ES 331/767 Lecture 19

James S. Aber

Table of Contents
Introduction Last millennium
Medieval optimum Medieval glaciation
Little ice age Modern optimum
Related sites References

These web pages remain available in archive status,
but the course will not be updated after May 2017.

Updated version of Holocene climate.


During the last 10,000 years, climatic conditions have varied substantially from present-day conditions. Holocene climatic events have been reconstructed from glacial deposits, pollen, fossil remains, tree-ring chronology (Cook et al. 1991), radiocarbon content of tree rings, and oxygen-isotope records. For example, climate was the main driver for regional changes in boreal forest composition in northern Europe during the Holocene (Kuosmanen et al. 2016). Climate of the Holocene is divisible into three main phases, which are widely documented around the world.

The exact timing of these main phases differed somewhat in various portions of the globe, but the overall pattern is broadly similar worldwide. The middle Holocene was an interval of marked regional and global aridity events. For example droughts in the Ethiopian Plateau led to reduced flows in the Nile River and Delta several times between ~6000 and ~3000 calendar years ago with undoubted effects on early civilization (Bernhardt et al. 2012).

In northeastern Greenland, the HTM took place between 7700 to 4400 calendar years ago with peak temperatures about 7000 calendar years ago (Wagner and Bennike 2015). Meanwhile in southern Greenland, atmospheric temperature was 2-4 °C warmer than today (Larsen et al. 2015). The Neoglaciation began in western Greenland about 5000 years ago, was followed by substantial cooling about 3700 years ago, and continued with additional cooling episodes and expansion of glaciers (Schweinsberg et al. 2017). These long-term trends were interrupted by “abrupt events” during which sharply colder climate happened, namely the cooling at 8200 years ago, known as the 8.2 ka event (e.g. Seppä et al. 2010; Nicolussi and Schlüchter 2012), and the Little Ice Age (ca. 800 to 100 years ago).

Evidence has accumulated for several climatic cycles during the latest Pleistocene and Holocene of North America (Viau et al. 2002). Climatic oscillations took place with a period of roughly 1650+500 years and caused changes in vegetation across North America. These cycles may represent changes in atmospheric circulation with global climatic consequences, which are documented in ice-core and marine-sediment records. The origin of millennium-scale cycles is uncertain, but many scientists consider solar forcing a likely mechanism—more in lecture 20.

Table 19-1. Holocene climatic cycles of North America. Adapted from Viau et al. (2002).
Date* Period Climatic Conditions
110 Modern Modern climatic optimum
600 Little Ice Age Coldest climate of Holocene
1650 Neo-Atlantic Medieval climatic optimum
2850 Sub-Atlantic continued cooling
4030 Sub-Boreal beginning Neoglaciation period
6700 Atlantic II mid-Holocene climatic optimum
8100 Atlantic I continued warming
10,190 Boreal early Holocene warming
12,900 Younger Dryas Cold late-glacial interval
13,800 Ålleröd Warm late-glacial interval

* Date is approximate beginning of each climatic
cycle in calendar years before present.

Climate of the last millennium

Contrary to the popular belief in climatic stability of recent times, the Earth's climate of the past 1500 years has changed significantly. Good historical documentation, particularly for western Europe, exists for this period. Based on comprehensive studies of both scientific and historical information from all parts of the world, we now have a reasonably complete understanding of climate for this time interval (Le Roy Ladurie 1971, 2004; Lamb 1981; Grove 1988). Three main phases are now recognized:

Dates given for these phases are approximate and vary considerably from place to place around the world (Ingram et al. 1981). Within these main phases shorter-term fluctuations lasting a few decades are common and, again, vary distinctly in different locales. For instance, Zhang et al. (2017) utilized pollen records from ice cores and proglacial lakes to establish climatic phases for the past 2500 years in the monsoonal portion of the southern Tibet Plateau. The following phases correspond with similar climatic episodes and glacier fluctuations in the European Alps and elsewhere.

