30
An Introduction to the Holocene
and Anthropic Disturbance
Pascale Braconnot and Pascal Yiou
The Major Trends of the Holocene
The Different Radiative Disturbances
The Holocene started about 10,000 years ago at the end of
the last glaciation. The last thousand years of this period is
marked by the growing impact of human activity (changes in
land use and atmospheric composition). At first sight, the
numerous data show us variations that are less spectacular
than the great upheavals engendered by deglaciation. Nevertheless, the general natural trend, driven by changes in
solar radiation at the top of the atmosphere, is characterized
by radical changes in the monsoon and the El Niño phenomenon in the tropics. In the mid-latitudes of the northern
hemisphere, the changes are seen in the characteristics of the
main modes of variability. Several abrupt events also
punctuate the unfolding of the process.
Slow variations in solar radiation at the top of the
atmosphere, caused by variations in orbital parameters, are
the driving force behind the evolution of the main climate
characteristics on this 10,000-year scale. Variation in the
obliquity, from 24.23° at the beginning of the Holocene to
23.44° at the present time, has brought about an increase in
the average annual solar radiation at high latitudes of
1.5 W/m
2 over this period. During the same period, solar
radiation in the low latitudes dropped by 1.10 W/m
2 . Added
to this effect of obliquity is the precession of the equinoxes.
At the beginning of the Holocene, the summer solstice was
located at the perihelion of the ecliptic. It is now at the
aphelion. Thus, the solar radiation received at the top of the
atmosphere (insolation) in June was 48 W/m
2 higher at
60° N at the beginning of the Holocene than it is today
(Fig. 30.1), and 5 W/m
2 higher about 3000 years ago. Also,
the amplitude of the seasonal cycle of insolation at the
beginning of the Holocene was greater in the northern
hemisphere but less in the southern hemisphere. This variation in amplitude is not symmetrical on either side of the
equator, with greater variations in the northern hemisphere
and in the tropics (Fig. 30.1). The precession also alters the
length of the seasons. According to Kepler’s laws, the boreal
summer, defined as the time between the spring and autumn
equinoxes, lasted 172 days 9500 years and 176 days
6000 years ago with 180 days currently. In the northern
hemisphere, summer insolation was therefore more intense
over a shorter period.
Greenhouse gases, volcanism and the solar constant are
other factors that have influenced the climate of the Holocene through their impact on the radiative balance
(Fig. 30.2). These are also the dominant factors of the last
2000 years, ever since insolation has reached approximate
current values. Indeed, the combined effect of these greenhouse gases has resulted in a reduction in the radiative
balance of the planet of around 0.5 W/m
2 from the beginning of the Holocene to the beginning of the industrial era.
This slight perturbation of the radiative balance is due to a
7 ppm increase in atmospheric carbon content at the
beginning of the Holocene, followed by a decrease of
20 ppm up to the beginning of the industrial era. Methane
levels decreased from 730 ppb at the beginning of the
Holocene to 580 ppb in the Middle Holocene (6000 years
ago), and gradually returned to early Holocene levels in the
pre-industrial era. Levels of atmospheric N 2 O follow the
variations of CO 2 and have varied between 2 and 10 ppb. In
more recent times, the evolution of greenhouse gases is
dominated by anthropic emissions. The combined effects of
human activity correspond to an increase of 1.6 W/m
2 in
the radiative balance. This estimate takes into account the
dominant effect of the increase in greenhouse gases and the
negative contribution of aerosols for the twentieth century
(Solomon et al. 2007; IPCC 2013).
