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1. The essential greenhouse effect
The Earth receives all its energy from the Sun. This energy is partly confined within
the atmosphere by certain gases, which prevent it from escaping and dissipating in
space. Thanks to these gases and their greenhouse effect, the lower layers of the
atmosphere warm up and reach temperatures suitable for life. Without them, the
heat would leave the atmosphere and return to space and the average temperature
on earth would be – 18°C instead of 15°C.
2. Human activity heats the planet
Since the beginnings of industrialization, man has used great quantities of fossil fuels (oil, coal, gas)
for transport, agriculture, industry, heating or electricity. Burning of these energy sources, resulting
in the emission of great quantities of carbon dioxide, CO2, has modified the atmosphere’s chemical
composition and caused an additional greenhouse effect which has led to further warming of the
whole planet.
The concentration of CO2, one of the main greenhouse gases, has risen by about 30% in a
century, causing a rise of our planet’s average temperature of about 0.85°C on land and 0.06°C in
the oceans, which is extremely rapid compared with rates attained during the climate’s history.
This is why we speak of climate change.
3. Is the ocean beneficial?
The oceanic carbon cycle
The ocean is earth’s principal carbon sink. Two main mechanisms
allow this reservoir to extract carbon from the atmosphere: the
Atmospheric CO2
Atmospheric CO2
through the oceanic circulation system, dragging surface waters
laden in dissolved CO2 down towards the deeper layers.
Phy
CO2
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The biological pump fixes carbon either in the tissues of organisms
via photosynthesis, or in the calcareous shells of certain
n
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Falling organic
debris
micro-organisms. A proportion of the carbon so fixed is then pulled
Carbon dissolved
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Dissolved
carbon
down to the ocean floor and stored in marine sediments.
Remineralization of
organic debris
At present, the ocean reabsorbs 90% of the calories produced by the additional
zon
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Upwelling
DR
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00
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Without the ocean, the temperature on earth would be 8°C higher! However, the experts
consider that the natural carbon sinks (oceans, biosphere) are tending to diminish overall with
global warming. In this way the efficiency of the southern ocean, the planet’s principal carbon “sponge”
260 °
has been declining noticeably for nearly 30 years.
250 °
m
Carbon dissolved
at depth
greenhouse effect and one third of anthropogenic CO2 emissions into the atmosphere.
240 °
Deep
convection
DR
physical pump and the biological pump. The physical pump works