Climatic phases in the the Tibet Plateau
Climate Age Period
Warm 1850-present Current warm period
Cold AD 1250-1850 Little Ice Age
Warm AD 600-1250 Medieval warm period
Cold AD 400-600 Dark Ages
Warm AD 300-400
200-50 BC
Iron/Roman Age
Cold AD 100-300
520-300 BC
Iron Age
cold epoch
Adapted from Zhang et al. (2017).

Medieval climatic optimum

This was a time of extremely favorable climate in northern Europe. Harvests were good, fishing was abundant, sea ice stayed far to the north, vineyards existed 500 km north of 20th century limits, and famine was rare. Tree-ring records from the French Alps indicate summer temperatures comparable to the 20th century (Corona et al. 2011). The warmest decade (810s) matched the warmest decade of the 20th century (1990s).

This was the period of great Viking expansion from Scandinavia—see Fig. 19-7. In addition to their warlike image, Vikings were also traders and colonists. Their settlements were based on cereal grains (wheat and barley), livestock (goats, sheep, pigs, and cattle), and hunting marine mammals (seals, whales, and walrus).

Oseberg Ship (left), a completely preserved Viking ship from a burial mound in southern Norway. The ship dates from about A.D. 1000. Ships of this kind were sailed across the North Atlantic to Iceland, Greenland, and North America. Detail of ship's prow (right), showing construction technique and ornate wood carving. From the Viking Ship Museum, Norway.

Iceland was settled beginning in AD 870 and soon became an independent republic. Greenland was colonized in AD 985 by Eirík the Red, according to tradition, and his son, Leif (the Lucky) Erikson, discovered Newfoundland (Vinland) around AD 1000. By the 12th century, two sizeable communities existed in southwestern Greenland, and the Norse colonies obtained their own Catholic bishop. Greenland was a viable European outpost that lasted some five centuries. Norse subsistence was based on pasturing livestock in pockets of inner fjord regions separated by deep valleys and rugged mountains as well as seasonal seal and caribou hunting (McGovern 1981).

Archeologic remains of Norse farmsted at Brattahlid, Greenland. This farm site presumably was founded by Eirík the Red during the initial Viking colonization of Greenland in the tenth century. At that time, icebergs were not common in coastal waters. Photo by Preben Jensen; reproduced by permission.
The remains of a large barn for dairy cattle can be seen in foreground (just to right of previous view). In the right background stands Thjodhild's church and cemetery. Many graves are preserved from the latter part of Viking settlement, because permafrost conditions developed. Photo by Preben Jensen; reproduced by permission.

Essential trade with Iceland, Norway, and the rest of Europe was maintained through supply of valuable, exotic products, namely walrus tusk ivory, furs, and live polar bears that were taken in the Nordrsetur (northern hunting ground in the Disko Bay vicinity). In exchange, Norse Greenlanders received wood, iron, salt, wine, and other necessary commodities as well as church bells, stained glass, rich vestments, and the latest European fashions (Zorich 2017).

The most important seasonal activity was the annual walrus hunt to obtain ivory tusks and hides (Floger and Jazbee 2017). Walrus ivory tusks from Greenland and Iceland were among the most valuable trading items in Medieval Europe and were the economic basis for the survival of Norse Greenland settlements. This rich trade is demonstrated by the churches built in Greenland.

A spectacular program of church construction was carried out between 1125 and 1300. The later churches of Norse Greenland are among the largest stone structures in the Atlantic Islands, and they were built by one of the smallest communities (McGovern 1981).

Stone walls of Hvalsey church, the best preserved of any Viking building in southwestern Greenland. At least twelve church districts were set up in the "eastern" settlement, including a cathedral at Gardar, and at least three more church districts existed in the "western" settlement (Krogh 1967). Photo by Preben Jensen; reproduced by permission.

In North America, pollen and charcoal in sediments from Chesapeake Bay record climatic changes over the last 1000 years (Brush 1991). During the Medieval climatic optimum, large influxes of charcoal, sediment, and metals indicate more frequent forest fires and higher rates of erosion in the surrounding basin. Forest in the Chesapeake basin recovered, and erosion diminished, during the following few centuries of cold climate. In southern Florida, sea level was at least ½ m higher than now from the first through tenth centuries (Froede 2002).

