Environmental Engineering Reference
In-Depth Information
Chapter
11
The global ocean
We live at a time of revitalized interest in Earth's oceans, often seen as the 'Cinderella'
of physical geography with consequential neglect or omission from many textbooks and
courses, despite the fact that they cover 70 per cent of Earth's surface. They were
perceived as a
black box
of largely invisible parts operating as a homogeneous, stable
system, with a monotonous surface except at the coastline and apparently subject only to
slow spatial and temporal change. Above all, oceans are not the habitat of humans and
the marine environment is hostile to human life. What is happening to change this
indifference? Our concern about global environmental change leads us increasingly to
appreciate the integrated nature and fast response times of many previously neglected
planetary systems - including oceans - and interest in our own future security does the
rest!
The global ocean is the largest single moderating influence on extremes of radiation
budgets and climate and the source of most precipitation. It is coupled geochemically
with the atmosphere, to buffer Earth between the extreme environments of lithosphere
and space, and receives and recycles sediments eroded from the continents. Biologically,
oceans house some of the world's simpler yet locally most productive ecosystems.
Oceans are instrumental in climatic change, leading or responding to changes in other
interactive systems. Six important oceanic parameters - geometry, volume, sea level,
composition (especially salinity), temperature and circulation - are now linked with the
atmosphere and climate in
Atmosphere-Ocean General Circulation Models
(AOGCMs)
and explain their role in global climate.
Vested human interest also focuses on the land-sea '
ecotone
' - the coastline - which
houses Earth's highest concentrations of human population, agriculture and industry
through choice or necessity (Plate 11.1). Many peoples - the British, Japanese,
Caribbeans, Indonesians and Polynesians, for example - are islanders; many others
occupy the more benign coastline of inhospitable desert, polar or mountainous regions.
All have a strong maritime thread to their lives and are aware of threats to coastline
integrity through rising sea level, and water quality through pollution. Globally, we view
oceans increasingly as a potential source of food and minerals as we outgrow terrestrial
resources. Geographers now recognize the need to understand our oceans better and this
chapter sets out the more important dynamic characteristics of the global ocean.
EVOLUTION OF EARTH'S OCEAN BASINS
AFTER PANGAEA: THE FORMATION OF MODERN OCEANS
Modern ocean-continent distributions reflect the breakup of Pangaea, which coalesced as
Earth's most recent supercontinent in the late Palaeozoic,
c
. 290 Ma ago. It survived for
c
. 100 Ma before restless tectonic stresses rifted it apart again. Rocks common to its now
separated parts reveal that it embraced
Gondwana
, centred between the south pole and
