from ice can past variability in marine biogenic emissions
potentially be reconstructed, since organisms like Phaeocystis do not leave any traces in marine sediments. The issue
is important because it contributes to our understanding of
the feedback taking place in high latitudes (regions highly
sensitive to global changes in climate), involving complex
processes between marine biology, sea ice, and climate.
Aerosols and Dust
Aerosols are small liquid or solid particles, ranging from a
few nm to 100 µm, in suspension in the atmosphere. Natural
aerosols include desert dust, sea salt, carbonaceous, sulphur
and nitrogen species, largely emitted from dry and vegetated
landscapes, the oceans, and volcanoes (Carslaw et al. 2010).
Primary aerosols are emitted directly from the surface of the
Earth, whereas secondary aerosols are formed from gaseous
precursors in the atmospheric environment. Aerosols are
washed out by precipitation, or removed by gravitational
settling and dry deposition, so that their lifetime in the
atmosphere is short, a few days only—except when they
reach the stratosphere, where they can stay for a few years,
as can happen during giant volcanic eruptions. Therefore,
unlike well-mixed GHGs, aerosols are considered to be
short-lived climate forcing agents, and their impacts are
characterized by a strong regional component (Boucher et al.
2013). Aerosol emissions vary depending on surface climate
conditions; on the other hand, aerosols impact the climate
system themselves, though direct and indirect (cloudmediated) interactions with the atmospheric radiation budget, by changing the surface albedo, as well as by means of
indirect impacts on global biogeochemical cycles (Mahowald et al. 2017).
Size, shape and composition define the specific interactions of aerosols with radiation, including absorption and
scattering of shortwave (solar) and longwave (terrestrial)
radiation. The scattering of shortwave radiation results in
cooling, but absorption can lead to warming when it is above
a highly reflective surface. Through their absorption of
outgoing longwave radiation, aerosols also behave like
GHGs. For some aerosol species, one particular effect is
dominant, in other cases, opposing effects coexist. Considering the variability of aerosol spatial distribution, the
coexistence and mixing of different aerosol species, and the
diversity of aerosol-radiation interactions, it is clear that
direct impacts of aerosols on climate constitute a complex
problem (Boucher et al. 2013).
Aerosols also interact with clouds. Changes in relative
humidity linked to the vertical stability of the atmospheric
column and the surface evapotranspiration balance, occurring as a rapid adjustment to direct aerosol forcing, can
influence cloud formation. This is called the semi-direct
effect. The indirect effects, on the other hand, involve
aerosols acting as cloud condensation (CCN) or ice nuclei
(IN), which means that water or ice aggregates around them.
This modifies the type, extent and lifetime of clouds. For
example, the presence of aerosols leads to smaller but more
numerous droplets, which yields a more reflective cloud than it
would be without aerosols. This also means that clouds formed
with aerosols will have a longer lifetime since the droplets are
smaller and won’t reach the critical size for precipitation. The
indirect effect can result in warming or cooling depending on
the altitude where clouds are formed. Because the effect of
aerosols on radiation and on clouds is complex and depends on
many parameters, it remains one of the main sources of
uncertainties in models (Boucher et al. 2013).
In addition, aerosols depositing back to the surface can
also modify the albedo. This is the case of dust, and especially black carbon; they can cause snow and ice to darken,
which reduces the albedo and leads to warming.
In virtue of their composition, aerosols also act as carriers
of specific elements, such as nitrogen, phosphorus, sulphur,
and iron, which are linked to important biogeochemical
cycles, including the carbon cycle (Mahowald et al. 2017).
In particular, phosphorus and iron are linked to the dust
cycle, and the peculiarity is that windblown inputs can be
fundamental to the mass budgets of those elements in remote
regions, far from the dust sources. For instance, dust-borne
phosphorus from North Africa replenishes the pool of this
element in the Amazon, where the loss by fluvial erosion
would otherwise deplete it, with implications for the rainforest. Iron, on the other hand, is a micronutrient for marine
ecosystems. Because its sources are the continents, remote
marine areas are depleted in this element. Where the abundance of macronutrients such as nitrogen and phosphorus is
accompanied by a relative scarcity of iron, which limits the
primary production at the ecosystem level, i.e. in
High-Nutrient Low-Chlorophyll (HNLC) areas, dust-borne
inputs of iron become of great importance in sustaining algal
blooms—this is notably the case of the Southern Ocean
(Jickells et al. 2005).
Natural Aerosols: Overview
In this section we will briefly describe the main natural
aerosol types. Note that mineral dust and sea salt are still the
most abundant primary aerosol species by mass in the present day atmosphere.
Mineral (desert) dust (Fig. 23.17) is emitted into the
atmosphere in response to wind erosion of the surface, in dry
and semi-dry areas, with low vegetation cover. Far-travelled
dust particles are mostly clays and fine silts below 10 µm in
diameter, and are composed mainly of silicates, along with
carbonates, gypsum, and metal oxides. Dust aerosols interact
23 Biogeochemical Cycles and Aerosols Over the Last Million Years
291
potentially be reconstructed, since organisms like Phaeocystis do not leave any traces in marine sediments. The issue
is important because it contributes to our understanding of
the feedback taking place in high latitudes (regions highly
sensitive to global changes in climate), involving complex
processes between marine biology, sea ice, and climate.
