THE MYTH OF EUROPE’S LITTLE ICE AGE
Eugeniy TISHCHENKO,
assistant of economics and finance chair,
Institute of Economics, Management and Law
National Research Irkutsk State Technical University
The Little Ice Age is generally seen as a major event in European history.
Analysing a variety of recent weather reconstructions, this column finds that
European weather appears constant from the Middle Ages until 1900, and that
events like the freezing of the Thames and the disappearance of English
vineyards have simpler explanations than changing climate. It appears instead
that the European Little Ice Age is a statistical artefact, where the standard
climatological practice of smoothing what turn out to be white noise data prior
to analysis gives the spurious appearance of irregular oscillation – a Slutsky
Effect.
The Little Ice Age – dated from the mid-14th century to the early 19th –
plays a large role in historical analyses. Anthropologist Brian Fagan suggests
that this climate swing demoralised the European peasantry, allowing for the rise
of despotic leaders (Fagan 2000). British and Dutch canals and rivers frequently
froze, allowing French land forces to invade the Netherlands while the Dutch
fleet was ice-locked. But was it really an ice age, or have historians been “fooled
by randomness” (Taleb 2005)?
In recent research we attempt to discover how much European weather
actually worsened during the Little Ice Age. Our conclusion – using a variety of
standard temperature reconstructions – is that there is little evidence that a
European Little Ice Age ever occurred (Kelly and Ó Gráda 2014).
Figure 1. The European Little Ice Age as Slutsky Effect: Netherlands
summer temperature, 1301-1980. The top panel shows the data smoothed with a
twenty-five year moving average and gives the appearance of marked cold
episodes between the 15th and 19th centuries. The lower panel shows the actual
data which are almost entirely random with a constant mean and variance.
Instead, European weather between the 14th and 19th centuries resembles
white noise: uncorrelated draws from a distribution with a constant mean and
variance (although there are occasional decades of markedly lower summer
temperature), with the same behaviour holding more tentatively back to the
eleventh century. Our results suggest that the existing consensus over a Little Ice
Age in Europe is a statistical artefact, where the standard climatological practice
of smoothing what turn out to be random data prior to analysis gives the
spurious appearance of irregular oscillations. This is an example of the ‘Slutsky
effect’ where filtering of purely random variations can produce spurious
cycles.[1]
Climate cycles in history
The idea that human history is driven by fluctuations in climate is a
tenacious one, with the collapse of most civilizations from Rome to Maya
having been attributed at some time or another to the diminishing resources and
increasing conflict caused by weather. While the implicit view of humanity as
passive victims of meteorological circumstance – preferring starvation to
adaptation as conditions change – led most mainstream historians to be sceptical
of such stories, the growing popularity of ‘big history’ has led to a revival of
climatic determinism, but the emphasis is decidedly on anecdote over formal
analysis (see McNeill 2014 for an enthusiastic overview).
Thanks to concerns about global warming, an increasing number of annual
winter and summer temperature reconstructions now exist:
 for Central Europe since 1500,
 for the Netherlands from 1201,
 for Switzerland from 1525, and
 for England from 1660.
These reconstructions offer the potential to assess the size and timing of
major swings in European climate since the Middle Ages – assuming, of course,
that they are more than figments of their authors’ imaginations. To assess the
reliability of the earliest (and presumably most conjectural) reconstructions we
compare the Netherlands estimates with known English wheat prices from 1211
to the beginning of the price revolution in 1500 and find a close match.
Given the widely-held belief that Europe experienced a Little Ice Age, the
statistical behaviour of these temperature reconstructions comes as a surprise. In
every case the mean and variance of the series remain almost constant until
around 1900 when temperatures start to rise steadily (particularly winter
temperatures). In no case do we find evidence of autocorrelation, trends, or
structural breaks in mean temperature. Most importantly, an array of powerful
martingale difference tests (portmanteau tests that look at the sum of
autoregressive coefficients in the data, variance ratio tests that look at how the
variance of a series grows as the number of observations rises, and spectral tests
that look for departures from a straight line spectrum) do not reject the null that
mean temperatures from the late middle ages to the late 19th century are
constant.
Given the lack of evidence for any sustained trends or shifts in European
temperatures before, 1900 how are we to account for the general belief among
climatologists that Europe experienced a Little Ice Age? The answer probably
lies in the practise in climatology of smoothing data using some sort of moving
average before analysing it. If the underlying data are white noise, as European
temperatures appear to be, then smoothing them will give the spurious
appearance of irregular oscillations as the filter is distorted by runs of high or
low values: a Slutsky effect. We illustrate this in Figure 1.
