rapid climate change in some science fiction writings as early as Stanley G. Weinbaum's 1937 short story "Shifting Seas" where the author described the freezing of Europe after the Gulf Stream was disrupted, and more recently in Kim Stanley Robinson's novels, particularly Fifty Degrees Below. It also underpinned the 1999 book, The Coming Global Superstorm. Likewise, the idea of rapid climate change caused by disruption of North Atlantic ocean currents creates the setting for 2004 apocalyptic science-fiction film The Day After Tomorrow. Similar sudden cooling events have featured in other novels, such as John Christopher's The World in Winter, though not always with the same explicit links to the Younger Dryas event as is the case of Robinson's work.

Solar cycle

From Wikipedia, the free encyclopedia

400 year sunspot history, including the Maunder Minimum

"The current prediction for Sunspot Cycle 24 gives a smoothed sunspot number maximum of about 69 in the late Summer of 2013. The smoothed sunspot number reached 68.9 in August 2013 so the official maximum will be at least this high. The smoothed sunspot number has been rising again towards this second peak over the last five months and has now surpassed the level of the first peak (66.9 in February 2012). Many cycles are double peaked but this is the first in which the second peak in sunspot number was larger than the first. We are currently over five years into Cycle 24. The current predicted and observed size makes this the smallest sunspot cycle since Cycle 14 which had a maximum of 64.2 in February of 1906."^[1] The monthly sunspot number was still rising as of March 2014.^[2]

The solar cycle or solar magnetic activity cycle is the nearly periodic 11-year change in the Sun's activity (including changes in the levels of solar radiation and ejection of solar material) and appearance (changes in the number of sunspots, flares, and other manifestations).
They have been observed (by changes in the sun's appearance and by changes seen on Earth, such as auroras) for centuries.

The changes on the sun cause effects in space, in the atmosphere, and on Earth's surface. While it is the dominant variable in solar activity, aperiodic fluctuations also occur.

Play media

Evolution of magnetism on the Sun.

Definition

Solar cycles have an average duration of about 11 years. Solar maximum and solar minimum refer respectively to periods of maximum and minimum sunspot counts. Cycles span from one minimum to the next.

Observational history

Samuel Heinrich Schwabe (1789–1875). German astronomer, discovered the solar cycle through extended observations of sunspots

Rudolf Wolf (1816–1893), Swiss astronomer, carried out historical reconstruction of solar activity back to the seventeenth century

The solar cycle was discovered in 1843 by Samuel Heinrich Schwabe, who after 17 years of observations noticed a periodic variation in the average number of sunspots.^[3] Rudolf Wolf compiled and studied these and other observations, reconstructing the cycle back to 1745, eventually pushing these reconstructions to the earliest observations of sunspots by Galileo and contemporaries in the early seventeenth century.

Following Wolf's numbering scheme, the 1755–1766 cycle is traditionally numbered "1". Wolf created a standard sunspot number index, the Wolf index, which continues to be used today.
The period between 1645 and 1715, a time of few sunspots,^[4] is known as the Maunder minimum, after Edward Walter Maunder, who extensively researched this peculiar event, first noted by Gustav Spörer.

In the second half of the nineteenth century Richard Carrington and by Spörer independently noted the phenomena of sunspots appearing at different latitudes at different parts of the cycle.

The cycle's physical basis was elucidated by Hale and collaborators, who in 1908 showed that sunspots were strongly magnetized (the first detection of magnetic fields beyond the Earth). In 1919 they showed that the magnetic polarity of sunspot pairs:

• Is constant throughout a cycle;
• Is opposite across the equator throughout a cycle;
• Reverses itself from one cycle to the next.

Hale's observations revealed that the complete magnetic cycle spans two solar cycles, or 22 years, before returning to its original state. However, because nearly all manifestations are insensitive to polarity, the "11-year solar cycle" remains the focus of research.

