Thursday 8 June 2023

What do we Know about the Medieval Warm Period?

I have written a great deal about the Little Ice Age (LIA) that lasted from about 1300 to 1850. It’s an important period for family historians as it encompasses most of the centuries during which a substantial number of records were created that assist us in identifying our ancestors. Prior to those years, documents that describe people are in short supply, especially regarding recording of their surnames upon which we base much of our family research.

In discussions about my articles and presentations, I get asked many questions concerning climate change: Where are we going? How warm could it get? What causes climate change? etc. The short answer to all of these is: No one really knows!

There are lots of theories, of course. But if we cannot predict weather more than a few days or weeks in advance, how can we predict climate change many decades in the future. Perhaps one way of understanding the current Modern Warm Period, and to put it into long-term perspective, is to look at what happened many centuries in the past, when previous warm climatic periods occurred.

Are areas around the world warmer now that they were during the 18th century? Yes, they are. Is warming proceeding still? Very possibly it is. Is the rate of such warming increasing? There the jury is still out as warming has been going on since at least the middle of the 17th century at a relatively constant rate. While there have been decades when temperatures rose at seemingly alarming rates, there were also years when it dropped significantly as well.

We can all accept that the world today is warmer than it was. What many people do not realize, though, is that it is not the first climatic period during which the Earth was very warm. Within just the last 10,000 years of human history, there have been at least six major warm periods, each period lasting several hundred years (Shepheard, 2018). These alternated with colder climatic periods.

Average near-surface temperatures of the Northern Hemisphere during the last 11,000 years based on analyses of the changes in 18O isotopes from Greenland ice cores; diagram demonstrates the alternating periods of warm (red) and cold (blue) that occurred throughout the Holocene Epoch; after Dansgaard et al, 1969

What is interesting in looking at the last 100 centuries is that there is a strong parallel between the warm and cold periods and the rise and demise of civilizations. During most of the warm periods average temperatures were high – estimated from scientific and archaeological studies to be much higher than today – and societies of those times flourished. During each of the cold periods: climate cooled, harvests often failed, famine was common and pressure on societal structures increased.

In the end major civilizations each collapsed, probably not coincidentally, as climate conditions worsened.

Of more particular note, we can learn about the effects of climate change and the present environmental conditions by reviewing what went on during the last warm phase, the Medieval Warm Period (MWP) which lasted from about 900 to 1300.

Overall, data is still too sparse and unevenly measured to conclude what temperatures were like during the past two millennia on a global or even hemispheric scale. Even for the relatively short period of the last 700 years, after the first instruments were invented to measure it, different parts of the Earth have shown different trends or anomalies, although the LIA is readily observed in all regions. The Northern Hemisphere shows a relatively steady temperature increase from the coldest part of the LIA in the late 17th century.

Composite temperature anomaly series (ºC) for North America, Europe, China and all the Northern Hemisphere, with reference to the period 1860-1959 (Modern Warm Period); source Bradley & Jones, 1993

Of necessity, scientists have had to use proxies to infer relative heat and cold for centuries before instrumental measurements were possible. Such elements include changes in oxygen isotopes derived from ice core samples, tree rings, fossil vegetation, corals, pollen, dinoflagellate remains, deep sea or lake sedimentary deposits and others.

Geological studies show how sea level fluctuated during the past several millennia. One recent article (Balco et al, 2023) concluded that the Antarctic ice sheet was likely 115 feet thinner thousand years ago and, through successive cold periods – the LIA being the last – thickened to the extent it is today. Over recent decades various studies have shown Antarctic ice sheets have occasionally shrunk in some regions but simultaneously expanded in others with the result that, overall, the ice shelves may now be larger than they were many years ago (Andreasen et al, 2023).

In a 2010 study which involved temperature reconstruction of the last two millennia, researcher Fredrik Ljungquist concluded that “[t]he highest average temperatures in the reconstruction are encountered in the mid to late tenth century and the lowest in the late seventeenth century.” These results are consistent with many other studies of climate although most have not used or compared the number of proxies of this study.

Estimations of extra-tropical Northern Hemisphere (90–30°N) decadal mean temperature variations (black line) AD 1–1999 relative to the 1961–1990 mean instrumental temperature from the variance adjusted CRUTEM3+HadSST2 90–30°N record with two standard deviation error bars (blue shading); source Ljungquist, 2010

Conditions of the MWP, particularly in Europe, featured higher overall temperatures and stable weather patterns over many decades. That resulted in expansion of arable land and population growth. Polar ice was rarely seen in the North Atlantic in contrast to during the LIA when ice packs often surrounding Iceland. Mountain passes in mountain ranges around the world were mostly free of ice, allowing frequent contact between communities and greater trade interactions. By 1300 almost all of Europe was part of an integrated and vibrant, international trading system doing business around the world.

