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Vulnerability of past societies to climate change and legacies of the “Medieval Warm Period” the “Little Ice Age” Last updated 12 January 2010 REQUIRED MATERIAL FOR THIS SECTION Dire Predictions pages 46-47 Coursepack Peterson, L.C. and Haug, G.H. 2005. Climate and the collapse of Maya civilization. American Scientist 93:322-329. Webpages Medieval Warm Period by the US National Oceanic and Atmospheric Administration www.ngdc.noaa.gov/paleo/globalwarming/medieval.html WHAT TO KNOW – you should be able to answer the following questions after covering material associated with this topic: ¾ Approximately when and where did the Medieval Warm Period and Little Ice Age occur? ¾ What evidence is there for each event? ¾ What processes have been proposed to be responsible for the climatic changes? ¾ Why might the timing of collapse of Mayan society vary regionally? ¾ What evidence is there for drought experienced by Mayans? ¾ Under what conditions would minor climate change have impacts on a society? Can you give some examples of how at least one society was particularly vulnerable to the Little Age or drought? INTRODUCTION We will begin our investigation of past climates, their evidence, and impacts with two of the most recent climatic “events”, the Medieval Warm Period and the Little Ice Age that followed it. There is considerable debate regarding the timing, cause and even existence of this pair of climatic fluctuations. Recent additions to this debate are studies of recurrent and extended droughts over the last few millennia. If these droughts are triggered by the same factors as the Medieval Warm Period and Little Ice Age then global “teleconnections” must be explained – a major focus of paleoclimate research today. The multiple occurrences of extended droughts provide greater opportunity to test linkages among factors such as solar irradiance and the oceanatmosphere climate system that will be covered in upcoming lectures. Archaeological evidence and historical (i.e., written) records provide indications of past climate condition and record the impact on and response of societies to climate change. So, despite the lack of records from meteorological instruments we can infer past climate from records of such things as harvests of crops which are dependent upon the length of the growing season (time of the last frost in spring and first frost in fall) and weather during the growing season (e.g., storms can damage the grass stems which support the developing seeds). Even tax records can provide useful data on characteristics of climate change! 1 THE MEDIEVAL WARM PERIOD The 12th and 13th centuries seemed to be a time of mild temperatures when agriculture in Europe flourished, thus called the Medieval Warm Period or Little Optimum. Note the change in temperature of ~0.25°C on the graph of temperature variation over the last 1,000 years in figure 1. Figure 1. Example of regional variations in surface air temperature for the last 1000 years, estimated from a variety of sources, including temperature-sensitive tree growth indices and written records of various kinds, largely from western Europe and eastern North America. Shown are changes in regional temperature in °C, from the baseline value for 1900. Compiled by R. S. Bradley and J. A. Eddy based on J. T. Houghton et al., Climate Change: The IPCC Assessment, Cambridge University Press, Cambridge, 1990 and published in EarthQuest, vol 5, no 1, 1991. During the so-called Medieval Warm Period vineyards in Britain extended 3-5° north in latitude (see figure 2 below) and 100-200 metres further up hillsides. (Temperatures tend to decrease with increasing altitude, so this was a critical change.) Based upon knowledge of the weather requirements for successful vineyards (e.g., length of growing season) some scientists assume that temperatures were 1-2°C higher than today (see table 1 below). Historical records of agricultural expansions exist from other continents, as well. For example, in China the lychee was north of its present range. Some would say that the Medieval Warm Period dates as far back as the 9th century A.D. Warmer temperatures would have meant that sea ice did not extend as far south and harbours of the northern Atlantic were ice free more often or for longer periods. These conditions in turn Table 1. Climate parameters relevant to successful vineyards. facilitated Norse Reproduced from Lamb 1965. (Viking) exploration (fig 3) and settlement of relatively marginal lands in Iceland, Greenland and at L'Anse aux Meadows in Newfoundland (fig 4). 2 Figure 2. Map showing the northern limits of British viticulture in the Roman and Medieval warm phases, the Little Ice Age, and the present day. From Seeley 2008. Figure 4. Reconstructed Viking dwellings at L’Anse aux Meadows, Newfoundland. Figure 3: Routes various Vikings traveled. (Source: McGovern and Perdikaris, 2000) 3 The study of tree rings (dendroclimatology) and ice cores are two types of “proxy” indicators that give annual records of past climate. This fine-scale temporal resolution is critical to the study of recent climatic events such as the Medieval Warm Period which lasted only centuries. We will cover these types of evidence in a future lecture. After pulling together the records from ice cores and tree rings some scientists conclude that the Medieval Warm Period was no warmer than the last half of the 20th century, but may still have been warmer