Download Medieval Warm Epoch, Little Ice Age

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
ƒ 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!
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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).
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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)
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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
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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,
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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.
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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.
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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.
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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.
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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.
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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
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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.
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