by the large-scale atmospheric circulation, shaped the geographical variability in dust cycle dynamics. The sharp
decline in dust deposition to Antarctica paralleling the rise in
atmospheric CO 2 levels, led oceanographer John Martin
(“Give me a tanker of iron, and I will give you an ice age”),
in the late eighties, to formulate the famous iron hypothesis:
the glacial increase in dust-borne iron inputs to the Southern
Ocean may have stimulated the ocean biological pump,
resulting in an increased productivity in HNLC areas and
subsequent carbon sequestration in the deep ocean (Jickells
et al. 2005). State of the art ESMs suggest that this mechanism may have been responsible for a decrease of *20
ppmv of atmospheric CO 2 , out of 80/100 ppmv—the overall
decrease in CO 2 concentrations during the LGM measured in
ice cores. On the other hand, the impact of the increased dust
burden on atmospheric radiation is estimated by models to
be at least −1 W/m
2 globally, compared to −6 W/m
2 forcing
by reduced GHGs atmospheric concentrations and changes
in surface albedo from decreased sea levels and the growth
of ice sheets in the Northern Hemisphere. Nonetheless, this
figure might hide much larger dust impacts of the opposite
sign, with strong cooling downwind of the major dust
sources, and warming over the bright, glaciated Arctic
regions (Albani et al. 2018).
During the course of the entire glacial period, millennial
scale variability expressed by Dansgaard-Oeschger and
Heinrich events in the d
18 O record in Greenland ice cores, is
almost paralleled by variations in dust deposition rates, with
colder phases associated to dustier conditions (Rasmussen
et al. 2014). The same picture emerges from the corresponding alternation of stadial/interstadial periods in
Antarctica (EPICA Community Members 2006).
The deglaciation followed, characterized by a nonmonotonic increase in global temperatures and CO 2 , mirrored by decreasing dust levels in both hemispheres. While
the Holocene was initially described as a relatively flat
period in terms of dust, based on polar records, in the last
three decades, a few studies of North Atlantic sediment cores
highlighted the large variability in North African dust
emissions; a reduction by a factor 2–5 corresponded to the
“Green Sahara” phase of the Early and mid-Holocene,
characterized by an enhanced summer monsoon, compared
to the drier late Holocene after *5 ka BP. The possible
(positive or negative) feedbacks between the monsoon system and variations in the North African dust cycle are a
subject of study by the modelling community (Albani et al.
2015).
Ironically, we know relatively less about the more recent
past. Marine sediments and loess profiles generally can’t
achieve a temporal resolution fine enough to resolve the last
millennia, and often surface layers of loess/paleosol deposits
are disturbed by agricultural practices, as they tend to be
very fertile soils. Ice cores do provide this kind of temporal
resolution, but we still have very little data. The top meters
of polar cores, corresponding to this time frame, are actually
made of firn, which complicates the analysis because of the
risk of contamination of the samples. Alpine ice cores, on
other hand, allow the analysis of dust concentration and can
have good chronologies for the last few decades, but reliable, quantitative estimates of dust mass accumulation rates
are hampered by the extreme spatial variability of snow
accumulation and post-depositional processes. Therefore, we
do not have a clear pre-industrial reference state for dust
(Carslaw et al. 2010). A few studies trying to address dust
trends during the twentieth century yield contrasting results;
a generally increasing trend over parts of the last century
suggested by some authors may have “masked” a fraction of
global temperature increase, due to the net cooling effect of
dust.
Other Aerosol Species in Past Climates
There is much less information about other aerosol species in
the past; unlike dust, solubility and volatility limit the
preservation of most species in many environmental settings,
and pose additional analytical challenges. Most of the
information we have is from ice cores (Wolff et al. 2006;
Preunkert and Legrand 2013).
Na
+ is a stable proxy for sea salts. Yet, we do not really
have information from lower latitudes; sea salt records from
polar ice cores (Fig. 23.18) show, similar to dust, increased
deposition rates in glacial climates, by a factor 3–5 in
Antarctica and 1.5–3 in Greenland. It is not fully clear to
what extent this was due to an increase in emissions or
transport from open waters, rather than to the expanded sea
ice source, although the latter seems to be the dominant
factor. Increases in sea salts could have had a negative
forcing, via direct and indirect effects, on the atmospheric
radiation budget.
Sulphur and nitrogen aerosol species present more
important issues with preservation, and are more difficult to
interpret. The baseline sulphate records from polar ice cores
(Fig. 23.18), mainly derived from DMS emissions, show a
flat signal in Antarctica and some variability in Greenland.