False-color Landsat TM image of Chesapeake Bay and Potomac Bay vicinity, Maryland and Virginia. Washington, D.C. is blue spot near scene center. Changing character of sediment accumulation in Chesapeake Bay reflects vegetation and climatic conditions in surrounding land areas. From NASA Goddard Space Flight Center.

For Pacific Islands, the period AD 750 to 1300 was a climatic optimum marked by warm temperature, high sea level, and probable aridity (Nunn and Britton 2001; Nunn 2003). This was the period of long-distance Polynesian migrations and colonization across vast oceanic distances. The Pacific climatic optimum during the 12th century is confirmed by tree-ring records of the Huon pine from Tasmania (Cook et al. 1991).

Medieval glaciation

Climatic deterioration began in the 1200s; glaciers expanded in Iceland and in the Alps. Vineyards had declined in Germany by the 1300s and had completely disappeared in England. Fishing replaced cereal grains as the main source of food in Iceland, sea ice expanded southward between Greenland and Iceland, and the diet in Greenland shifted from crops and livestock to wild seal (Floger and Jazbee 2017). Thule people in Greeland began migrating southward along the western coast and came into increasing contact and combat with Norsemen.

Around 1350 the more northerly of the two Greenland communities was abandoned to the native Thule. In the years 1346-1353, bubonic plague swept through Europe and killed one in three people. It is unknown whether the plague reached either Iceland or Greenland, but more than half the population died in Norway, which was Greenland's economic lifeline to Europe (Floger and Jazbee 2017). After 1369, no trade ships were sent to Greenland (Zorich 2017). Furthermore, Portugal began trade routes into sub-Sarahan Africa for high-quality elephant ivory. Thus the environmental and economic basis for Norse Greenland collapsed.

The last reliable account of Norsemen living in Greenland comes from 1408, when a wedding took place and also a burning for witchcraft, both at Hvalsey Church—see Fig. 19-8. As the ivory trade waned and living conditions became more difficult, it seems likely many Greenlanders gradually and simply moved back home to family and relations in Iceland and Norway. The archaeologic ruins suggest an orderly retreat, as no items of value were left behind. For instance, the couple married at Hvalsey Church in 1408, Sigrid Bjornsdottir and Thorstein Olafsson, eventually settled in Iceland in 1424 (Floger and Jazbee 2017).

Based on archeologic evidence, it seems that a few Norsemen continued to live in Greenland until about 1480. However, when the region was next visited, by German merchants in 1510, only Inuits were found living among the ruins. The harsh climate after 1300 was undoubtedly a factor in the demise of the Norse settlements. Cold climate reduced dairy production, hampered hunting success, and extensive sea ice restricted essential trade with Europe. Furthermore, the last Norse Greenlanders held to their cultural, religious, and subsistence traditions without attempting to adapt to changing conditions (McGovern 1981).

Interior view of Hvalsey church. A wedding in 1408 at this church is the last recorded event in the history of Viking Greenland. Photo by Preben Jensen; reproduced by permission.

From the mid-1400s to the mid-1500s climatic conditions in western Europe improved somewhat. This episode was too little and too late, apparently, to save the doomed Norse settlements in Greenland. Meanwhile in Scotland, unfavorable climate impacted cereal crops, and large areas were permanently abandoned for agriculture during the period 1200 to 1700 (Parry 1981). Elsewhere in Europe, life went on with no recognition of climatic change or its effects.

Across the Pacific Islands, the period AD 1270-1475 was a transitional interval, often called the "AD 1300 event" (Nunn 2000). Sea level fell, perhaps in two stages by more than 1 m, and temperature declined an average 1½ °C. El Niño increased in frequency, and precipitation increased. These climatic changes resulted in a serious decline in productivity for near-shore coral reefs, and significant shifts in human culture took place. Most notably, the long-distance voyages of the previous period came to an end.

Little Ice Age

Cold climate and glacier expansion during the Little Ice Age are documented from all continents (except Antarctica) and on major islands from New Zealand to Svalbard (Grove 1988). The best historical evidence comes from the Alps, Scandinavia, and Iceland. The Little Ice Age was not a single, uniformly cold climatic episode. Distinct variations in climate and in glacier activity took place on a regional basis with complex patterns of warming and cooling in different places (Ingram et al. 1981). In Europe and North America, at least six phases of glacier expansion occurred and were separated by milder intervals.

