and presented armadas of icebergs linked to the HS1 event
(see Chap. 29). Following this event, around 14 ka, northern
hemisphere warming was strongly amplified with a culmination during the Bølling-Allerød, while temperatures in the
southern hemisphere stabilized and even dropped in
Antarctica during the so-called Antarctic Cold Reversal
(Fig. 21.12). During this early phase of the deglaciation, the
North-South antiphase is similar to what is observed during
abrupt events of the last ice age, with the exception of the
general deglacial warming trend. Recent studies point to
CO 2 as a key mechanism of global warming during the last
deglaciation. An anti-phased hemispheric temperature
response to ocean circulation changes, superimposed on
globally in-phase warming driven by increased CO 2 concentrations, is an explanation for much of the temperature
change at the end of the most recent ice age (Barker et al.
2009; Shakun et al. 2012).
However, at the end of the Bølling-Allerød warm event,
at about 12.5 ka, the ice caps stopped melting, the sea level
stabilized and the deglaciation stopped: this was the
Younger Dryas period (Fig. 21.12), characterized by a return
to very cold conditions for about 1.5 kyr, despite insolation
reaching maximum values. This return to almost ice age
conditions still raises many questions. The most commonly
accepted explanation is a sudden change in the path taken by
meltwater from the Laurentide ice sheet (Leydet et al. 2018).
Until about 12.5 ka, this huge flow of water was transported
by the Mississippi River. Released into the Gulf of Mexico,
the fresh water was drawn in by the circulation of the surface
currents of the Atlantic (Gulf Stream followed by the North
Atlantic Drift) and was very gradually diluted by the salty
tropical waters without any major climate impact. During the
Younger Dryas, however, the flow rate of the Mississippi
River dropped considerably, which led to the hypothesis that
the watershed of the meltwater plume changed and flowed
instead through the St. Lawrence River to the northwest of
the Atlantic Ocean. The salinity in this higher latitude area
was reduced, interrupting deep water formation and thus the
thermohaline circulation, and causing cooling and the
growth of some glaciers. This hypothesis has been supported
by simple ocean circulation models, although marine sediment cores recovered from the likely North Atlantic zone of
evacuation of meltwater have, as of yet, failed to yield traces
of this event. Recent study suggests multiple causes of the
Younger Dryas cold period: a weakened Atlantic Meridional
Overturning Circulation, moderate negative radioactive
forcing and an altered atmospheric circulation (Renssen et al.
2015). The detailed study of this event could help us to
better understand the interactions between ocean, ice and
atmosphere under conditions of strong insolation.
Interglacial Periods, the Holocene and the Last
Two Millennia
In order to explain the succession of glacial and interglacials
periods over the last million years, Milankovitch developed
the astronomical theory of paleoclimate. Since then, conceptual models have been able to describe the general trends,
as well as the dominant periodicities centered around 100, 40
and 20 kyr fairly accurately (see Chap. 28). Although
changes in ice cap volume during glacial periods and the
time constants of their response to changes in insolation are
relatively well understood, the same cannot be said for the
evolution of climate during interglacial periods. In particular,
the mechanisms causing the differences in duration, in
temperature of the atmosphere and ocean, and in ocean
circulation are not well understood, even though differences
in forcing are precisely calculated (Past InterGlacialS
Working Group of PAGES 2016). This lack of understanding is derived in part from the small ocean temperature
differences between past interglacial periods and the present
day, with temperature changes that remain close to the error
of temperature reconstructions with the usual tracers
(Sect. “Sea Surface Temperature”). A further complication
arises because the internal mechanisms in the climate system
must be investigated through its various components (atmosphere, ocean, continent), which involves the construction of time scales common to the various archives used to
reconstruct each of them, and makes the study of the interglacial periods prior to the Holocene particularly difficult.
In this section, we limit ourselves to the analysis of the
last two interglacial periods: the Last Interglacial (also called
the Eemian), about 125 ka ago, and the Holocene, period in
which we now live. Eemian and Holocene, the terms used in
this chapter, are names borrowed from palynologists to
identify these two interglacial periods. A short subsection
will finally be devoted to results recently obtained for the last
two millennia, which has the advantage of presenting a wide
range of continental and marine records that can, in some
cases, be compared with recorded meteorological data.
The Last Interglacial Period
Before presenting our understanding of this period of time, it
is important to define what an interglacial is. It may in fact
be defined in a number of ways depending on whether one
considers, for example, variability in flora, ocean circulation,
atmospheric temperature or ocean temperature (Past
InterGlacialS Working Group of PAGES 2016). If we take
ice volume as a marker, an interglacial period sensu stricto is
the time interval during which the ice volume is at its
minimum and remains constant for several millennia.