In 1961 the father-and-son team of Harold and Horace Babcock established that the solar cycle is a spatiotemporal magnetic process unfolding over the Sun as a whole. They observed that the solar surface is magnetized outside of sunspots; that this (weaker) magnetic field is to first order a dipole; and that this dipole undergoes polarity reversals with the same period as the sunspot cycle. Horace's Babcock model described the Sun's oscillatory magnetic field, with a quasi-steady periodicity of 22 years.^{[2] [3]} It covered the oscillatory exchange of energy between poloidal and toroidal solar magnetic field ingredients. The two halves of the 22-year cycle are not identical, typically alternating cycles show higher (lower) sunspot counts (the "Gnevyshev–Ohl Rule."^[5])

Cycle history

Reconstruction of solar activity over 11,400 years. Period of equally high activity over 8,000 years ago marked.

Sunspot numbers over the past 11,400 years have been reconstructed using Carbon-14-based dendroclimatology. The level of solar activity beginning in the 1940s is exceptional – the last period of similar magnitude occurred around 9,000 years ago (during the warm Boreal period).^[6][7][8] The Sun was at a similarly high level of magnetic activity for only ~10% of the past 11,400 years. Almost all earlier high-activity periods were shorter than the present episode.^[7]

Solar activity events recorded in radiocarbon. Present period is on right. Values since 1900 not shown.

Major events and approximate dates

Event	Start	End
Homeric minimum[9]	950BC	800BC
Oort minimum	1040	1080
Medieval maximum	1100	1250
Wolf minimum	1280	1350
Spörer Minimum	1450	1550
Maunder Minimum	1645	1715
Dalton Minimum	1790	1820
Modern Maximum	1900	present

A list of historical Grand minima of solar activity^[6] came around 690 AD, 360 BC, 770 BC, 1390 BC, 2860 BC, 3340 BC, 3500 BC, 3630 BC, 3940 BC, 4230 BC, 4330 BC, 5260 BC, 5460 BC, 5620 BC, 5710 BC, 5990 BC, 6220 BC, 6400 BC, 7040 BC, 7310 BC, 7520 BC, 8220 BC and 9170 BC. Since observations began, cycles have ranged from 9–14 years. Significant amplitude variations also occur.

It was first thought that 28 cycles had spanned the 309 years between 1699 and 2008, giving an average length of 11.04 years, but recent research has showed that the longest of these (1784–1799) seems actually to have been two cycles,^[10][11] meaning that one of the two had to have lasted less than 8 years.

Recent cycles

Cycle 24

The current solar cycle began on January 4, 2008, with minimal activity until early 2010.^[12][13] It is on track to have the lowest recorded sunspot activity since accurate records began in 1750. The cycle featured a "double-peaked" solar maximum. The first peak was reached 99 in 2011 and the second in early 2014 at 101.^[14]

Cycle 23[edit]

This cycle lasted 11.6 years, beginning in May 1996 and ending in January 2008. The maximum smoothed sunspot number (monthly number of sunspots averaged over a twelve-month period) observed during the solar cycle was 120.8 (March 2000), and the minimum was 1.7.^[15] A total of 805 days had no sunspots during this cycle.^[16][17][18]

Phenomena

Various solar phenomena follow the solar cycle, including sunspots and coronal mass ejections.

Sunspots

A drawing of a sunspot in the Chronicles of John of Worcester.

The Sun's apparent surface, the photosphere, radiates more actively when there are more sunspots. Satellite monitoring of solar luminosity revealed a direct relationship between the Schwabe cycle and luminosity with a peak-to-peak amplitude of about 0.1%.^[19] Luminosity decreases by as much as 0.3% on a 10-day timescale when large groups of sunspots rotate across the Earth's view and increase by as much as 0.05% for up to 6 months due to faculae associated with large sunspot groups.^[20]

The best information today comes from SOHO (a cooperative project of the European Space Agency and NASA), such as the MDI magnetogram, where the solar "surface" magnetic field can be seen.