than the Little Ice Age. Review the graph and the associated text at the U.S. National Oceanic and Atmospheric Administration (NOAA) Paleoclimate Web pages (see readings listed above) on the Medieval Warm Period. This paleotemperature record indicates yearly average temperatures. A year could have colder winters and hotter summers, but the yearly average would be the same. An average annual temperature record might not reflect a climate change which was expressed in shorter growing seasons but overall milder winter temperatures (and seasons of ice-free harbours). It may be that the expression of the Medieval Warm Period was limited to certain regions either because the process causing the climate change was spatially restricted OR sensitivity of regions varied. If there was no Medieval Warm Period, than why did Viking exploration and settlement expand west? Anthropologist Robert McGhee suggests a motivation. Viking chiefs may have been resisting pressures to relinquish their powers in Norway and were losing too many battles in Europe to continue their exploitation to the south. THE LITTLE ICE AGE “On August 1562 a thunderstorm hit central Europe. The sky darkened at noontime as if it were night and a severe storm began, destroying roofs and windows. Some hours later the thunderstorm turned into a hailstorm (figure 5) which lasted until midnight. The hailstorm destroyed crops and vineyards, killed birds and other animals, including some unprotected horses and cows. The next day trees stripped of their leaves and branches were observed and the fields were a picture of devastation (Wahrhafftiger und gruendlicher Bericht, 1562). Travellers Figure 5. An etching made in 1562 depicting a recognized the unusual strength of hailstorm over a city. Reproduced from the journal Climatic Change. the hailstorm. A nobleman on a journey from Vienna to Brussels reported seeing severe storm damage whilst traveling the postal route (Weyer, 1586). The meteorological front must have covered an area of several hundred kilometers in 4 diameter. A printed newsletter reported that many people feared the beginning of the Last Judgement. Since observers of the period had no recollection of similar climatic disasters many considered this thunderstorm as unnatural and looked for explanations.” Behringer 1999 Figure 6. An etching from 1592 depicting a witch conjuring up bad weather. Historical records provide evidence of periods of bad weather, with colder winters and summers, as well as increased storminess a few centuries following the Medieval Warm Period. Weather is especially important in an agrarian society. In some years there was crop failure. As local populations were dependent upon the food they grew, famine often followed crop failure in parts of Europe. Crop failure also caused increases in prices, malnutrition, rising infant mortality and finally, epidemics. In some regions the bad weather was a trigger for early witch-hunts as people searched for someone to blame for their climatic misfortune. Hailstorms that were quite frequent in some regions were thought to have been conjured up by witches (figure 6). The historical records from Europe demonstrate a variety of impacts the cooling had. The cooling is thought to be the cause of advances of mountain glaciers in many parts of the world, 5 hence this event is sometimes referred to as the neoglaciation. In Switzerland the Aletsch glacier advanced over part of an aqueduct. In Norway requests for tax relief for damage due to glacier advances, avalanches, landslides and floods increased (figure 7). The northernmost vineyards in Britain were abandoned. Although there was year-to-year fluctuation in temperatures (as seen on both temperature graphs) average temperature decreased about 0.5°C from a baseline temperature set at 1900 A.D. The decrease was even greater if the comparison is made to the peak temperatures of the Medieval Warm Period. This colder period that followed the Medieval Warm Period is more widely recognized, but still debated among scientists. Even scientists who believe there was a “Little Ice Age” have varied opinions regarding the time interval it spans. Its beginning has been placed anywhere from the 14th to the 16th century A.D. and its end during the 18th or 19th century. . During the Little Ice Age, Viking settlements in Greenland were abandoned (those in Newfoundland lasted only a few years). The livelihood of local Inuit (Eskimos) was adapted to these colder conditions and they survived, but the Vikings were unwilling or unable to adopt Figure 7. The incidence of mass movement, the critical lifestyle changes. The limit of flooding, etc. revealed by requests for tax relief in parishes on the margins of the Jostedalsbreen sea ice now extended further south, glacier in southern Norway. restricting communications and fishing. McGhee suggests that the decline in the Viking’s Greenland settlement was due to a combination of factors such as deteriorating climate and economic forces, including harassment by European pirates. There is a mystery surrounding the final end to the colony. There are no remains of the last settlers and no records of their departure, or of their burial by visitors. Archeological investigations suggest a rather sudden abandonment. Perhaps members of the small population of Norse that were left in those last days of the settlement were no longer able to defend themselves against pirates who kidnapped the remaining few. 6 CAUSES OF THE