P. Braconnot (&) Á P. Yiou
Laboratoire des Sciences du Climat et de l’Environnement
LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay,
91191 Gif-sur-Yvette, France
e-mail: pascale.braconnot@lsce.ipsl.fr
© Springer Nature Switzerland AG 2021
G. Ramstein et al. (eds.), Paleoclimatology, Frontiers in Earth Sciences,
https://doi.org/10.1007/978-3-030-24982-3_30
423
An Introduction to the Holocene
and Anthropic Disturbance
Pascale Braconnot and Pascal Yiou
The Major Trends of the Holocene
The Different Radiative Disturbances
The Holocene started about 10,000 years ago at the end of
the last glaciation. The last thousand years of this period is
marked by the growing impact of human activity (changes in
land use and atmospheric composition). At first sight, the
numerous data show us variations that are less spectacular
than the great upheavals engendered by deglaciation. Nevertheless, the general natural trend, driven by changes in
solar radiation at the top of the atmosphere, is characterized
by radical changes in the monsoon and the El Niño phenomenon in the tropics. In the mid-latitudes of the northern
hemisphere, the changes are seen in the characteristics of the
main modes of variability. Several abrupt events also
punctuate the unfolding of the process.
Slow variations in solar radiation at the top of the
atmosphere, caused by variations in orbital parameters, are
the driving force behind the evolution of the main climate
characteristics on this 10,000-year scale. Variation in the
obliquity, from 24.23° at the beginning of the Holocene to
23.44° at the present time, has brought about an increase in
the average annual solar radiation at high latitudes of
1.5 W/m
2 over this period. During the same period, solar
radiation in the low latitudes dropped by 1.10 W/m
2 . Added
to this effect of obliquity is the precession of the equinoxes.
At the beginning of the Holocene, the summer solstice was
located at the perihelion of the ecliptic. It is now at the
aphelion. Thus, the solar radiation received at the top of the
atmosphere (insolation) in June was 48 W/m
2 higher at
60° N at the beginning of the Holocene than it is today
(Fig. 30.1), and 5 W/m
2 higher about 3000 years ago. Also,
the amplitude of the seasonal cycle of insolation at the
beginning of the Holocene was greater in the northern
hemisphere but less in the southern hemisphere. This variation in amplitude is not symmetrical on either side of the
equator, with greater variations in the northern hemisphere
and in the tropics (Fig. 30.1). The precession also alters the
length of the seasons. According to Kepler’s laws, the boreal
summer, defined as the time between the spring and autumn
equinoxes, lasted 172 days 9500 years and 176 days
6000 years ago with 180 days currently. In the northern
hemisphere, summer insolation was therefore more intense
over a shorter period.
Greenhouse gases, volcanism and the solar constant are
other factors that have influenced the climate of the Holocene through their impact on the radiative balance
(Fig. 30.2). These are also the dominant factors of the last
2000 years, ever since insolation has reached approximate
current values. Indeed, the combined effect of these greenhouse gases has resulted in a reduction in the radiative
balance of the planet of around 0.5 W/m
2 from the beginning of the Holocene to the beginning of the industrial era.
This slight perturbation of the radiative balance is due to a
7 ppm increase in atmospheric carbon content at the
beginning of the Holocene, followed by a decrease of
20 ppm up to the beginning of the industrial era. Methane
levels decreased from 730 ppb at the beginning of the
Holocene to 580 ppb in the Middle Holocene (6000 years
ago), and gradually returned to early Holocene levels in the
pre-industrial era. Levels of atmospheric N 2 O follow the
variations of CO 2 and have varied between 2 and 10 ppb. In
more recent times, the evolution of greenhouse gases is
dominated by anthropic emissions. The combined effects of
human activity correspond to an increase of 1.6 W/m
2 in
the radiative balance. This estimate takes into account the
dominant effect of the increase in greenhouse gases and the
negative contribution of aerosols for the twentieth century
(Solomon et al. 2007; IPCC 2013).
P. Braconnot (&) Á P. Yiou
Laboratoire des Sciences du Climat et de l’Environnement
LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay,
91191 Gif-sur-Yvette, France
e-mail: pascale.braconnot@lsce.ipsl.fr
© Springer Nature Switzerland AG 2021
G. Ramstein et al. (eds.), Paleoclimatology, Frontiers in Earth Sciences,
https://doi.org/10.1007/978-3-030-24982-3_30
423