Africa was Wetter During the Medieval Warm Period

Hydroclimate in Africa during the Medieval Climate Anomaly

Authors:

Lüning et al

Abstract:

The Medieval Climate Anomaly (MCA) is a recognized period of distinct pre-industrial climate change, with a core period of 1000–1200 CE. The field of palaeoclimatology has made major progress over the past 15 years during which a great number of high- and medium-resolution case studies were published, reconstructing climate change of the past millennia. In many parts of the world, regional data coverage has now reached a point which allows compiling palaeoclimate maps for well-defined time intervals. Here we present hydroclimatic trend maps for the MCA in Africa based on 99 published study locations. Key hydroclimatic proxy curves are visualized and compared in a series of 16 correlation panels. Proxy types are described and possible issues discussed. Based on the combined MCA dataset, temporal and spatial trends are interpreted and mapped out. Three areas have been identified in Africa in which rainfall seems to have increased during the MCA, namely Tunisia, western Sahel and the majority of southern Africa. At the same time, a reduction in precipitation occurred in the rest of Africa, comprising of NW and NE Africa, West Africa, Eastern Africa and the Winter Rainfall Zone of South Africa. MCA hydroclimate change in Africa appears to have been associated with characteristic phases of ocean cycles, as also supported by modern climate observations. Aridity in Morocco typically coincides with the positive phase of the North Atlantic Oscillation (NAO), whilst increased rainfall in the western Sahel is often coupled to the positive phase of the Atlantic Multidecadal Oscillation (AMO). Reduction in rainfall in the region Gulf of Aden/southern Red Sea to Eastern Africa could be linked to a negative Indian Ocean Dipole (IOD) or a derived long-term equivalent Indian Ocean cycle parameter. The Intertropical Convergence Zone (ITCZ) appears to have been shifted pole-wards during the MCA, for both the January and July positions. MCA hydroclimate mapping revealed major data gaps in the Sahara, South Sudan, Somalia, Central African Republic, Democratic Republic of Congo, Angola, northern Mozambique, Zambia and Zimbabwe. Special efforts are needed to fill these gaps, e.g. through a dedicated structured research program in which new multiproxy datasets are created, based on the learnings from previous African MCA studies.

Greenland may NOT Have Been Warmer When Norse Colonization?

What is known: Vikings sailed to Greenland. They homesteaded there for a few hundred years, and likely experienced multiple famines. Many died. Some returned to European shores. And all of this happened during a time in Europe known to geoscientists as the Medieval Warm Period. The warmer, milder conditions that defined this time eventually ended too.

For many years, scientists have pondered if the Vikings' diaspora to Greenland was made easier by the warmer temperatures of the Medieval Warm Period. Climate data extracted from shells had indicated that this warm period extended to Greenland, but new research looking at glacial movements and using isotope data from terminal moraines suggests this may not necessarily be so.

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Medieval Warm Period Wasn't. At Least in Greenland?

A new study questions the popular notion that 10th-century Norse people were able to colonize Greenland because of a period of unusually warm weather. Based upon signs left by old glaciers, researchers say the climate was already cold when the Norse arrived--and that climate thus probably played little role in their mysterious demise some 400 years later. On a larger scale, the study adds to building evidence that the so-called Medieval Warm Period, when Europe enjoyed exceptionally clement weather, did not necessarily extend to other parts of the world.

"It's becoming clearer that the Medieval Warm Period was patchy, not global," said lead author Nicolás Young, a glacial geologist at Columbia University's Lamont-Doherty Earth Observatory. "The concept is Eurocentric--that's where the best-known observations were made. Elsewhere, the climate might not have been the same." Climate scientists have cited the Medieval Warm Period to explain anomalies in rainfall and temperature in far-flung regions, from the U.S. Southwest to China. The study appears today in the journal Science Advances.

Norse, or Vikings, led by Erik the Red, first sailed from recently settled Iceland to southwestern Greenland around 985, according to Icelandic records. Some 3,000 to 5,000 settlers eventually lived in Greenland, harvesting walrus ivory and raising livestock. But the colonies disappeared between about 1360 and 1460, leaving only ruins, and a longstanding mystery as to what happened. The native Inuit remained, but Europeans did not re-inhabit Greenland until the 1700s.

The Greenlandic Vikings' apogee coincided with the Medieval Warm Period (also known as the Medieval Climate Anomaly), generally dated from about 950-1250; their disappearance followed the onset of the Little Ice Age, which ran from about 1300-1850. Both periods are firmly documented in European and Icelandic historical records. Thus, popular authors and some scientists have fixed on the idea that nice weather drew the settlers to Greenland, and bad weather froze and starved them. But there are no early historical climate records from Greenland. Recently, historians have proposed more complex factors in addition to, or instead of, climate: hostilities with the Inuit, a decline in ivory trade, soil erosion caused by the Vikings' imported cattle, or a migration back to Europe to farms depopulated by the Black Plague.

In the new study, the scientists sampled boulders left by advancing glaciers over the last 1,000-some years in southwest Greenland, and on neighboring Baffin Island, which the Norse may also have occupied, according to newly uncovered evidence. Glacial advances during the Little Ice Age have wiped out most evidence of where the glaciers were during the Norse settlement. But Young and his colleagues were able to find traces of a few moraines--heaps of debris left at glaciers' ends--that, by their layout, they could tell predated the Little Ice Age advances. Using newly precise methods of analyzing chemical isotopes in the rocks, they showed that these moraines had been deposited during the Viking occupation, and that the glaciers had neared or reached their later maximum Little Ice Age positions between 975 and 1275. The strong implication: it was at least as cold when the Vikings arrived as when they left. "If the Vikings traveled to Greenland when it was cool, it's a stretch to say deteriorating climate drove them out," said Young.

