1. The Medieval Climate Anomaly and the Little Ice Age
Reconstructions of past Northern Hemisphere temperature. Data sources include satellites, tree rings, ice cores etc. Source: IPCC Temperatures were barely 0.1ºC above the mean in the ‘medieval warm period’ (around 1000AD) – significantly lower than current temperatures

2. The Medieval Climate Anomaly
There is a nice discussion about vineyards in England and the MCA at
The Medieval Warm Period saw warm conditions over a large part of the North Atlantic, Southern Greenland, the Eurasian Arctic, and parts of North America. In these regions, temperature appears to be warmer than the 1961–1990 baseline. In some areas, temperatures were even as warm as today. However, certain regions such as central Eurasia, northwestern North America, and the tropical Pacific are substantially cooler compared to the 1961 to 1990 average.
Peak warmth occurred at different times for different regions, indicating that the Medieval Warm Period was not a time of globally uniform change
Possible causes of the Medieval Warm Period include increased solar activity, decreased volcanic activity, and changes to ocean circulation driven by the El Nino southern oscillation and the North Atlantic Oscillation. IPCC 4th assessment report, WG1, 2007, section 6.6

3. The Vikings Explore…
Colonised Iceland between AD
Farms prosperous in medieval times were engulfed by glaciers by 1700
Erik the Red, Thorvaldsson reached Greenland about 985AD
The settlers experienced above average temperatures for the first crucial years
Eastern Settlement on the south coast, abandoned by 1500
Western settlement near current Nuuk, abandoned around Sea-ice and snow probably played a role.

4. The Vikings Explore…
Leif Eriksson, Erik the Red’s son, arrived in North America in 1000AD.
He establishd Vínland (wineland), Helluland (slab land), Skaeling land (wretch land) and Markland (forest land).
Travel between Greenland and North America would only have been possible for 2 months of the year.
In years of heavy sea-ice, travel would have been perilous.
Conflicts with indigenous people probably led to the abandonment of the American settlements after a few years.

5. The Little Ice Age – When was it?
Three particularly cold intervals: one beginning about 1650, another about 1770, and the last in 1850, each separated by intervals of slight warming
May have been independent events at different places at different times
Peak cooling in Europe was in the 17th Century
Peak cooling in N. America was in 19th Century
Useful references:
Little Ice Age
Michael E Mann
Volume 1, The Earth system: physical and chemical dimensions of global environmental change,
Pp 504–509
Brian Fagan: The Little Ice Age, How Climate made History,

6. The Little Ice Age – Where was it?
Estimated relative temperature variations during the past millennium for different regions
What we think of as the LIA in Europe doesn’t really show up in other places.
What evidence is available suggests generally colder conditions anywhere from the 13th through to the
19th century, quite variable in timing from region to region, and in most cases punctuated with intermittent periods
of warmth. For example, the 17th century, the coldest century in Europe, does not appear to have been
unusually cold in North America. By contrast, during the 19th century, as Europe was recovering from Little Ice Age
conditions, North America was experiencing some of its coldest temperatures. There was increased glaciation in certain
regions of the world outside Europe prior to the 20th century, including Alaska in North America, and New Zealand
and Patagonia in the Southern Hemisphere). However, the precise timing of glacial advances in these regions (and even between the western and eastern Alps) differs considerably from region to region, suggesting the possibility that they represent roughly coincident, but independent regional climate changes, rather than globally synchronous increased glaciation.
Slightly preceding the increased growth of mountain glaciers across Europe during the 17th–19th centuries is evidence of a depression of temperatures in the region (Central England – panel e) relative to modern levels by approximately 0.4 °C during the period 1500–1800 and by more than 0.6 °C during the 17th century period of peak cold. The conclusion that the region exhibited its coldest conditions during the 17th century, and began warming significantly during the 19th century, is independently confirmed from tree ring reconstructions of European summer temperatures

