The Medieval Climate Anomaly: A Myth Debunked
The Medieval Climate Anomaly: A Myth Debunked
Christopher J. Mignano
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THE MEDIEVAL CLIMATE ANOMALY: A MYTH DEBUNKED
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The Medieval Climate Anomaly: A Myth Debunked
The evidence is overwhelming; the 21st century Earth is warmer than the Earth of the
medieval period. The Medieval Climate Anomaly refers to a period of time that stretched from
the end of the 10th century to the end of the 15th. This period was marked by regional climate
instability most notably in Europe, but also in various parts of the Northern and Southern
Hemispheres (Kitchen, 2014). The Medieval Climate Anomaly is also known as the “Medieval
Warm Period” and came to an end with the arrival of the “Little Ice Age” (Kitchen, 2014). As
the name implies, the Medieval Warm Period was marked by alternating periods of warming that
was regional and transient in nature (Kitchen, 2014). During this period a warming climate in
the northern hemisphere allowed populations there to grow and expand into new lands
previously uninhabitable as was the case with Viking expansion into places such as Iceland,
Greenland and even North America (Skeptical Science, 2014). In sharp contrast, the Little Ice
Age was a period of cooler temperatures dated between the 15th and 17th centuries (Kitchen,
2014). This Little Ice Age bore witness to expanding glaciers and contracting human populations
brought on by shrinking arable land, famine, disease and war (Kitchen, 2014). The focus of this
work will be the Medieval Warm Period.
The Myth
Global Climate change skeptics erroneously point to data regarding the Medieval Warm
Period as evidence that the current warming observed by scientists globally is part of a natural,
cyclical climate phenomenon and therefore not anthropic. These sceptics often use a practice
known as “cherry picking” to mine data that support their arguments while ignoring the
remaining data that could potentially refute it (Mandia, 2014). The climate change skeptic’s
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invalid argument often goes; the Medieval Warm Period was warmer than today; historical
records and modern proxy data prove this. Since we know it to be true that medieval period
humans lacked SUVs and large scale fossil fuels based industry and agriculture, it must follow
that the warming observed in the Middle Ages was natural and not human caused therefore the
warming we observe today must also be non-anthropic.
At the very core of all science is the scientific method. The scientific method demands
that every hypothesis be tested rigorously to determine its merit and anthropogenic climate
change is no exception. Although the evidence is overwhelming, there does still exist a small
cadre of scientists who question human caused global climate change (Mandia, 2014). As is the
case in all aspects of scientific discourse, scholarly debate and careful investigation should be
both encouraged and welcomed. However, the motives of many skeptics who hang their hats on
the myth that the Medieval Climate Anomaly was warmer than today are disingenuous,
fundamentally incorrect and motivated more by their world view than any data which refutes it
(Mania 2014). Acknowledging human activity as the driver of modern climate change places the
burden of action squarely on the shoulders of people living today and their direct descendants. If
this is true, then it stands to reason that our current generation and subsequent generations must
bear the costs of said actions which many find to be inconvenient or in direct conflict with their
sociopolitical ideology, particularly adherents of the hierarchal-individualist worldview (Mandia,
2014).
In the following paragraphs I will debunk the myth that the world was warmer during the
Medieval Warm Period. I will briefly describe the modern consensus of the Medieval Warm
Period’s causes; namely higher than average solar activity (sunspots), decreased volcanic
activity, and changes in the North Atlantic Oscillation (Skeptical Science, 2014). Finally, I will
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make the case that in fact, global temperatures are warmer now than at any time during the
Medieval Climate Anomaly or the last 20,000 yrs. and that the current warming is 100%
attributed to anthropogenic greenhouse gases.
The Data
At the core of many false claims to non-anthropic causes of climate change are facts and
figures cherry picked from proxy data such as ice cores, tree rings and coral as well as
documentation from the Medieval Climate Anomaly period, such as it is (Skeptical Science,
2014). These data do indeed show temperatures anomalies in certain and disparate locations that
meet or even exceed the high temperatures witnessed in Eurasia and North America today.
Proven, fundamental science provides us with an understanding of the primary causes of the
Medieval Warm Period. If the investigation into the Medieval Warm Period was a criminal one,
the primary suspect would be the Medieval Solar maximum. The Medieval Solar maximum is
defined as a period of increased solar activity, measured via proxies such as the levels of the
cosmogenic isotope Carbon-14 found in ice cores dated from the middle ages (Kitchen, 2014).
In ice cores, reduced measurement of Carbon-14 isotopes reflect increased solar activity
(Kitchen, 2014). Increased solar radiation interacts with the Earth’s magnetic field and
atmosphere where complex chemistry, (beyond the scope of this work) takes place to reduce
levels of Carbon-14 (iceandclimate, 2014). In addition, a slight decrease in volcanic activity
during the period and a subsequent decrease in albedo due to the absence of light reflecting
aerosols could have served as a positive feedback, increasing warming (Kitchen, 2014). Finally,
there is evidence that the North Atlantic Oscillation may have seen increased activity; this would
have had the effect of bringing increased amounts of warmer water to the North Atlantic thereby
THE MEDIEVAL CLIMATE ANOMALY: A MYTH DEBUNKED
warming the regional climate (Mandia, 2014). Climate sensitivity is the expected increase in
global temperature when CO2 is doubled from 280 ppm to 560 ppm and is likely to be between
2°C and 4.5°C (Mandia, 2014). If these small variation in natural phenomena were indeed the
cause of the medieval warm period, it stands to reason that the imbalance caused by
anthropogenic greenhouse gases will result in even greater climate instability; climate skeptics
can not have it both ways (Mandia, 2014).
