The following article has been published as a discussion paper in ESDD,
the scientific discussion forum of ESD:
Author(s): F. D. Oeste, R. de Richter, T. Ming, S. Caillol
Climate engineering by mimicking the natural dust climate control: the
Iron Salt Aerosols method.
The paper is accessible and open for interactive public discussion at:
http://www.earth-syst-dynam-discuss.net/esd-2016-32/
Some features of the paper may characterized like follows:
It is known that the dust concentration in the troposphere increased
during every cold period in ice ages and reached a multiple of today’s
levels (Martinez-Garcia et al. 2011), and that dust deposition in the
Southern Ocean during glacial periods was 3 to 10 times greater than
during interglacial periods (Lamy et al., 2014).
In the review we propose to increase the iron aerosols concentration as
a mean to reduce global warming. Our proposal to increase the iron
aerosols concentration makes sense, not only because it “mimics the
natural dust climate control”, but also as it is feasible because it
currently occurs. This allows quantitative estimates of the “iron salt
aerosols” needs to stabilize the Earth climate.
It is worth being mentioned that some authors estimate that deposition
of soluble iron from combustion processes already contributes from 20 to
100% of the soluble iron deposition over many ocean regions (Luo et al.
2008). That means that already the anthropogenic combustion emissions
play a significant role in the atmospheric input of soluble iron to the
surface ocean (Sedwick et al., 2007).
A well known cooling process induced by “iron salt aerosols” consists in
a phytoplankton fertilizing stage. Ocean iron fertilisation has been
proposed as a geoengineering scheme (Güssow et al. 2010).
But thanks to new interpretations of existing results and by augmenting
our comprehension of the full Iron cycle process, we found that this
cooling effect is only part of a cascade of at least 12 climate cooling
stages presented in our review.
For instance:
a) methane in the troposphere is currently destroyed mainly by the
hydroxyl radical °OH. But about 3 to 4 % of CH4 (25 Tg/yr) are oxidized
by the chlorine radical °Cl in the troposphere (Allan et al. 2007), and
the °Cl is generated by a Fe(II)/Fe(III) photocatalytic reaction. Thus
increasing the amount of “iron salt aerosols” can fight global warming
by reducing the CH4 concentration in the atmosphere.
b) also, by reflecting sunlight radiation back to space, some types of
aerosols increase the local albedo, producing a cooling effect. In order
to enhance the cooling effects of low altitude clouds, marine cloud
brightening has been proposed (Latham et al., 2012) by injecting sea
salt aerosols over the oceans.
c) iron, the fourth most abundant element in the Earth’s crust, is
essential in biology and a key micronutrient for phytoplankton growth in
the surface ocean. Yet, in today’s oceans, iron is a vanishingly rare
element: its concentration (typically < 1 nM) is so low that iron
scarcity limits biological productivity across large areas of the
Earth’s surface (Martin and Fitzwater, 1988).
There are large areas of the world ocean (equatorial Pacific and much of
the southern oceans) where the concentrations of nitrate and phosphates
nutrients are high and yet chlorophyll concentration is low. Primary
productivity in these oceanic regions is limited by the availability of
iron. Deposition of iron to these regions has important implications for
the CO2 budget. Increases in iron to the oceans may result in increased
productivity and hence a decrease of CO2 in the atmosphere, thus a
cooling effect by atmospheric GHG removal.
During winter, sea ice acts as a reservoir for iron and releases the
trace metal to the surface ocean in spring and summer, which increases
phytoplankton production during these seasons. But Arctic sea ice cover
has been declining rapidly and a significant reduction of sea ice in
both hemispheres is projected in future climate scenarios.
Climate change will impact desert dust sources and humans will impact
the transport of the bioavailable fraction of iron to the oceans by
reducing acid and mineral emissions coming out of the chimneys of fossil
fuel power plants. These aspects are also reviewed in the article.
The atmospheric dust cycle with iron inputs to the ocean (Mahowald et
al. 2005) and the biogeochemical cycle of iron in the ocean (Boyd et al.
2010) are is described in the literature. In the review we describe the
“global iron cycle” in the atmosphere, in the ocean, in sediments and in
the Earth crust.
This review completes the current knowledge of “the global iron cycle”,
provides a global vision of the numerous cooling effects of the iron
cycle and allows the proposal of a concrete method that might be helpful
in the fight against global warming and to help stabilise the average
Earth temperature, probably the major environmental challenge humanity
will face in the 3rd millennium.
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