As each cycle begins, sunspots appear at mid-latitudes, and then closer and closer to the equator until solar minimum is reached. This pattern is best visualized in the form of the so-called butterfly diagram. Images of the Sun are divided into latitudinal strips, and the monthly-averaged fractional surface of sunspots calculated. This is plotted vertically as a color-coded bar, and the process is repeated month after month to produce this time-series diagram. As there are peaks in sunspot number around 1955–58, James T. Struck argued for a Struck Maximum, given his discovery of the peak at this point, like Maunder and Dalton's work.^{[citation needed]}

The sunspot butterfly diagram. This modern version is constructed (and regularly updated) by the solar group at NASA Marshall Space Flight Center.

While magnetic field changes are concentrated at sunspots, the entire sun undergoes analogous changes, albeit of smaller magnitude.

Time vs. solar latitude diagram of the radial component of the solar magnetic field, averaged over successive solar rotation. The "butterfly" signature of sunspots is clearly visible at low latitudes. Diagram constructed (and regularly updated) by the solar group at NASA Marshall Space Flight Center.

Coronal mass ejection

The solar magnetic field structures the corona, giving it its characteristic shape visible at times of solar eclipses. Complex coronal magnetic field structures evolve in response to fluid motions at the solar surface, and emergence of magnetic flux produced by dynamo action in the solar interior. For reasons not yet understood in detail, sometimes these structures lose stability, leading to coronal mass ejections into interplanetary space, or flares, caused by sudden localized release of magnetic energy driving emission of ultraviolet and X-ray radiation as well as energetic particles. These eruptive phenomena can have a significant impact on Earth's upper atmosphere and space environment, and are the primary drivers of what is now called space weather.
The occurrence frequency of coronal mass ejections and flares is strongly modulated by the cycle. Flares of any given size are some 50 times more frequent at solar maximum than at minimum. Large coronal mass ejections occur on average a few times a day at solar maximum, down to one every few days at solar minimum. The size of these events themselves does not depend sensitively on the phase of the solar cycle. A case in point are the three large X-class flares that occurred in December 2006, very near solar minimum; an X9.0 flare on Dec 5 stands as one of the brightest on record.^[21]

Patterns

An overview of three solar cycles shows the relationship between the sunspot cycle, galactic cosmic rays, and the state of our near-space environment.^[22]

The Waldmeier effect names the observation that cycles with larger maximum amplitudes tend to take less time to reach their maxima than cycles with smaller amplitudes;^[23] maximum amplitudes are negatively correlated to the lengths of earlier cycles, aiding prediction.^[24]

Solar maxima and minima also exhibit fluctuations at time scales greater than solar cycles. Increasing and decreasing trends can continue for periods of a century or more.

The 87 year (70–100 year Gleissberg cycle, named after Wolfgang Gleißberg, is thought to be an amplitude modulation of the Schwabe Cycle,^[5][25][26] The Gleisberg cycle implied that the next solar cycle have a maximum smoothed sunspot number of about 145±30 in 2010 (instead 2010 was just after the cycle's solar minimum) and that the following cycle have a maximum of about 70±30 in 2023.^[27]

Associated centennial variations in magnetic fields in the Corona and Heliosphere have been detected using Carbon-14 and beryllium-10 cosmogenic isotopes stored in terrestrial reservoirs such as ice sheets and tree rings^[28] and by using historic observations of Geomagnetic storm activity, which bridge the time gap between the end of the usable cosmogenic isotope data and the start of modern satellite data.^[29]

These variations have been successfully reproduced using models that employ magnetic flux continuity equations and observed sunspot numbers to quantify the emergence of magnetic flux from the top of the solar atmosphere and into the Heliosphere,^[30] showing that sunspot observations, geomagnetic activity and cosmogenic isotopes offer a convergent understanding of solar activity variations.

2,300 year Hallstatt solar variation cycles.

