Global Change and the Earth System: A planet under pressure Page: 9
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Linkages and connectivities
One of the most ubiquitous features of the planetary
machinery is the suite of linkages that bind processes
in one region to consequences in others thousands
of kilometers away. Atmospheric and oceanic
circulation play a major role in the transport of
heat from the tropics to the poles. The horizontal
movement of water in rivers is another important
transport process that couples seemingly isolated
parts of the planet.
The atmospheric transport of materials, often
considered only in the context of air pollution, also
plays a role in natural biogeochemical cycles by lin-
king land and ocean processes across long distances.
In southern Africa, for example, recirculating air
flows pick up dust from arid lands and smoke from
industrial areas and eventually transport them
over the southern Indian Ocean towards Australia.
Portions of these plumes subside regularly over
certain patches in the ocean, depositing iron-laden
dust on the sea surface. The iron, a micro-nutrient
for phytoplankton, probably acts as an intermittent
fertiliser triggering planktonic blooms and accoun-
ting for the observed 'hot spots' of carbon uptake
in precisely these areas of the southern Indian
Ocean (Fig. 6).
Horizontal transport of materials also occurs via
river networks, which provide corridors that link
mountains and coastal areas, land and water ecosys-
tems, across landscapes regional to subcontinental
in scale. Riverine fluxes have been amajor pathway by which carbon, phosphorus, and
other elements cycle on geologic timescales. Prior
to significant human disturbance, nutrient cycling in
terrestrial ecosystems was usually tightly constrained
within the ecosystem itself with much recycling;
there was little leakage to river networks. Leakage
occurred only when terrestrial nutrient cycles were
briefly perturbed by rare events or sequences of
rare events.
Abrupt changes and critical thresholds
Because human societies have developed and
flourished over a very short period of time from an
Earth System perspective, and because the period
of instrumental observation and modern scientific
enquiry is even shorter, a narrow view of the Earth's
environment has developed.The notion that a single
stable equilibrium is the natural state of Earth's
environment is not supported by observations of
past global changes. The behaviour of the Earth
System is typified not by stable equilibria, but by
OI- o " sourlno - n aeasNetFux (12 grams y noeach4 x area)
20S
30S
40S
50S20 40 60 80 100
1200E
Figure Top paneL: Mean annual exchange of CO2 across the sea surface. Blue and purple colours denote regions in which the ocean
takes up Large amounts of CO2. Bottom paneL: The trajectory of iron-laden aerosols from southern Africa over the southern
Indian Ocean and their deposition on areas of observed carbon uptake.
Sources: Takahashi et al. (1999) Proc. 2nd Intl. Symp. on CO2 in Oceans, pp. 9-15. Piketh et al. (2000) Sth. Afr. J. Sci. 96, 244-246.Planetary Machinery
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IGBP SCIENCE No. 4
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Global Environmental Change Programmes. Global Change and the Earth System: A planet under pressure, text, 2001; Stockholm, Sweden. (https://digital.library.unt.edu/ark:/67531/metadc12041/m1/11/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; .