Drought and famine were not unknown especially in those parts of the world where temperatures were very warm, and precipitation was very low. For example, Central and Western North America experienced several mega-droughts during the 10th to 13th century. Many indigenous societies disappeared or were forced to migrate away from homelands they had occupied for hundreds of years.

There were certainly droughts during the LIA, however, these were mostly resulted from lower precipitation levels over extended periods, not from excessive heat and evaporation. What rain did occur in many regions was often not sufficient to recharge aquifers or raise groundwater levels.

Left: Drought Area Index (DAI; percent area covered by PDSI < -1 each year) over time in the West as reconstructed by tree rings, both annual in pale brown and 60-year low-pass filtered in black. The dashed blue curves are two-tailed 95% confidence intervals for the latter. The red and blue lines are mean DAI for the MCA (ca. AD 900–1300) and the 20th century out to 2003, respectively. This record shows that the MCA (ca. AD 900–1300) was much more arid on average than the 20th century; source Cook et al, 2009; Right: gridded summer Palmer Drought Severity Index (PDSI), reconstructed for period 1146–1155; source Woodhouse et al, 2009

As the MWP came to a close during the 13th century major storms became more frequent, pummeling European coastlines in particular. These destructive events continued into the LIA period beginning in the 14th century.

Rising sea level, resulting from melting glaciers and polar ice during the MWP, had led to building of dikes and land drainage in the Low Countries of northern Flanders, Belgium, the Netherlands and Luxembourg to protect farming operations developed during the preceding decades. These would come under major attack by storms in the lead up to and early decades of the LIA.

The MWP was a time when societies advanced, population expanded, food production grew, and commercial ventures developed worldwide. Families became more prosperous and life spans increased. While records are sparser than in subsequent centuries, this is the time when genealogical studies can commence.

In the analysis of any area in which one’s ancestors lived it may be valuable to research what the climatic conditions were through scientific and historical studies. Going back decades before existing family records will also show conditions of the environment that perhaps attracted or kept families in the areas in which future generations lived.



Drought Atlases:

·         Old World –

·         North American –

·         Latin American and Caribbean –

·         Eastern Australia and New Zealand –

Andreasen, Julia R., Anna E. Hogg & Heather L. Selley. (2023). Change in Antarctic ice shelf area from 2009 to 2019. The Cryosphere, 17 pp. 2059-2072.

Balco, Greg, Nathan Brown, Keir Nichols, Ryan A. Venturelli, Jonathan Adams, Scott Braddock, Seth Campbell, Brent Goehring, Joanne S. Johnson, Dylan H. Rood, Klaus Wilcken, Brenda Hall & John Woodward. (2023). Reversible ice sheet thinning in the Amundsen Sea Embayment during the Late Holocene. The Cryosphere, 17 pp. 1787-1801.

Bradley, Raymond S. & Philip D. Jones. (1993). Little Ice Age summer temperature variations: their nature and relevance to recent global warming trends. The Holocene, 3(4), pp. 367-376.

Cook, Edward R., Richard Seager, Richard R. Heim Jr., Russell S. Vose, Celine Herweijer & Connie Woodhouse. (2009). Megadroughts in North America: placing IPCC projections of hydroclimatic change in a long-term palaeoclimate context. Journal of Quaternary Science, 25, pp. 48-61. file:///F:/Downloads/2009_Cook_IPCC_paleo-drought%20(2).pdf

Dansgaard, W., Johnsen, S. J., Moller, J. (1969). One thousand centuries of climatic record from Camp Century on the Greenland Ice Sheet. Science, 166(3903), pp. 377-381.

Lungqvist, F. C. (2010). A new reconstruction of temperature variability in the extra-tropcial Northern Hemisphere during the last two millennia. Geografiska Annaler, Series A92, pp. 339-351.

Le Roy Ladurie, E. (1967). Histoire du Climat depuis l'an mil. Flammarion, Paris. (translated by B. Bray as Times of Feast, Times of Famine: A History of Climate Since the Year 1000, Doubleday and Co., 1971).

Shepheard, W. Wayne. (2018). Surviving Mother Nature’s Tests: The effects climate change and other natural phenomena have had on the lives of our ancestors. St. Agnes, South Australia: Unlock the Past, 179 pp.

Woodhouse, Connie A., David M. Meko, Glen M. MacDonald, Dave W. Stahle & Edward R. Cook. (2010). A 1,200-year perspective of 21st century drought in southwestern North America. Proceedings of the National Academy of Sciences (PNAS),