Hypothesized cycles

Periodicity of solar activity with periods longer than the sunspot cycle has been proposed,^[5] including:

The 210 year Suess cycle (a.k.a. "de Vries cycle").^[26] This cycle is recorded from radiocarbon studies, although "little evidence of the Suess Cycle" appears in the 400-year sunspot record.^[5])

The Hallstatt cycle is hypothesized to extend for approximately 2,300 years.^[31][32]

An as yet unnamed cycle may extend over 6,000 years.^[33]

In carbon-14 cycles of 105, 131, 232, 385, 504, 805 and 2,241 years have been observed, possibly matching cycles derived from other sources.^[34] Damon and Sonett^[35] proposed carbon 14-based medium- and short-term variations of periods 208 and 88 years; as well as suggesting a 2300-year radiocarbon period that modulates the 208-year period.^[36]

During the Upper Permian 240 million years ago, mineral layers created in the Castile Formation show cycles of 2,500 years.^{[citation needed]}

Solar magnetic field

The Sun's magnetic field structures its atmosphere and outer layers all the way through the corona and into the solar wind. Its spatiotemporal variations lead to various measurable solar phenomena. Other solar phenomena are closely related to the cycle, which serves as the energy source and dynamical engine for the former.

Effects

Solar[edit]

Activity cycles 21, 22 and 23 seen in sunspot number index, TSI, 10.7cm radio flux, and flare index. The vertical scales for each quantity have been adjusted to permit overplotting on the same vertical axis as TSI. Temporal variations of all quantities are tightly locked in phase, but the degree of correlation in amplitudes is variable to some degree.

Surface magnetism

Sunspots eventually decay, releasing magnetic flux in the photosphere. This flux is dispersed and churned by turbulent convection and solar large-scale flows. These transport mechanisms lead to the accumulation of magnetized decay products at high solar latitudes, eventually reversing the polarity of the polar fields (notice how the blue and yellow fields reverse in the Hathaway/NASA/MSFC graph above).

The dipolar component of the solar magnetic field reverses polarity around the time of solar maximum and reaches peak strength at the solar minimum.

Space

Spacecraft

CMEs (coronal mass ejections) produce a radiation flux of high-energy protons, sometimes known as solar cosmic rays. These can cause radiation damage to electronics and solar cells in satellites. Solar proton events also can cause single-event upset (SEU) events on electronics; at the same, the reduced flux of galactic cosmic radiation during solar maximum decreases the high-energy component of particle flux.

CME radiation is dangerous to astronauts on a space mission who are outside the shielding produced by the Earth's magnetic field. Future mission designs (e.g., for a Mars Mission) therefore incorporate a radiation-shielded "storm shelter" for astronauts to retreat to during such an event.

Gleißberg developed a CME forecasting method that relies on consecutive cycles.^[37]

On the positive side, the increased irradiance during solar maximum expands the envelope of the Earth's atmosphere, causing low-orbiting space debris to re-enter more quickly.

Galactic cosmic ray flux

The outward expansion of solar ejecta into interplanetary space provides overdensities of plasma that are efficient at scattering high-energy cosmic rays entering the solar system from elsewhere in the galaxy. The frequency of solar eruptive events is modulated by the cycle, changing the degree of cosmic ray scattering in the outer solar system accordingly. As a consequence, the cosmic ray flux in the inner solar system is anticorrelated with the overall level of solar activity. This anticorrelation is clearly detected in cosmic ray flux measurements at the Earth's surface. The effect amounts to several percent variation over the solar cycle, greater than the typically 0.1% variation in total solar irradiance.^[38][39]

Some high-energy cosmic rays entering Earth's atmosphere collide hard enough with molecular atmospheric constituents to cause occasionally nuclear spallation reactions. Fission products include radionuclides such as ¹⁴C and ¹⁰Be that settle on the Earth's surface. Their concentration can be measured in ice cores, allowing a reconstruction of solar activity levels into the distant past.^[40] Such reconstructions indicate that the overall level of solar activity since the middle of the twentieth century stands amongst the highest of the past 10,000 years, and that epochs of suppressed activity, of varying durations have occurred repeatedly over that time span.