Creating an ice age with 25 year moving averages
The top panel of Figure 1 shows Low Countries’ summer temperatures from
1301, smoothed using a twenty-five year moving average. The Little Ice Age is
immediately evident as a declining trend from the mid 15th to the early 19th
centuries, with markedly cold episodes in the late 16th, late 17th, and early 19th
centuries. But looking at the unsmoothed graph of summer temperatures in the
panel below, the impression is one of randomness, without any major trends,
cycles, or breaks: something that our statistical tests confirm.
Glaciers can be seen as a physical embodiment of the Slutsky effect: their
extent represents a moving average process of temperature and precipitation
over preceding years, and can show considerable variation through time even
though the annual weather processes that drive them are random. For example,
while annual Swiss winter temperatures and precipitation are close to random
until the late 19th century, Swiss glaciers fluctuate notably, expanding from the
mid-15th century until 1650, contracting until 1750, and then expanding again
until 1850.
A word of caution
Note when interpreting our results that local weather conditions are a noisy
signal of global ones. Just as the fact that European temperatures did not rise
much during the twentieth century does not imply that global temperatures were
constant (rises were concentrated in the Arctic and Siberia), so their constancy
between the 14th and 19th centuries does not imply that global average
temperatures did not experience notable falls during this time associated
primarily with known falls in solar output.
While sustained shifts in climate do not appear to have occurred in northern
Europe since the late middle ages, and more tentatively (using German data that
categorizes each year as good, average, or bad) since AD 1000, there have been
decades of notably poor weather. Using a Bayesian changepoint procedure –
which is robust to runs of outliers that distort most filters and lead to Slutsky
effects – we can identify in Figure 2 decades of noticeably poor summer weather
– in particular the 1690s when harvests failed repeatedly across Europe, and the
1810s. None of the European series, or recent Northern Hemisphere temperature
reconstructions for that matter, shows evidence of any decline in the period
1635-1665 which the excitable but influential study of Parker (2013) claims to
have been a period of “catastrophic” weather.
Figure 2. Historical series of central European, Netherlands, Swiss, and
English summer temperatures, showing mean estimated by a Bayesian change
point procedure.
The anecdotal evidence
In Kelly and Ó Gráda (2013) we examine some of the anecdotal evidence
used to support claims of a European Little Ice Age.
 The freezing of the Thames – which for most people is the most
salient fact about the Little Ice Age – was caused by Old London Bridge,
whose twenty arches effectively acted as a dam, creating a large pool of
still water that froze twelve times between 1660 and 1815.
Tidal stretches of the river have not frozen since the bridge was replaced
with five-arched one in 1831 – even during 1963, which was the third coldest
winter since records begin in 1660 (after 1684 and 1740).
 Grape growing in England was uncommon in the eleventh century –
a lot of confusion stems from mistaking the word vivarium (fish pond) for
vinarium (vineyard) – and disappeared entirely after Bordeaux passed to
the English crown in 1152, suggesting that comparative advantage may
have played a larger role than climate.
Similarly, the decline of wheat and rye cultivation in Norway from the 13th
century may owe more to lower German cereal prices than temperature change.
 Regarding Greenland’s Viking settlements, competition for
resources with the indigenous Inuit, the decline of Norwegian trade in the
face of an increasingly powerful German Hanseatic League, the greater
availability of African ivory as a cheaper substitute for walrus ivory,
overgrazing, plague, and marauding pirates probably all played some role
in their demise.
Most importantly, the fact that the colony had survived previous episodes of
cold weather points to the insufficiency of the standard explanation that the
Vikings there simply ‘got cold and died’.
Conclusion
While the idea that Europe experienced a Little Ice Age is widespread, its
statistical basis is at best exiguous, and appears to stem from inappropriate
efforts to smooth data that are actually random. At the same time, most of the
anecdotal evidence admits more simple explanations than climate change
BIBLIOGRAPHY
1. Fagan, B (2000), The Little Ice Age: How Climate Made History, 1300–
1850, New York: Basic Books.
2. Kelly, M and C Ó Gráda, (2013), “The Waning of the Little Ice Age”,
Journal of Interdisciplinary History 44(2): 301—325.
3. Kelly, M and C Ó Gráda (2014), “Change Points and Temporal
Dependence in Reconstructions of Annual Temperature: Did Europe
Experience a Little Ice Age?”, Annals of Applied Statistics 8(3):1372—
1394.
4. Mahon, J and P Davies (2009), “The Meaning of Slutsky”,
https://www.minneapolisfed.org/publications/the-region/the-meaning-ofslutsky.
5. McNeill, J R (2014), Changing Climates of History, Public Books.
6. Parker, G N (2013), Global Crisis: War, Climate Change and Catastrophe
in the Seventeenth Century, New Haven: Yale University Press.
7. Taleb, N N (2005), Fooled by Randomness: The Hidden Role of Chance
in Life and in the Markets, Random House.