grey areas concerning this period. Nevertheless, as insolation
seasonality varied gradually, with reduced insolation during
boreal summer, over the course of the Holocene, the rapid
establishment of these dry conditions are the result of nonlinear interactions between climates and ecosystems.
Medieval Warm Period
Through the study of historical archives, the British meteorologist Lamb reconstructed in the 1960s, the severity of the
winters and the humidity of the summers since the year 1000
(Le Roy Ladurie 1967). He concluded that Europe experienced an escalation of dry summers during the period 1080–
1200, unparalleled since then. He named this period the
Medieval Warm Period. It is important to note that there are
no direct temperature measurements for this time, since the
thermometer did not yet exist. More recent studies (Jones
and Mann 2004) suggest that Lamb’s conclusions cannot be
generalized to the entire planet.
From more extensive datasets, several studies have confirmed that many regions of the northern hemisphere experienced warmer conditions during the eleventh and twelfth
centuries. However, several regions do not present signs of
having experienced this optimum, or else did at a different
time in history. For example, it appears that even though the
summers were warmer at that time, winters were often very
harsh in Western Europe until 1170. These apparent contradictions, probably due to a small number of observations,
may be completed by new climate reconstruction programs
in the future. Meanwhile, it should not be ruled out that this
Medieval Warm Period was purely a regional phenomenon.
It is also not a direct analogue to the current warming, as it
was not associated with an increase in the atmospheric
content of greenhouse gases.
The Little Ice Age
The Medieval Warm Period was interrupted at the beginning
of the fifteenth century by the arrival of the Little Ice Age,
which took hold until 1850. Again, there is no precise date
for the beginning of this period: the Scandinavians felt a
cooling at the end of the nineteenth century, whereas in
France, the degradation of the weather was felt around 1430
(Bradley 1999; Le Roy Ladurie 1967). During this Little Ice
Age in France there was a growth of alpine glaciers, very
harsh winters and a succession of terrible summers. For
example, the Bossons glacier, near Chamonix, descended
1000 m lower than presently. Though possibly influenced by
the artistic trends of the time, the study of old engravings and
paintings shows a striking difference with the present period.
The strongest cooling was undoubtedly in northern Europe,
with lakes being systematically frozen over in winter.
There are also signs of cooling in Equatorial America and
New Zealand (Bradley 1999). This relatively cold period (on
average, 1 °C cooler than currently in France) was responsible for the loss of crops and spikes in wheat prices, and
often for insurrections, as well as an increase in the number
of witches burned alive in public squares (Le Roy Ladurie
1967). After 1850, the glaciers began to retreat and the
temperature to rise steadily, marking the end of the Little Ice
Age.
The Little Ice Age is marked by a few particularly cold
decades in Europe between 1650 and 1720. A decrease in
the number of sunspots was observed at this time. These
decades are called the Maunder Minimum, after the American astronomer Edward W. Maunder (1851–1928), who
studied the relationship between sunspots and sun activity. It
should be noted that it was the German astronomer
Friederich W. G. Spörer (1822–1895) who first noticed the
decrease in the number of sunspots between 1650 and 1720.
History bestowed the Spörer Minimum between 1420 and
1570 on him. The variation in temperature caused by the
direct influence of solar activity on the radiative balance at
the Earth’s surface (0.5 W/m
2 ) is less than 0.08 °C for the
northern hemisphere (Jansen 2007) and is not sufficient to
explain a cooling of between 0.2 and 0.5 °C in the northern
hemisphere.
The Little Ice Age was also marked by intense volcanic
eruptions that affected temperatures in a visible but not very
durable manner (Fig. 30.4). Eruptions having a global effect
generally took place in the tropics (Tambora, Krakatoa,
Agung, Pinatubo etc.), and injected enough dust into the
stratosphere to be homogeneously distributed globally
(Robock 2000). Extratropical volcanoes (e.g., Laki in Iceland in 1783, St Helens in the United States in 1980) have
had significant effects regionally in the northern hemisphere
due to dust transport in the troposphere. However, their
impact on a global scale was minor, because the volcanic
dust they emitted did not reach the stratosphere.
The Anthropocene Era
The Anthropocene is a recently devised term that refers to
the period when the climate and the environment became
influenced by human activity. It is accepted that this period
started at the beginning of the industrial era (in the middle of
the nineteenth century), which also coincides with the first
functioning weather service networks. It is possible, however, to argue that man began to impact on the climate from
the middle of the Holocene. Land-clearing, the start of
farming and slash-and-burn cultivation have emitted enough
30 An Introduction to the Holocene and Anthropic Disturbance
427
seasonality varied gradually, with reduced insolation during
boreal summer, over the course of the Holocene, the rapid
establishment of these dry conditions are the result of nonlinear interactions between climates and ecosystems.