Atmospheric

Solar irradiance

The total solar irradiance (TSI) is the amount of solar radiative energy incident on the Earth's upper atmosphere. TSI variations were undetectable until satellite observations began in late 1978. A series of radiometers were launched on satellites from the 1970s to the 2000s.^[41] TSI measurements varied from 1360 to 1370 W/m2 across ten satellites. One of the satellites, the ACRIMSAT was launched by the ACRIM group. The controversial 1989-1991 "ACRIM gap" between non-overlapping satellites was interpolated by an ACRIM composite showing +0.037%/decade rise. Another series based on ACRIM data is produced by the PMOD group. Its series shows a -0.008%/decade downward trend.^[42] This 0.045%/decade difference impacts climate models.
Solar irradiance varies systematically over the cycle,^[43] both in total irradiance and in its relative components (UV vs visible and other frequencies). The solar luminosity is an estimated 0.07 percent brighter during the mid-cycle solar maximum than the terminal solar minimum. Photospheric magnetism appears to be the primary cause (96%) of 1996-2013 TSI variation.^[44] The ratio of ultraviolet to visible light varies.^[45]

TSI varies in phase with the solar magnetic activity cycle^[46] with an amplitude of about 0.1% around an average value of about 1361.5 W/m^2[47] (the "solar constant"). Variations about the average of up to −0.3% are caused by large sunspot groups and of +0.05% by large faculae and the bright network on a 7-10-day timescale^[48] (see TSI variation graphics).^[49] Satellite-era TSI variations show small but detectable trends.^[50][51]

TSI is higher at solar maximum, even though sunspots are darker (cooler) than the average photosphere. This is caused by magnetized structures other than sunspots during solar maxima, such as faculae and active elements of the "bright" network, that are brighter (hotter) than the average photosphere. They collectively overcompensate for the irradiance deficit associated with the cooler, but less numerous sunspots. The primary driver of TSI changes on solar rotational and sunspot cycle timescales is the varying photospheric coverage of these radiatively active solar magnetic structures.^{[citation needed]}

Energy changes in UV irradiance involved in production and loss of ozone have atmospheric effects. The 30 HPa Atmospheric pressure level changed height in phase with solar activity during solar cycles 20-23. UV irradiance increase caused higher ozone production, leading to stratospheric heating and to poleward displacements in the stratospheric and tropospheric wind systems.^[52]

Short-wavelength radiation

A solar cycle: a montage of ten years' worth of Yohkoh SXT images, demonstrating the variation in solar activity during a sunspot cycle, from after August 30, 1991, to September 6, 2001. Credit: the Yohkoh mission of ISAS (Japan) and NASA (US).

With a temperature of 5870 K, the photosphere emits a proportion of radiation in the extreme ultraviolet (EUV) and above. However, hotter upper layers of the Sun's atmosphere (chromosphere and corona) emit more short-wavelength radiation. Since the upper atmosphere is not homogeneous and contains significant magnetic structure, the solar ultraviolet (UV), EUV and X-ray flux varies markedly over the cycle.

The photo montage to the left illustrates this variation for soft X-ray, as observed by the Japanese satellite Yohkoh from after August 30, 1991, at the peak of cycle 22, to September 6, 2001, at the peak of cycle 23. Similar cycle-related variations are observed in the flux of solar UV or EUV radiation, as observed, for example, by the SOHO or TRACE satellites.

Even though it only accounts for a minuscule fraction of total solar radiation, the impact of solar UV, EUV and X-ray radiation on the Earth's upper atmosphere is profound. Solar UV flux is a major driver of stratospheric chemistry, and increases in ionizing radiation significantly affect ionosphere-influenced temperature and electrical conductivity.