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Medieval Climate Anomaly Allowed Polynesians to Expand to New Zealand, Easter Island and Other Isolated Locales

Climate windows for Polynesian voyaging to New Zealand and Easter Island

Authors:

Goodwin et al

Abstract:

Debate about initial human migration across the immense area of East Polynesia has focused upon seafaring technology, both of navigation and canoe capabilities, while temporal variation in sailing conditions, notably through climate change, has received less attention. One model of Polynesian voyaging observes that as tradewind easterlies are currently dominant in the central Pacific, prehistoric colonization canoes voyaging eastward to and through central East Polynesia (CEP: Society, Tuamotu, Marquesas, Gambier, Southern Cook, and Austral Islands) and to Easter Island probably had a windward capacity. Similar arguments have been applied to voyaging from CEP to New Zealand against prevailing westerlies. An alternative view is that migration required reliable off-wind sailing routes. We investigate the marine climate and potential voyaging routes during the Medieval Climate Anomaly (MCA), A.D. 800–1300, when the initial colonization of CEP and New Zealand occurred. Paleoclimate data assimilation is used to reconstruct Pacific sea level pressure and wind field patterns at bidecadal resolution during the MCA. We argue here that changing wind field patterns associated with the MCA provided conditions in which voyaging to and from the most isolated East Polynesian islands, New Zealand, and Easter Island was readily possible by off-wind sailing. The intensification and poleward expansion of the Pacific subtropical anticyclone culminating in A.D. 1140–1260 opened an anomalous climate window for off-wind sailing routes to New Zealand from the Southern Austral Islands, the Southern Cook Islands, and Tonga/Fiji Islands.

Medieval Warm Period Was Not Uniformly Warm

Summers on the Norwegian archipelago of Svalbard are now warmer than at any other time in the last 1,800 years, including during medieval times when parts of the northern hemisphere were as hot as, or hotter, than today, according to a new study in the journal Geology.

"The Medieval Warm Period was not as uniformly warm as we once thought--we can start calling it the Medieval Period again," said the study's lead author, William D'Andrea, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory. "Our record indicates that recent summer temperatures on Svalbard are greater than even the warmest periods at that time."

The naturally driven Medieval Warm Period, from about 950 to 1250, has been a favorite time for people who deny evidence that humans are heating the planet with industrial greenhouse gases. But the climate reconstruction from Svalbard casts new doubt on that era's reach, and undercuts skeptics who argue that current warming is also natural. Since 1987, summers on Svalbard have been 2 degrees to 2.5 degrees C (3.6 to 4.5 degrees F) hotter than they were there during warmest parts of the Medieval Warm Period, the study found.

Researchers produced the 1,800 year climate record by analyzing levels of unsaturated fats in algae buried in the sediments of Kongressvatnet lake, in western Svalbard. In colder water, algae make more unsaturated fats, or alkenones; in warmer water, they produce more saturated fats. Like pages in a book, the unsaturation level of fats can provide a record of past climate. So far, most Arctic climate records have come from ice cores that preserve only annual layers of cold-season snowfall, and thus cold-season temperatures. But lake sediments, with their record of summertime temperatures, can tell scientists how climate varied the rest of the year and in places where ice sheets are absent.

"We need both ice core and lake sediment records," said Elisabeth Isaksson, a glaciologist at the Norwegian Polar Institute who was not involved in the study. "Here, Billy has found something that tells a different, more detailed story."

In looking at how summers on Svalbard varied, researchers also discovered that the region was not particularly cold during another recent anomalous period--the "Little Ice Age" of the 18th and 19th centuries, when glaciers on Svalbard surged to their greatest extent in the last 10,000 years and glaciers in many parts of Western Europe also grew. They suggest that more snow, rather than colder temperatures, may have fed the growth of Svalbard glaciers.

Evidence from tree rings and ice cores shows that southern Greenland and parts of North America were warmer from 950 to 1250 than today, with the Vikings taking advantage of ice-free waters to settle Greenland. Some regions also saw prolonged drought, including California, Nevada and the Mississippi Valley, leading some scientists to coin the term Medieval Climate Anomaly to emphasize the extreme shift in precipitation rather than temperature. A natural increase in solar radiation during this time was responsible for warming parts of the northern hemisphere, with a rise in volcanic activity from 1100 to 1260 causing milder winters, University of Massachusetts scientist Ray Bradley explained in a 2003 Perspective piece in Science. Bradley is a co-author of the Svalbard lake sediment study.

Western Svalbard began to gradually warm in 1600, the researchers found, when the northern arm of the Gulf Stream, known as the West Spitsbergen Current, may have brought more tropical water to the region. In 1890, the warming began to accelerate, with researchers attributing most of the warming since about 1960 to rising industrial greenhouse gas levels. Ice cores from Svalbard, by contrast, show a slight cooling over the last 1,800 years. The conflicting evidence suggests that temperatures may have fluctuated more sharply between winter and summer, said Anne Hormes, a quaternary geologist at the University Centre in Svalbard who was not involved in the study.

D'Andrea and his colleagues dated their lake cores by analyzing grains of glass spewed by volcanoes hundreds of miles away in Iceland. Those past eruptions-- Snæfellsjökull in 170, Hekla in 1104 and Öræfajökull in 1362--all left unique chemical time markers on Svalbard's lake sediments. "We know fairly precisely when these eruptions occurred, which is rare in the geologic record," said study co-author Nicholas Balascio, a scientist at University of Massachusetts, Amherst.

Gosh. Climate complicated? ;)