Introduction

• Climatic amelioration of early Holocene—Preboreal and Boreal periods.

• Climatic optimum of mid-Holocene, also known as the Holocene thermal maximum (HTM). This interval corresponds to the Atlantic period in northern Europe. In North America it has been called the altithermal or hypsithermal, a time of relatively warmer and drier climatic conditions. Sand dunes were particularly active during this period in the Great Plains region (Miao et al. 2007).

  Central Sand Hills, near Seneca, western Nebraska. Middle Loop River valley is visible in foreground and a massive sand ridge is seen on horizon. Sand dunes form ridges up to 100 m high and several km long. The Nebraska Sand Hills are the largest dune field in the western hemisphere, now stabilized by grass cover. Photo © by J.S. Aber. More about the Nebraska Sand Hills.

• Neoglaciation of late Holocene—Subboreal and Subatlantic periods. Climate was wetter and cooler in the southwestern United States (Polyak et al. 2001) and elsewhere. Insect remains in northern Sweden, for example, indicate a climatic deterioration began about 2400 years ago (Khorasani et al. 2015).

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).

Climate of the late Holocene

• Medieval climatic optimum (AD 700-1200).
  
• Medieval glaciation (AD 1200-1460) and Little Ice Age (AD 1560-1890).
  
• Modern climatic optimum (AD 1890-present).

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.

Medieval climatic optimum

A brief cold spell took place 820-834, when the Grindelwald Glacier advanced in the Swiss Alps, the only such advance between AD 600 and 1050. This cooling episode likely resulted from a series of large volcanic eruptions, including Katla in Iceland in AD 822-23, which is confirmed by tree rings of Scandinavia, sulfate deposition in Greenland ice cores, and historical evidence from Europe and China (Büntgen et al. 2017).

The Medieval climatic optimum was the period of great Viking expansion from Scandinavia. 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.

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.

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.

Medieval glaciation

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. 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.

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

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.

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. 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, 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.

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).

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.

Modern climatic optimum

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.

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).