The Resources of the Earth - Thanatia: The Destiny of the Earth's Mineral Resources - page 144

northern Pacific and brings warmer, fresher water in return. This global circulation
is driven by density differences, which depend on temperature and salinity. The
salinity and temperature gradients arise from heating/cooling at the sea surface
and from the surface freshwater fluxes (evaporation and sea ice formation enhance
salinity; precipitation, runoff and ice-melt decrease salinity).
In the North Atlantic warm and salty water that has been transported north
from tropical regions is cooled, forming pure ice and thereby, increasing the salinity
of the remaining unfrozen water. The denser saline waters drop to the floor of the
ocean and begin a great circuit through the world's oceans (see Fig. 6.3). In the
Pacific, the current mixes with warmer water, where it undergoes upwelling and
warming once again. When this warmer, saltier water reaches the high northern
latitudes once more, it chills, and eventually becomes North Atlantic deep water,
completing the circuit.
The volume transport of the overturning circulation at latitude 24 N has been
estimated from hydrographic section data at around 17 10 16 m 3 =s and its heat
transport at 1,200 TW (Roemmich and Wunsch, 1985). Munk and Wunsch (1998)
meanwhile estimated the heat transport at 2,000 TW. The corresponding exergy
flow assuming a difference of 20 K is about 100 TW transferred to the thermal
gradient (Hermann, 2006). Unfortunately, there is currently no technology which is
capable of extracting such energy.
The climatic effect of the THC, still to some extent under discussion, is due to
the heat transport of this circulation (Rahmstorf, 2006). The amount of heat trans-
ported into the northern North Atlantic (north of 24 N) should warm this region
by around 5 o C (the difference in sea surface temperature in the North Atlantic as
compared to the North Pacific at similar latitudes). Global surface air temperatures
show that over the three main deep water formation regions of the world ocean, air
temperatures are warmer by up to around 10 o C compared to the latitudinal mean.
Concerns surrounding the possible collapse of the THC through the anthro-
pogenic greenhouse effect have increased recently. A THC collapse is widely dis-
cussed as one of a number of “low probability - high impact” risks associated with
global warming (Rahmstorf, 2006). This is because when the strength of the haline
forcing increases due to excess precipitation, runoff, or ice melt, the THC could
weaken or even stop, eventually leading to a more acute global climate change.
6.6.4.2 Ocean waves
Waves are another expression of solar energy. They are formed from winds blowing
over the ocean, and their energy content is many thousands of times greater than
that found in tides. To use an example, a single wave of 1.8 metres high and
moving in water 9 metres deep generates around 10 kW for each metre of wave
front (Skinner, 1986). The momentum given to currents and surface gravity waves
transferred by the wind is estimated at 60 TW (Wang and Huang, 2004), a figure