Solar radio flux

Emission from the Sun at centimetric (radio) wavelength is due primarily to coronal plasma trapped in the magnetic fields overlying active regions.^[53] The F10.7 index is a measure of the solar radio flux per unit frequency at a wavelength of 10.7 cm, near the peak of the observed solar radio emission. F10.7 is often expressed in SFU or solar flux units (1 SFU = 10⁻²² W m⁻² Hz⁻¹). It represents a measure of diffuse, nonradiative coronal plasma heating. It is an excellent indicator of overall solar activity levels and correlates well with solar UV emissions.

Sunspot activity has a major effect on long distance radio communications, particularly on the shortwave bands although medium wave and low VHF frequencies are also affected. High levels of sunspot activity lead to improved signal propagation on higher frequency bands, although they also increase the levels of solar noise and ionospheric disturbances. These effects are caused by impact of the increased level of solar radiation on the ionosphere.

10.7 cm solar flux could interfere with point-to-point terrestrial communications.^[54]

Clouds

The cosmic ray changes over the cycle potentially have significant atmospheric effects. Speculations about cosmic rays include:

• Changes in ionization affect the aerosol abundance that serves as the condensation nucleus for cloud formation.^[55] During solar minima more cosmic rays reach Earth, potentially creating ultra-small aerosol particles as precursors to Cloud condensation nuclei.^[56] Clouds formed from greater amounts of condensation nuclei are brighter, longer lived and likely to produce less precipitation.
• A change in cosmic rays could cause an increase in certain types of clouds, affecting Earth's albedo.^{[citation needed]}
• Particularly at high latitudes, with less shielding from Earth's magnetic field, cosmic ray variation may impact terrestrial low altitude cloud cover (unlike a lack of correlation with high altitude clouds), partially influenced by the solar-driven interplanetary magnetic field (as well as passage through the galactic arms over longer timeframes).^{[38][39][57][58]} A 2002 paper rejected this hypothesis.^[59]

Later papers claimed that production of clouds via cosmic rays could not be explained by nucleation particles. Accelerator results failed to produce sufficient, and sufficiently large, particles to result in cloud formation;^[60][61] this includes observations after a major solar storm.^[62] Observations after Chernobyl do not show any induced clouds.^[63]

Terrestrial

Organisms

The impact of the solar cycle on living organisms has been investigated (see chronobiology). Some researchers claim to have found connections with human health.^[64][65]

The amount of ultraviolet UVB light at 300 nm reaching the Earth varies by as much as 400% over the solar cycle due to variations in the protective ozone layer. In the stratosphere, ozone is continuously regenerated by the splitting of O₂ molecules by ultraviolet light. During a solar minimum, the decrease in ultraviolet light received from the Sun leads to a decrease in the concentration of ozone, allowing increased UVB to reach the Earth's surface.^[66]

Radio communication

Skywave modes of radio communication operate by bending (refracting) radio waves (electromagnetic radiation) through the Ionosphere. During the "peaks" of the solar cycle, the ionosphere becomes increasingly ionized by solar photons and cosmic rays. This affects the propagation of the radio wave in complex ways that can either facilitate or hinder communications. Forecasting of skywave modes is of considerable interest to commercial marine and aircraft communications, amateur radio operators and shortwave broadcasters. These users occupy frequencies within the High Frequency or 'HF' radio spectrum that are most affected by these solar and ionospheric variances. Changes in solar output affect the maximum usable frequency, a limit on the highest frequency usable for communications.

Climate

Both long-term and short-term variations in solar activity are hypothesized to affect global climate, but it has proven extremely challenging to quantify the link between solar variation and climate.^[67]
Early research attempted to correlate weather with limited success,^[68] followed by attempts to correlate solar activity with global temperature. The cycle also impacts regional climate. Measurements from the SORCE's Spectral Irradiance Monitor show that solar UV variability produces, for example, colder winters in the US and southern Europe and warmer winters in Canada and northern Europe during solar minima.^[69]
Three hypothetical mechanisms mediate solar variations' climate impacts:

• Total solar irradiance ("Radiative forcing").
• Ultraviolet irradiance. The UV component varies by more than the total, so if UV were for some (as yet unknown) reason having a disproportionate effect, this might affect climate.
• Solar wind-mediated galactic cosmic ray changes, which may affect cloud cover.