246
T. Caley et al.
(see Chap. 29). Following this event, around 14 ka, northern
hemisphere warming was strongly amplified with a culmination during the Bølling-Allerød, while temperatures in the
southern hemisphere stabilized and even dropped in
Antarctica during the so-called Antarctic Cold Reversal
(Fig. 21.12). During this early phase of the deglaciation, the
North-South antiphase is similar to what is observed during
abrupt events of the last ice age, with the exception of the
general deglacial warming trend. Recent studies point to
CO 2 as a key mechanism of global warming during the last
deglaciation. An anti-phased hemispheric temperature
response to ocean circulation changes, superimposed on
globally in-phase warming driven by increased CO 2 concentrations, is an explanation for much of the temperature
change at the end of the most recent ice age (Barker et al.
2009; Shakun et al. 2012).
However, at the end of the Bølling-Allerød warm event,
at about 12.5 ka, the ice caps stopped melting, the sea level
stabilized and the deglaciation stopped: this was the
Younger Dryas period (Fig. 21.12), characterized by a return
to very cold conditions for about 1.5 kyr, despite insolation
reaching maximum values. This return to almost ice age
conditions still raises many questions. The most commonly
accepted explanation is a sudden change in the path taken by
meltwater from the Laurentide ice sheet (Leydet et al. 2018).
Until about 12.5 ka, this huge flow of water was transported
by the Mississippi River. Released into the Gulf of Mexico,
the fresh water was drawn in by the circulation of the surface
currents of the Atlantic (Gulf Stream followed by the North
Atlantic Drift) and was very gradually diluted by the salty
tropical waters without any major climate impact. During the
Younger Dryas, however, the flow rate of the Mississippi
River dropped considerably, which led to the hypothesis that
the watershed of the meltwater plume changed and flowed
instead through the St. Lawrence River to the northwest of
the Atlantic Ocean. The salinity in this higher latitude area
was reduced, interrupting deep water formation and thus the
thermohaline circulation, and causing cooling and the
growth of some glaciers. This hypothesis has been supported
by simple ocean circulation models, although marine sediment cores recovered from the likely North Atlantic zone of
evacuation of meltwater have, as of yet, failed to yield traces
of this event. Recent study suggests multiple causes of the
Younger Dryas cold period: a weakened Atlantic Meridional
Overturning Circulation, moderate negative radioactive
forcing and an altered atmospheric circulation (Renssen et al.
2015). The detailed study of this event could help us to
better understand the interactions between ocean, ice and
atmosphere under conditions of strong insolation.
Interglacial Periods, the Holocene and the Last
Two Millennia
In order to explain the succession of glacial and interglacials
periods over the last million years, Milankovitch developed
the astronomical theory of paleoclimate. Since then, conceptual models have been able to describe the general trends,
as well as the dominant periodicities centered around 100, 40
and 20 kyr fairly accurately (see Chap. 28). Although
changes in ice cap volume during glacial periods and the
time constants of their response to changes in insolation are
relatively well understood, the same cannot be said for the
evolution of climate during interglacial periods. In particular,
the mechanisms causing the differences in duration, in
temperature of the atmosphere and ocean, and in ocean
circulation are not well understood, even though differences
in forcing are precisely calculated (Past InterGlacialS
Working Group of PAGES 2016). This lack of understanding is derived in part from the small ocean temperature
differences between past interglacial periods and the present
day, with temperature changes that remain close to the error
of temperature reconstructions with the usual tracers
(Sect. “Sea Surface Temperature”). A further complication
arises because the internal mechanisms in the climate system
must be investigated through its various components (atmosphere, ocean, continent), which involves the construction of time scales common to the various archives used to
reconstruct each of them, and makes the study of the interglacial periods prior to the Holocene particularly difficult.
In this section, we limit ourselves to the analysis of the
last two interglacial periods: the Last Interglacial (also called
the Eemian), about 125 ka ago, and the Holocene, period in
which we now live. Eemian and Holocene, the terms used in
this chapter, are names borrowed from palynologists to
identify these two interglacial periods. A short subsection
will finally be devoted to results recently obtained for the last
two millennia, which has the advantage of presenting a wide
range of continental and marine records that can, in some
cases, be compared with recorded meteorological data.
The Last Interglacial Period
Before presenting our understanding of this period of time, it
is important to define what an interglacial is. It may in fact
be defined in a number of ways depending on whether one
considers, for example, variability in flora, ocean circulation,
atmospheric temperature or ocean temperature (Past
InterGlacialS Working Group of PAGES 2016). If we take
ice volume as a marker, an interglacial period sensu stricto is
the time interval during which the ice volume is at its
minimum and remains constant for several millennia.
246
T. Caley et al.