  1. 1560-1610 Major advances by all glaciers.
  2. 1640-1650 Glacier maximum in Switzerland.
  3. 1670-1705 Glacier maximum in Austria.
  4. 1720-1750 Glacier maximum in Norway.
  5. 1816-1825 Minor advances by all glaciers.
  6. 1850-1890 Glacier maximum in Canada/Iceland.

These advances during the Little Ice Age resulted in adverse conditions for farms and villages located in mountain valleys below the glaciers. Many farms and some villages were destroyed by a combination of glacier advance, melt-water floods, landslides, and related disasters. Population in the affected mountain regions declined significantly, due to emigration and death, whereas population elsewhere in "lowland" Europe continued to grow in general during the Little Ice Age.

Glacier advances in the vicinity of Mont Blanc, France, destroyed three villages and heavily damaged a fourth between 1600 and 1610. The oldest of these villages had existed since the 1200s. From the late 1600s until 1920, summer temperature of the French Alps averaged 0.7 °C lower than the mean for the late 20th century (1961-1990), according to tree-ring data (Corona et al. 2011).

Likewise in Norway, outlet glaciers of Jostedalsbreen ice cap advanced markedly in the 1700s and destroyed many farms—see Figs. 19-9, 19-10 and 19-11. The local population was reduced to eating bread made with a mixture of ground wheat chaff, straw, and pine bark. Taxes were reduced on farms that suffered physical damage—see Fig. 19-12, and many people were forced to migrate out of the region or become beggars.

Large lateral moraine of the Little Ice Age in vicinity of Hornsund, southern Spitsbergen, Svalbard. Photo © by J.J. Zeeberg; used here by permission.
Jostedalsbreen is the ice cap on the distant horizon. The deep valley is Jostedal, and a "summer farm" is seen to the right. Summer farms are used for tending dairy cattle that graze on the high pasture. During the Little Ice Age such summer farms were unproductive. Outlet glaciers of Jostedalsbreen descended into lower valleys in the distance and destroyed many farms. Photo date 6/87; © by J.S. Aber.

The Little Ice Age was a time of exceptional poverty, misery and suffering in Iceland, as a result of severe winters, major volcanic eruptions, and oppressive Danish colonial rule. Famine and pestilence ravaged the country. The human population of Iceland, which had reached about 70,000 around AD 1100, had dwindled to only 34,000 by 1708—less than half the Viking peak (Magnusson 1987). Following a huge volcanic eruption in 1783, there was serious discussion of evacuating the remaining inhabitants to live in Denmark, but this did not actually happen.

Climatic and human consequences of the Little Ice Age are best documented in western Europe. Therefore, some climatologists have concluded naively that this climatic episode was a regional anomaly, not of global significance. This point of view is contradicted strongly by evidence from all other parts of the world. For instance, lake sediments from the Lvliang Mountains of northern China document cold and dry conditions between AD 1230 and 1880 (Liu et al. 2014).

The Quelccaya Ice Cap in the Andes Mountains of southern Peru is among the most important records of late Holocene climate. Ice cores provide direct physical evidence for colder climate between AD 1500 and 1900 (Thompson et al. 1986). In fact, the Quelccaya Ice Cap reached its maximum Holocene advance during this period (Kelly et al. 2012).

Ice-cores from Quelccaya Ice Cap, Peru
Location map and ice-cap margin
Solar-powered drilling equipment and ice core
Oxygen-isotope and accumulation records
Climatic record and prehistoric civilization

The climatic changes recorded in the Quelccaya ice cap correspond closely with prehistoric cultures of Peru. Farther south, Lake Titicaca rose significantly during the 16th-19th centuries as a result of more humid, cooler conditions (pers. comm. J. Argollo, 1996).