The sunspot cycle variation of 0.1% has small but detectable effects on the Earth’s climate.^[70][71][72] Camp and Tung suggest that solar irradiance correlates with a variation of 0.18 K ±0.08 K (0.32 °F ±0.14 °F) in measured average global temperature between solar maximum and minimum.^[73]

The current scientific consensus is that solar variations do not play a major role in driving global warming,^[67] since the measured magnitude of recent solar variation is much smaller than the forcing due to greenhouse gases.^[74] Also, solar activity in the 2010s was not higher than in the 1950s (see above), whereas global warming had risen markedly. Otherwise, the level of understanding of solar impacts on weather is low.^[75]

Causes

The basic causes of solar cycles are debated. While the proximate cause is a solar dynamo, the forces driving its behavior are less clear. Possibilities include a link with the tidal forces due to the gas giants Jupiter and Saturn,^[76][77] or due to solar inertial motion.^[78][79] Another cause of sunspots may be solar jet stream "torsional oscillation".

Models

Single dynamo

The 11-year sunspot cycle is half of a 22-year Babcock–Leighton solar dynamo cycle, which corresponds to an oscillatory exchange of energy between toroidal and poloidal solar magnetic fields. At solar-cycle maximum, the external poloidal dipolar magnetic field is near its dynamo-cycle minimum strength, but an internal toroidal quadrupolar field, generated through differential rotation within the tachocline, is near its maximum strength. At this point in the dynamo cycle, buoyant upwelling within the Convection zone forces emergence of the toroidal magnetic field through the photosphere, giving rise to pairs of sunspots, roughly aligned east–west with opposite magnetic polarities. The magnetic polarity of sunspot pairs alternates every solar cycle, a phenomenon known as the Hale cycle.^[80][81]
During the solar cycle’s declining phase, energy shifts from the internal toroidal magnetic field to the external poloidal field, and sunspots diminish in number. At solar minimum, the toroidal field is, correspondingly, at minimum strength, sunspots are relatively rare and the poloidal field is at maximum strength. During the next cycle, differential rotation converts magnetic energy back from the poloidal to the toroidal field, with a polarity that is opposite to the previous cycle. The process carries on continuously, and in an idealized, simplified scenario, each 11-year sunspot cycle corresponds to a change in the polarity of the Sun's large-scale magnetic field.^[82][83]

Double dynamo

In 2015, a new model of the solar cycle was published. The model draws on dynamo effects in two layers of the Sun, one close to the surface and one deep within its Convection zone. Model predictions suggest that solar activity will fall by 60 per cent during the 2030s to conditions last seen during the 'Little ice age' that began in 1645. Prior models included only the deeper dynamo.^[84]

The model features paired magnetic wave components. Both components have a frequency of approximately 11 years, although their frequencies are slightly different and temporally offset. Over the cycle, the waves fluctuate between the Sun's northern and southern hemispheres.^[84]

The model used principal component analysis of the Magnetic field observations from the Wilcox Solar Observatory. They examined magnetic field activity from solar cycles 21-23, covering 1976-2008. They also compared their predictions to average Sunspot numbers. The model was 97% accurate in predicting solar activity fluctuations.^[84]

Exponential model

Perry and Hsu (2000) proposed a model based on emulating harmonics by multiplying the basic 11-year cycle by powers of 2, which produced results similar to Holocene behavior. Extrapolation suggested a gradual cooling during the next few centuries with intermittent minor warmups and a return to near-Little Ice Age conditions within the coming 500 years. This cool period then may be followed approximately 1,000 years later by a return to altithermal conditions similar to the previous Holocene Maximum.^[85]