MEDIEVAL WARM EPOCH AND THE LITTLE ICE AGE What can cause climate variation over the time scale of centuries? Weather-making counts among the traditional abilities of witches. During the late 14th and 15th centuries the traditional conception of witchcraft was transformed into the idea of a great conspiracy of witches to explain “unnatural” climate phenomena. Extended witch-hunts took place at various peaks of the Little Ice Age because a part of society held the witches directly responsible for the high frequency of climatic anomalies and their impacts. Scientists have suggested other explanations. Volcanic Activity and the Ice Record Volcanoes release ash (dust) into the air that blocks sunlight, thus cooling air temperatures. These particulates, or dust, can also serve as nuclei for cloud formation. Increased cloud cover can also reduce temperatures. However, the impact of both are dependent upon how broadly the dust is dispersed and long it stays in the atmosphere (its residence time). What is more important is the release of sulfur into the atmosphere where, as sulfuric acid it causes back-scatter of incoming solar radiation, thus cooling. It also may encourage cloud formation it is more widely dispersed than dust and has a longer residence time (1-5 yr). Figure 8. A volcano spewing dust into the atmosphere. The history of volcanic eruptions is reflected in the chemistry of ice held in glaciers and ice sheets. Ice Core Records The Greenland Ice Core Project (GRIP) drilled to a depth of 3,028.8 metres in the ice sheet (yes, that’s 3 kilometres of ice). Not to be outdone, a U.S. team, whose project was named GISP (Greenland Ice Sheet Project), drilled about 25 metres deeper at a nearby site the following year. (We will see a video in class of the project, including both teams dancing together in the European’s party room dug out of the glacier.) These projects cost their respective governments millions of dollars, but provide the longest paleo-environmental record (>100,000 years) ever compiled for the northern hemisphere. The ice core record is particularly useful because annual layers of snowfall in ice cores can be counted. Check out the layers in the photograph (figure 9) from an article by Dr. Kendrick Taylor published in the magazine American Scientist. 7 When snow falls, it collects insoluble dust particles, soluble compounds and the water in the snow itself. In some places more snow falls in a year than melts or sublimates away. (Sublimation is when water goes back to the atmosphere directly as a gas from ice.) Annual layers of snow accumulate, with atmospheric gases filling the open pores between snow crystals. The weight of accumulating snow compresses the pores in the snow below, turning the snow into ice and trapping the atmospheric gases. Figure 10 presents a schematic diagram of snow recrystallizing to Figure 9. In this snow pit at Greenland’s summit, light from an uncovered, adjacent snow pit shines through the layers of snow. The thick, dark bands of snow were deposited during the winter; the sequences of thin, lighter bands were deposited during the summer. These visual differences occur because the summer and winter snow have a different density and crystal shape. (Photograph by Kendrick Taylor.) firn, which becomes ice. The dust, chemicals and gases in the ice reflect the environment along the water’s journey from distant sources the glacier. They record how cold it was, how much snow fell in a year, what the concentration of atmospheric gases was and what the atmospheric circulation patterns were. We can identify annual layers in the ice because the concentration of sea salts, nitrate, mineral dust and the gas content in winter snow are different than in summer snow. We count the annual layers to determine the age of the ice. The gas trapped between ice crystals offers a sample of annual changes in the ancient atmosphere. Volcanic activity is observed in the annual layers of the Greenland Ice Sheet as “acidity profiles”, detected through measurements of electrical conductivity (figure 11). Low acidity means few or minor eruptions and high acidity indicates intensive and persistent volcanic activity. Note general correspondence of low volcanic activity to the Medieval Warm Period and high, persistent activity during the Little Ice Age. There is also good correspondence to the lowering of snow caps on mountains. Periods of increased volcanic activity are shown to correspond to decreased width of tree rings (more Figure 10. Diagrammatic depiction of the about these later) in the gradual lithification of the snow which falls on polar ice caps. The snow White Mountains of New recrystallized to firn which has passages Hampshire (USA) and through which air can circulate. When the firn is fully lithified (solid ice), the air decreased temperatures in passages are sealed off becoming central England, as well as bubbles. (From Broecker, 1995) in New Hampshire. 