Medieval Warm Period
Through the study of historical archives, the British meteorologist Lamb reconstructed in the 1960s, the severity of the
winters and the humidity of the summers since the year 1000
(Le Roy Ladurie 1967). He concluded that Europe experienced an escalation of dry summers during the period 1080–
1200, unparalleled since then. He named this period the
Medieval Warm Period. It is important to note that there are
no direct temperature measurements for this time, since the
thermometer did not yet exist. More recent studies (Jones
and Mann 2004) suggest that Lamb’s conclusions cannot be
generalized to the entire planet.
From more extensive datasets, several studies have confirmed that many regions of the northern hemisphere experienced warmer conditions during the eleventh and twelfth
centuries. However, several regions do not present signs of
having experienced this optimum, or else did at a different
time in history. For example, it appears that even though the
summers were warmer at that time, winters were often very
harsh in Western Europe until 1170. These apparent contradictions, probably due to a small number of observations,
may be completed by new climate reconstruction programs
in the future. Meanwhile, it should not be ruled out that this
Medieval Warm Period was purely a regional phenomenon.
It is also not a direct analogue to the current warming, as it
was not associated with an increase in the atmospheric
content of greenhouse gases.
The Little Ice Age
The Medieval Warm Period was interrupted at the beginning
of the fifteenth century by the arrival of the Little Ice Age,
which took hold until 1850. Again, there is no precise date
for the beginning of this period: the Scandinavians felt a
cooling at the end of the nineteenth century, whereas in
France, the degradation of the weather was felt around 1430
(Bradley 1999; Le Roy Ladurie 1967). During this Little Ice
Age in France there was a growth of alpine glaciers, very
harsh winters and a succession of terrible summers. For
example, the Bossons glacier, near Chamonix, descended
1000 m lower than presently. Though possibly influenced by
the artistic trends of the time, the study of old engravings and
paintings shows a striking difference with the present period.
The strongest cooling was undoubtedly in northern Europe,
with lakes being systematically frozen over in winter.
There are also signs of cooling in Equatorial America and
New Zealand (Bradley 1999). This relatively cold period (on
average, 1 °C cooler than currently in France) was responsible for the loss of crops and spikes in wheat prices, and
often for insurrections, as well as an increase in the number
of witches burned alive in public squares (Le Roy Ladurie
1967). After 1850, the glaciers began to retreat and the
temperature to rise steadily, marking the end of the Little Ice
Age.
The Little Ice Age is marked by a few particularly cold
decades in Europe between 1650 and 1720. A decrease in
the number of sunspots was observed at this time. These
decades are called the Maunder Minimum, after the American astronomer Edward W. Maunder (1851–1928), who
studied the relationship between sunspots and sun activity. It
should be noted that it was the German astronomer
Friederich W. G. Spörer (1822–1895) who first noticed the
decrease in the number of sunspots between 1650 and 1720.
History bestowed the Spörer Minimum between 1420 and
1570 on him. The variation in temperature caused by the
direct influence of solar activity on the radiative balance at
the Earth’s surface (0.5 W/m
2 ) is less than 0.08 °C for the
northern hemisphere (Jansen 2007) and is not sufficient to
explain a cooling of between 0.2 and 0.5 °C in the northern
hemisphere.
The Little Ice Age was also marked by intense volcanic
eruptions that affected temperatures in a visible but not very
durable manner (Fig. 30.4). Eruptions having a global effect
generally took place in the tropics (Tambora, Krakatoa,
Agung, Pinatubo etc.), and injected enough dust into the
stratosphere to be homogeneously distributed globally
(Robock 2000). Extratropical volcanoes (e.g., Laki in Iceland in 1783, St Helens in the United States in 1980) have
had significant effects regionally in the northern hemisphere
due to dust transport in the troposphere. However, their
impact on a global scale was minor, because the volcanic
dust they emitted did not reach the stratosphere.
The Anthropocene Era
The Anthropocene is a recently devised term that refers to
the period when the climate and the environment became
influenced by human activity. It is accepted that this period
started at the beginning of the industrial era (in the middle of
the nineteenth century), which also coincides with the first
functioning weather service networks. It is possible, however, to argue that man began to impact on the climate from
the middle of the Holocene. Land-clearing, the start of
farming and slash-and-burn cultivation have emitted enough
30 An Introduction to the Holocene and Anthropic Disturbance
427