Aerosols and Dust
Aerosols are small liquid or solid particles, ranging from a
few nm to 100 µm, in suspension in the atmosphere. Natural
aerosols include desert dust, sea salt, carbonaceous, sulphur
and nitrogen species, largely emitted from dry and vegetated
landscapes, the oceans, and volcanoes (Carslaw et al. 2010).
Primary aerosols are emitted directly from the surface of the
Earth, whereas secondary aerosols are formed from gaseous
precursors in the atmospheric environment. Aerosols are
washed out by precipitation, or removed by gravitational
settling and dry deposition, so that their lifetime in the
atmosphere is short, a few days only—except when they
reach the stratosphere, where they can stay for a few years,
as can happen during giant volcanic eruptions. Therefore,
unlike well-mixed GHGs, aerosols are considered to be
short-lived climate forcing agents, and their impacts are
characterized by a strong regional component (Boucher et al.
2013). Aerosol emissions vary depending on surface climate
conditions; on the other hand, aerosols impact the climate
system themselves, though direct and indirect (cloudmediated) interactions with the atmospheric radiation budget, by changing the surface albedo, as well as by means of
indirect impacts on global biogeochemical cycles (Mahowald et al. 2017).
Size, shape and composition define the specific interactions of aerosols with radiation, including absorption and
scattering of shortwave (solar) and longwave (terrestrial)
radiation. The scattering of shortwave radiation results in
cooling, but absorption can lead to warming when it is above
a highly reflective surface. Through their absorption of
outgoing longwave radiation, aerosols also behave like
GHGs. For some aerosol species, one particular effect is
dominant, in other cases, opposing effects coexist. Considering the variability of aerosol spatial distribution, the
coexistence and mixing of different aerosol species, and the
diversity of aerosol-radiation interactions, it is clear that
direct impacts of aerosols on climate constitute a complex
problem (Boucher et al. 2013).
Aerosols also interact with clouds. Changes in relative
humidity linked to the vertical stability of the atmospheric
column and the surface evapotranspiration balance, occurring as a rapid adjustment to direct aerosol forcing, can
influence cloud formation. This is called the semi-direct
effect. The indirect effects, on the other hand, involve
aerosols acting as cloud condensation (CCN) or ice nuclei
(IN), which means that water or ice aggregates around them.
This modifies the type, extent and lifetime of clouds. For
example, the presence of aerosols leads to smaller but more
numerous droplets, which yields a more reflective cloud than it
would be without aerosols. This also means that clouds formed
with aerosols will have a longer lifetime since the droplets are
smaller and won’t reach the critical size for precipitation. The
indirect effect can result in warming or cooling depending on
the altitude where clouds are formed. Because the effect of
aerosols on radiation and on clouds is complex and depends on
many parameters, it remains one of the main sources of
uncertainties in models (Boucher et al. 2013).
In addition, aerosols depositing back to the surface can
also modify the albedo. This is the case of dust, and especially black carbon; they can cause snow and ice to darken,
which reduces the albedo and leads to warming.
In virtue of their composition, aerosols also act as carriers
of specific elements, such as nitrogen, phosphorus, sulphur,
and iron, which are linked to important biogeochemical
cycles, including the carbon cycle (Mahowald et al. 2017).
In particular, phosphorus and iron are linked to the dust
cycle, and the peculiarity is that windblown inputs can be
fundamental to the mass budgets of those elements in remote
regions, far from the dust sources. For instance, dust-borne
phosphorus from North Africa replenishes the pool of this
element in the Amazon, where the loss by fluvial erosion
would otherwise deplete it, with implications for the rainforest. Iron, on the other hand, is a micronutrient for marine
ecosystems. Because its sources are the continents, remote
marine areas are depleted in this element. Where the abundance of macronutrients such as nitrogen and phosphorus is
accompanied by a relative scarcity of iron, which limits the
primary production at the ecosystem level, i.e. in
High-Nutrient Low-Chlorophyll (HNLC) areas, dust-borne
inputs of iron become of great importance in sustaining algal
blooms—this is notably the case of the Southern Ocean
(Jickells et al. 2005).
Natural Aerosols: Overview
In this section we will briefly describe the main natural
aerosol types. Note that mineral dust and sea salt are still the
most abundant primary aerosol species by mass in the present day atmosphere.
Mineral (desert) dust (Fig. 23.17) is emitted into the
atmosphere in response to wind erosion of the surface, in dry
and semi-dry areas, with low vegetation cover. Far-travelled
dust particles are mostly clays and fine silts below 10 µm in
diameter, and are composed mainly of silicates, along with
carbonates, gypsum, and metal oxides. Dust aerosols interact
23 Biogeochemical Cycles and Aerosols Over the Last Million Years
291