8 Figure 11. Inferred correlation of Greenland Ice Sheet acidity record with evidence of Late Holocene glacier variations in the Northern Hemisphere (after Porter 1986). Solar Radiation Some have attributed the decreased temperatures of the Little Ice Age to a series of cycles in which solar activity was diminished. Variation in solar activity is corresponds to the cycle of sunspots observed – colder periods coincide with periods of low sunspot numbers. During periods of reduced solar activity there is an increased production of the radioactive carbon-14 and beryllium-10 in the atmosphere (more on this in later lectures), as there is less solar magnetic shielding against cosmic rays. In 2000 Crowley examined the variation of carbon-14 and beryllium-10 over the period from the beginning of the supposed Medieval Warm Period to the present. Variation is expressed as net radiative forcing in watts per square metre over the last 2,000 years and is shown in figure 112. The plots of beryllium-10 (blue) and carbon-14, the latter calculated two different ways (shown in red and green) are not exact replicates of each other but do show a similar pattern. The net radiative forcing (solar energy) received from 1100 to 1400 A.D. was generally greater than the period following. This is evidence that changes in solar radiation caused warmer conditions during the Medieval Warm Period. 9 1000 1200 1400 1600 1800 2000 Year A.D. Figure 12. Here Crowley plots two indicators (proxies) of solar energy (expressed as “Net radiative forcing in watts per square metre) over time. The indicators are beryllium (blue) and carbon-14, the latter calculated two different ways (shown in red and green). Although the three records are not exact replicates of each other they show a similar pattern. The net radiative forcing (solar energy) received from 1100 to 1400 A.D. was generally greater than the period following. This is evidence for warmer conditions during the Medieval Warm Period. Crowley also calculated relationships of volcanism (e.g., frequency of eruptions) and solar radiation over time to the paleotemperature estimates derived from proxy data, such as historical records, ice cores, and tree rings. Results vary depending on choice of records and time period analysed. Some analyses show that 49% of the temperature variability can be explained by volcanism and 45% by solar radiation. Thus, both factors played important roles in climate change over the last 1000 years. Note that the two forcings have opposite effects. (More volcanism is cooling, and more solar radiation is warming. What happens when volcanism is decreased? What else must be taken into account?) As Crowley had mathematical functions that represent the relationship between solar radiation and volcanism with temperature he could combine these with his historical reconstructions (of solar radiation and volcanism) to estimate the pattern of past temperature. Figure 13 displays Crowley’s estimates of variability in temperature over the last 1000 years after removal of the effect (the “forcing”) of the variability in solar radiation and volcanism. Random variability (the “chaos” factor) and the greenhouse gas, CO2, are the only things not controlled for (i.e., removed from the calculations) in this graph. The horizontal dotted lines indicate the maximum uncertainty using various estimates. The heavy dashed line pointing up to the right represents the predicted response of temperature to assumed increases in greenhouse gases (without the extra complication of other climatic controls). Crowley concluded that the warming over the past century is unprecedented in the last 1000 years and that greenhouse warming is already here. 10 Figure 13. Estimation of variability in temperature over the last 1,000 years after removal of the effect of the variability in solar radiation and volcanism. Droughts and Collapse of Mayan Society Mayans of the Yucatan Peninsula were also living “on the edge” in a regions where water supplies were limited, thus sensitive to multi-year droughts, as described in your coursepack reading by Peterson and Haug. The series of droughts that contributed to the collapse of the Mayan society on the Yucatan have been tentatively linked to the same type of variation in solar radiation experienced during the Medieval Warm Period and Little Ice Age. Light Links Websites Windows to the Universe: The Little Ice Age www.windows.ucar.edu/tour/link=/earth/climate/little_ice_age.html Past and future of Britain’s wine industry www.winelandsofbritain.co.uk/index.htm The Smithsonian National Museum of Natural History Exhibit “Vikings: the North Atlantic Saga” www.mnh.si.edu/VIKINGS/start.html 11 Newfoundlands’s Viking Trail www.vikingtrail.org Videos The History Channel’s “Little Ice Age: Big Chill” first shown on November 20, 2005. Loaded Links Ice cores www.ngdc.noaa.gov/paleo/icecore/greenland/summit/document/gispinfo.htm Solar radiation NASA’s website on sunspots http://solarscience.msfc.nasa.gov/SunspotCycle.shtml Literary Links: journal articles, magazine articles and books Meehl, G.A. et al. 2009. Amplifying the Pacific Climate System Response to a Small 11-Year Solar Cycle Forcing. Science 325(5944):1114 – 1118. One of the mysteries regarding Earth’s climate system response to variations in solar output Issue 43 No. 1 of the journal Climatic Change is dedicated to the Little Ice Age and Medieval Warm Epoch. It also is available on line through the McGill library. Crowley, T.J. 2000. Causes of Climate Change Over the Past 1000 Years. Science 289(5477): 270-277. Ladurie, E.L. 1971. Times of Feast, Times of Famine. Doubleday, New York. Lamb, H.H. 1965. The early Medieval Warm Epoch and its sequel. Palaeoclimatology, Palaeoeclogy 1:13-37. Palaeogeography, Lamb, H.H. 1977. Climate, History and the Futures, vol. 2. Climate: Present, Past and Future. Methuen and Co. Ltd., London, England. Selley, R.C. 2008. The Winelands of Britain: Past, Present & Prospective, 2nd Edn. Petravin, Surrey, England. 12