Page 173 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 151

natural systems. Rainfall patterns change as temperatures increase and future
generations will experience an increase in the size of arid and semi-arid areas,
leading to a greater risk of desertification. There will also be an increase in the
incidence of disease, insect attack on existing forests, rising sea levels and
coastal erosion.

Oceanic heat flow
The oceans cover about 71% of the total Earth's surface and play a major role in
both local supply of heat energy to the atmosphere and regional transfer of heat
energy from the tropics to polar-regions. The ocean contains an enormous
amount of heat energy that is both gained and lost gradually. In addition to the
energising effects of oceanic heat on the atmosphere, the oceans also act as
moderating influences on air temperature, weather and climate. Minor surface
currents and eddies along continental shorelines have important effects on local
weather conditions. Continental weathering is closely linked to the interchange
of heat between land and sea and with its effects upon a wide range of factors.
Important amongst these are air composition and temperature, relative humidity,
insolation, altitude, motion and atmospheric pollution, nature of the water cycle
operating in the particular area, rock composition, vegetation, topography and
time.
Global transport of surplus solar energy from the tropics to regions of
deficient solar heat in the higher latitudes is now thought to be greater in the
oceans than in the atmosphere. This is due to the enormous capacity of oceans to
absorb and store heat during periods of surplus solar heat. Ehricke (1989)
suggests that annually, some 28% of the total energy reaching the Earth is
expended in evaporating water and lifting it into the atmosphere. It is estimated
3
that of this amount, some 369,000 km (85.5%) is evaporated from the oceans
3
and 62,500 km (14.5%) by evapotranspiration from the land. Estimates of total
oceanic and atmospheric transports given in Fig. 3.10 demonstrate the
dominance of atmospheric heat transport north of about 30ë N but that oceanic
transport is responsible for most of the heat transfer from the tropics.
Globally, the general circulation of surface and deep-sea ocean currents
depends upon such large-scale systems as the Gulf Stream, the Equatorial
Counter-Current and East Australia Current. Wind is the major driving force for
surface currents. Deep circulating currents result partly from wind stress and
partly from the influence of Coriolis forces, density differences and seabed
topography. The effects of the increased contribution of zero-temperature water
from melting of ice caps and sea ice on deep sea ocean currents suggests that
they may be a factor of vital importance affecting global warming. For example,
the largest glacier in South America, the Upsala Glacier is now retreating at the
rate of 200 m a year. Most scientists agree that sea level could rise as much as
70 mm if present levels of melting of the Greenland and Antarctic ice sheets and