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environment include changes in marine ecosystem productivity,
oceanic CO2 uptake, oceanic oxygen concentrations and shifts
in fisheries. Adaptation to MOC-related impacts is very likely
to be difficult if the impacts occur abruptly (e.g., on a decadal
time-scale). Overall, there is high confidence in predictions of a
MOC slowdown during the 21st century, but low confidence in
the scale of climate change that would cause an abrupt transition
or the associated impacts. However, there is high confidence that
the likelihood of large-scale and persistent MOC responses in-
creases with the extent and rate of anthropogenic forcing.
Dense shelf water cascading is a type of marine current driven
exclusively by seawater density contrast. The cascading pro-
cess is normally seasonal and triggered by the formation, on
the shelf, of dense water by cooling and/or evaporation and its
sinking down slope towards deeper offshore areas.
There are a number of places around the world where dense
water masses flow ‘over the edge’ of the continental shelf into
the deep sea, often using and carving submarine canyons. This
margin exchange process provides an essential link/exchange
between shallow and deep waters and involves water and con-
siderable particulate and dissolved loads, especially when oper-
ating in a ‘flushing’ pattern.
Due to their proximity to land areas, continental shelves are the
locus of input, transit and accumulation of land born particu-
late substances, including pollutants. Dense shelf water cascad-
ing transports these particulate substances for recycling into
the deep sea. Any future climate change driven alterations in
the temperature regime of the oceans, such as the predicted in-
crease in the horizontal layering (‘stratigraphy’) of water mass-
es, will have a significant impact in the frequency and intensity
of cascading events, and thereby on the biogeochemical bud-
gets of shallow waters and the ventilation of deep water areas.
Scientists working under the large deep-sea research project
HERMES (Hotspot Ecosystem Research on the Margins of Eu-
Figure 21. Coastal regions in the World where dense shelf water cascading ‘flushing’ has been observed. Knowledge
and mapping of these processes is still scarce due to uneven research effort. The map shows sites with known dense
shelf water cascading phenomena, which often may involve the ‘flushing’ effect (Ivanov et al., 2004; Durrieu et al., 2005;
Heussner et al, 2006). It is most likely that this phenomenon is also active off the coast of Alaska, Chile, Argentina and
West and southern Africa and in parts of the Indian Ocean. Dense shelf water cascading is highly sensitive to increases
in temperature, and hence, climate change. Data from Canals et al. (2006).