7. The Little Ice Age - Volcanoes
Volcanic forcing of climate over the past 1500 years: An improved ice core-based index for climate models 2008, Chaochao Gao, Alan Robock, and Caspar Ammann
The amount of sulphate aerosol in the stratosphere in the northern hemisphere (top), southern hemisphere(middle) and whole stratosphere (bottom) for the last 1500 years. This is from explosive volcanic eruptions, powerful enough to blast material into the stratosphere.
There was a major volcanic event in the 1200s which may have triggered the start of the Little Ice Age. Together with subsequent volcanoes, by allowing more Arctic sea-ice to form. The ‘year with no summer’ at the end of the LIA was triggered by the eruptions of Tambora, Indonesia, 1815, La Soufriere, Saint Vincent (1812) and Mayon in the Philippines (1814)
Wikipedia suggests:
A recent study found that an especially massive tropical volcanic eruption in 1258, possibly of Mount Rinjani, followed by three smaller eruptions in 1268, 1275, and 1284 did not allow the climate to recover. This may have caused the initial cooling, and the 1452–53 eruption of Kuwae in Vanuatu triggered a second pulse of cooling. The cold summers can be maintained by sea-ice/ocean feedbacks long after volcanic aerosols are removed.
Other volcanoes that erupted during the era and may have contributed to the cooling include Billy Mitchell (ca. 1580), Huaynaputina (1600), Mount Parker (1641), Long Island (Papua New Guinea) (ca. 1660), and Laki (1783). The 1815 eruption of Tambora in Indonesia blanketed the atmosphere with ash; the following year, 1816, came to be known as the Year Without a Summer, when frost and snow were reported in June and July in both New England and Northern Europe.

8. The Little Ice Age – The Sun
See related PowerPoint on MetLink on how the Sun can influence the climate.

9. The Little Ice Age – Carbon Dioxide and Smallpox
As this graph shows, during the Little Ice Age there was less carbon dioxide in the atmosphere.
In part, this is because temperatures were already cooler – as the temperature falls, the amount of carbon dioxide in the atmosphere falls, which in turn causes the temperature to fall further.
However, there was something else going on too – after Christopher Columbus arrived in the Caribbean in 1492, European colonization of South and Central America brought diseases to which the local population had no resistance, such as smallpox. About 95% of indigenous inhabitants died. As a result, areas which had been deforested grew forests once more. The regrowth of Neotropical forests following the Columbian encounter
led to terrestrial biospheric carbon sequestration on the order of 2 to 5 Pg C, thereby contributing to the well documented
decrease in atmospheric CO2 recorded in Antarctic ice cores from about 1500 through 1750.
The amount of carbon dioxide in the air, as measured from air bubbles trapped in ice at the Law Dome in Antarctica.

10.
Many European glaciers advanced at times during the Little Ice Age.
Evidence from mountain glaciers does suggest increased glaciation in a number of widely spread regions outside Europe prior to the twentieth century, including Alaska, New Zealand and Patagonia. However, the timing of maximum glacial advances in these regions differs considerably, suggesting that they may represent largely independent regional climate changes, not a globally-synchronous increased glaciation. Thus current evidence does not support globally synchronous periods of anomalous cold or warmth over this interval, and the conventional terms of "Little Ice Age" and "Medieval Warm Period" appear to have limited utility in describing trends in hemispheric or global mean temperature changes in past centuries... [Viewed] hemispherically, the "Little Ice Age" can only be considered as a modest cooling of the Northern Hemisphere during this period of less than 1°C relative to late twentieth century levels
IPCC, 2001

11. The Little Ice Age – Conclusions
Changes in the Sun and volcanic activity, coupled with changes in the North Atlantic Oscillation (the pressure difference between Iceland and the Azores), the ocean circulation in the North Atlantic, the north Atlantic storm track and feedbacks involving the quantity of polar ice and the natural release of greenhouse gases combined to give the Little Ice Age, at least in Europe and at best in the northern Hemisphere.
It is also possible that changes in the numbers of people – through, for example, the Black Death, may have had an impact on agriculture and forestation, which in turn may have had an impact on the climate.