This has dampened some early enthusiasm for the idea that
DMS could be a dominant feedback in driving
glacial-interglacial variability, given the strong cooling
effects associated with sulphate aerosols. Sulphate concentration spikes are associated with inputs from large volcanic
eruptions, and are sometimes associated with the presence of
tephra. In certain cases, a lag of one or two years was
observed in the peaking of the two signals, suggesting a
longer stratospheric residence time of sulphates. There are
294
N. Bouttes et al.
decline in dust deposition to Antarctica paralleling the rise in
atmospheric CO 2 levels, led oceanographer John Martin
(“Give me a tanker of iron, and I will give you an ice age”),
in the late eighties, to formulate the famous iron hypothesis:
the glacial increase in dust-borne iron inputs to the Southern
Ocean may have stimulated the ocean biological pump,
resulting in an increased productivity in HNLC areas and
subsequent carbon sequestration in the deep ocean (Jickells
et al. 2005). State of the art ESMs suggest that this mechanism may have been responsible for a decrease of *20
ppmv of atmospheric CO 2 , out of 80/100 ppmv—the overall
decrease in CO 2 concentrations during the LGM measured in
ice cores. On the other hand, the impact of the increased dust
burden on atmospheric radiation is estimated by models to
be at least −1 W/m
2 globally, compared to −6 W/m
2 forcing
by reduced GHGs atmospheric concentrations and changes
in surface albedo from decreased sea levels and the growth
of ice sheets in the Northern Hemisphere. Nonetheless, this
figure might hide much larger dust impacts of the opposite
sign, with strong cooling downwind of the major dust
sources, and warming over the bright, glaciated Arctic
regions (Albani et al. 2018).
During the course of the entire glacial period, millennial
scale variability expressed by Dansgaard-Oeschger and
Heinrich events in the d
18 O record in Greenland ice cores, is
almost paralleled by variations in dust deposition rates, with
colder phases associated to dustier conditions (Rasmussen
et al. 2014). The same picture emerges from the corresponding alternation of stadial/interstadial periods in
Antarctica (EPICA Community Members 2006).
The deglaciation followed, characterized by a nonmonotonic increase in global temperatures and CO 2 , mirrored by decreasing dust levels in both hemispheres. While
the Holocene was initially described as a relatively flat
period in terms of dust, based on polar records, in the last
three decades, a few studies of North Atlantic sediment cores
highlighted the large variability in North African dust
emissions; a reduction by a factor 2–5 corresponded to the
“Green Sahara” phase of the Early and mid-Holocene,
characterized by an enhanced summer monsoon, compared
to the drier late Holocene after *5 ka BP. The possible
(positive or negative) feedbacks between the monsoon system and variations in the North African dust cycle are a
subject of study by the modelling community (Albani et al.
2015).
Ironically, we know relatively less about the more recent
past. Marine sediments and loess profiles generally can’t
achieve a temporal resolution fine enough to resolve the last
millennia, and often surface layers of loess/paleosol deposits
are disturbed by agricultural practices, as they tend to be
very fertile soils. Ice cores do provide this kind of temporal
resolution, but we still have very little data. The top meters
of polar cores, corresponding to this time frame, are actually
made of firn, which complicates the analysis because of the
risk of contamination of the samples. Alpine ice cores, on
other hand, allow the analysis of dust concentration and can
have good chronologies for the last few decades, but reliable, quantitative estimates of dust mass accumulation rates
are hampered by the extreme spatial variability of snow
accumulation and post-depositional processes. Therefore, we
do not have a clear pre-industrial reference state for dust
(Carslaw et al. 2010). A few studies trying to address dust
trends during the twentieth century yield contrasting results;
a generally increasing trend over parts of the last century
suggested by some authors may have “masked” a fraction of
global temperature increase, due to the net cooling effect of
dust.
Other Aerosol Species in Past Climates
There is much less information about other aerosol species in
the past; unlike dust, solubility and volatility limit the
preservation of most species in many environmental settings,
and pose additional analytical challenges. Most of the
information we have is from ice cores (Wolff et al. 2006;
Preunkert and Legrand 2013).
Na
+ is a stable proxy for sea salts. Yet, we do not really
have information from lower latitudes; sea salt records from
polar ice cores (Fig. 23.18) show, similar to dust, increased
deposition rates in glacial climates, by a factor 3–5 in
Antarctica and 1.5–3 in Greenland. It is not fully clear to
what extent this was due to an increase in emissions or
transport from open waters, rather than to the expanded sea
ice source, although the latter seems to be the dominant
factor. Increases in sea salts could have had a negative
forcing, via direct and indirect effects, on the atmospheric
radiation budget.
Sulphur and nitrogen aerosol species present more
important issues with preservation, and are more difficult to
interpret. The baseline sulphate records from polar ice cores
(Fig. 23.18), mainly derived from DMS emissions, show a
flat signal in Antarctica and some variability in Greenland.
This has dampened some early enthusiasm for the idea that
DMS could be a dominant feedback in driving
glacial-interglacial variability, given the strong cooling
effects associated with sulphate aerosols. Sulphate concentration spikes are associated with inputs from large volcanic
eruptions, and are sometimes associated with the presence of
tephra. In certain cases, a lag of one or two years was
observed in the peaking of the two signals, suggesting a
longer stratospheric residence time of sulphates. There are
294
N. Bouttes et al.