A Warmer World
The final blow to the climate skeptic’s argument is the fact that the planet as a whole is
warmer now than at any time during the Medieval Warm Period (Skeptical Science, 2014). The
following graphs, obtained from the Skeptical Science 2014 web site show reconstructed global
temperatures from the Medieval Warm Period against that of today’s temperatures.
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These graphs plainly illustrate that today’s warming is global in scope, (whereas the warming
experienced during the Medieval Warm Period was regional) and global temperatures then, were
cooler than today. In fact, recent research conducted by Shaun A. Marcott et al and published in
the journal Science suggest global temperatures are warmer today than at any time in the last
20,000 years (Skeptical Science, 2014).
This graph further illustrates that point:
The data is irrefutable: the planet is warmer today than at any time during the Medieval Warm
Period or the last 20,000 yrs. and anthropogenic greenhouse gases are to blame.
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The Guilty Party
Once we eliminate solar activity (sunspots) and understand that Earth’s current orbital
position as related to Milankovitch Cycles, (changes in obliquity, eccentricity and precession that
govern glaciation and interglacial cycles by affecting the level of solar insolation received at the
top of the atmosphere (Kitchen, 2014), fosters cooling not warming; the only suspects left are
greenhouse gases, especially CO2. Since the start of the Industrial Revolution, c.1760 global
temperatures have risen by 1°C (Mandia, 2014). This rise in temperature corresponds with the
rise of CO2 concentrations in the atmosphere. Pre-industrial CO2 levels have been measured at
approximately 280 parts per million or ppm (Mandia, 2014). Today’s measurements hover
around 400 ppm and some estimates project CO2 levels at an astounding 900 ppm by the end of
the century unless measures are taken to reduce current and future CO 2 emissions (Mandia,
2014). An imbalance in the atmosphere due to radiative forcing, (which is defined as the
difference between incoming and outgoing radiation) exists as a result of anthropogenic
greenhouse gases (Kitchen, 2014). CO2, along with other greenhouse gases such as methane and
nitrous oxide to name a few, prevent the earth from losing its excess heat by trapping infrared
light or thermal energy from escaping the atmosphere but do nothing to stop incoming light
(Mandia, 2014).
The Verdict
In conclusion, to bring our investigative analogy to completion we must suppose the case
had been brought to trial. Our main suspects: anthropogenic greenhouse gases, would now be
made to stand as the judge reads the verdict. You, anthropogenic CO2 and your coconspirators
NO2 and CH4 have been found guilty of creating an energy imbalance that has warmed the planet
more than at any time during the Medieval Warm Period or at any time in the past tens of
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thousands of years. By carefully reviewing the evidence gathered meticulously from tree rings,
ice cores and coral samples using fundamental, proven science as well as written records from
the time, we have determined that an increase in sunspot activity along with a decrease in
volcanic activity and possibly a change in the North Atlantic Oscillation are responsible for the
warming experienced during the Medieval Warm Period. Furthermore, since these conditions do
not exist today we find the only other plausible suspects, CO2, CH4 and NO2 must be the cause of
the warming experienced today. One might imagine that the jury would sentence the offending
gases to 1000 yrs. sequestration or some other similar draconian punishment but that alone
would not solve the problem. After all, it is human activity that is ultimately responsible for
100% of the warming experienced today and as entertaining as the court room analogy may be it
does not strike at the heart of who is truly guilty; humanity. As many experts of both the
physical and social sciences have noted; we are now living in the Anthropocene; an era that is
now and will continue to be characterized by human activity above all else. In order to return the
planet’s energy budget to balance, human activities must be curtailed. Human societies must
learn to do more with less or drastically reduce population (the methods by which this could ever
be achieved in time sufficient enough to have a measurable impact most would find morally
reprehensible). Societies the world over must redirect funding away from fossil fuels and into
renewables like wind, solar, tidal and even nuclear. Local communities must rethink and rework
the ways in which they use energy and the methods by which that energy is distributed.
Individuals must also make changes in their everyday lives to reduce their carbon footprint and
better utilize precious energy resources, particularly in the developed world.
As a student of history I am reminded of a period of Roman history the ranged from AD
235 to AD 284 known as the Crisis of the 3rd Century and I wonder if future historians will look
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back at our time and regard it as the Crisis of the 21st Century. While the causes of those crisis
times were not primarily climatic in origin, (there is some evidence it played a role), the
challenges they faced then may well prove to be similar to those humanity will face over the next
hundred years unless action is taken now to mitigate the impact of climate change in the future.
War, famine and disease as well as widespread political, social and economic instability all
conspired to end the Roman Empire in the Third Century, but for the rise of Diocletian and
others of his ilk who were committed to making the hard choices and changes necessary to save
their cherished way of life, (however repugnant a modern reader may have found it). Similarly,
short of a global change in consciousness, it will take strong leadership, not by emperors, tyrants
or dictators guided by mysticism and magical thinking but by democratically elected officials
and modern governments who employ logic and science to reach conclusions. It will require
science educators and science communicators with the tenacity to inform the masses, not with
what they want to hear or what will best help them sleep at night, but with that which they need
to know, namely the facts, guided by evidence and reason.
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References
Mandia, Scott. (2014). MET 103 Global Climate Change, [PDF document]. Retrieved from
Lecture Notes Online Web site: http://www2.sunysuffolk.edu/mandias/
Kitchen, David. E. (2014). Global Climate Change; Turning Knowledge Into Action. University
of Richmond: Pearson.
Tamblyn, G., Sweet, M., Blackburn, A., et al., (November 2014). How does the Medieval Warm
Period compare to current global temperatures? Retrieved from
www.skepticalscience.com/medieval-warm-period.htm
University of Copenhagen, Centre for Ice and Climate, (November 2014). Retrieved from
www.iceandclimate.nbi.ku.dk/research/past_atmos/cosmogenic_isotopes