The Little Ice Age was in fact a worldwide event with distinct regional variations (Nesje and Dahl 2000). It is documented from the southern hemisphere to Spitsbergen in the far north (Svendsen and Mangerud 1997). Based on many forms of historical, archaeological and geological evidence, global average temperature was 1-2 °C cooler than today (Grove 1988). This climatic episode was not recognized at the time; its true character has become clear only since the Little Ice Age ended.

Special lecture on Late Holocene climate.

Modern climatic optimum

The Little Ice Age ended in some parts of the world as early as 1860, in other regions not until the 1930s. A marked difference is apparent for climatic change in the northern and southern hemispheres—see Fig. 19-13. In any case, without question the 20th century was noticeably warmer for most regions than for any time since the 12th century. However, 20th century climate did not recover to the level of warmth that existed during the Medieval climatic optimum a millennium ago (Robinson et al. 2007). Furthermore, the modern climatic optimum remains well below conditions experienced during the mid-Holocene thermal maximum (Larsen et al. 2015, Wagner and Bennike 2015).

Entrance to an ice cave high on the side of the Tennengebirge mountains, south of Salzburg, Austria. The natural ice formations inside are maintained today only by careful human regulation of air flow during winter and summer. July 2007 © J.S. Aber.

Glaciers and ice caps have experienced negative mass balances and have been retreating since the end of the Little Ice Age. This is a general condition for glaciers of all types in nearly all geographic locations, with the possible exception of Antarctica. The local timing of deglaciation may vary considerably, however, depending on many factors as detailed below.

Table 19-2. Comparison of response
rates for glaciers in different settings.
Rapid Response Slow Response
High altitude (mountains) Low altitude (lowlands)
Continental climatic zone Maritime climatic zone
Sea- or lake-based glaciers Land-based glaciers
Small glaciers & ice caps Large glaciers & ice caps
Atlantic Ocean regime Pacific Ocean regime
Northern hemisphere Southern hemisphere

The end of the Little Ice Age occurred earliest—mid-1800s—for interior mountains of northern mid-latitudes, such as the European Alps, and took place latest—early 1900s—on islands of the South Pacific, as in New Zealand. The end of the Little Ice Age is just beginning to have an effect in Antarctica. Meanwhile, the late 20th century was a period of positive mass balance and expansions for small glaciers in many places, for example Iceland and Norway, as a result of increased winter precipitation (Nesje and Dahl 2000).

Table 19-3. Dynamics of Virkisjökull and Falljökull, southeastern Iceland.
Date range Glacier-front behavior
1932-1934 Advancing ice margin
1935-1948 Rapid retreat
1949-1972 Slow retreat or stationary
1973-1990 Advancing ice margin
1991-1997 Slow retreat or stationary
1998-2011 Rapid retreat
Based on Bradwell et al. (2013).

Since the end of the Little Ice Age, glaciers have experienced many lesser periods of ice advance and retreat that happened at different times in separate parts of the world. This scenario indicates that global climatic change takes place with distinct regional variations, which are probably the results of lag effects caused by differences in heat transfer and storage at the Earth's surface.

Aerial views of Svartisen, a double ice cap in northern Norway near the Arctic Circle. Norwegian ice caps and outlet glaciers shrank rapidly during the first half of the twentieth century. However, they stablized or even expanded late in the century, as a result of slightly cooler summers and more winter accumulation. The activity of some Norwegian glaciers has become dangerous, because of snow-covered crevasses and sudden melt-water floods. Photographs courtesy of J.J. Zeeberg (1996).

One consequence of warming climate is steadily rising global sea level caused by melting glaciers and thermal expansion of water. Low-lying coastal areas and small islands are threatened by increasing sea level. However, sea level is not rising uniformly around the world, and some areas are actually declining due to subtle tectonic changes in the Earth's geoid (gravity field). Highest rates of sea-level increase are on the order of one cm per year in the western Pacific, southern Indian and southern Atlantic oceans (see MSL map). The Funafuti Atoll, for example, has experienced ~5 mm/year rise in sea level totaling ~30 cm (one foot) over the past 60 years. Nonetheless, the islands actually have gained land area during the past century, which demonstrates that coral-reef islands may adjust to changing conditions (Kench et al. 2015).

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ES 331/767 © J.S. Aber (2017).