• Over two-thirds of Earth's surface is ocean.
• Global climate patterns are influenced by oceanic circulation which in turn is controlled by the physical characteristics of the oceans.
• The evolution of coastal landforms depends on the interaction of wave action with processes in the continental interior and human actions.

ver two-thirds (~71%) of the planet is covered in seawater. The world’s oceans are unevenly distributed and lie mostly in the Southern Hemisphere (88% seawater). The three major oceans (Pacific, Atlantic, Indian Oceans) are connected together along their southern margins by the Southern Ocean that encircles Antarctica. Smaller, enclosed or partially enclosed water bodies are termed seas or gulfs (Fig. 1). All of these features are created by plate tectonic processes and many continue to increase or decrease in size as plates diverge or converge (see the Plate Tectonics chapter for more on plate tectonics and the seafloor).

It is in the various small seas that we can observe the most obvious negative consequences of human activity on the ocean realm in the form of overfishing, pollution, eutrophication, and habitat destruction. These same consequences are visited on the margins of the major oceans.

Figure 1. The distribution of the world's oceans and selected seas. Nearly 90% of the Southern Hemisphere is covered by oceans.

Today we appreciate the oceans for their direct and indirect impacts on human activity. This chapter is divided into two halves. The first part considers the physical characteristics of the oceans and begins with the depth of the ocean floor. The floor of the ocean is a few hundred meters below sea level along the margins of the continents but averages over 3 km (2 miles) depth between the major oceans and reaches a maximum depth of 11 km (7 miles) in the western Pacific. The ocean floor has the potential to be a great source of mineral wealth in the future and mining companies are already staking claims to potentially lucrative sections of subsea real estate. 

Ocean currents are influenced not only by the extent and depth of the oceans but also the salinity and temperature of ocean waters. Ocean waters are thought to have originated from meteorites and icy comets colliding with the early Earth. Both temperature and salinity change with depth and latitude and each is influenced by atmospheric weather patterns. Each factor is examined in the section titled salinity and temperature of the oceans.

The final section of the first half of the chapter examines how depth, salinity, and temperature combine to generate characteristic patterns of oceanic circulation. Surface currents are controlled by dominant wind patterns that are in turn linked to the rotation of the planet. Deepwater circulation patterns are controlled by the density of ocean waters and the distribution of landmasses. Both sets of currents redistribute the Earth's heat budget and play a crucial role in controlling climate.

In the second half of the chapter we focus on how human activity impacts the ocean margins and enclosed seas. Coastlines represent the fragile strip of land that borders the ocean. Developed coastal areas are threatened with potential loss of life and billions of dollars in property damage as a result of storm impacts and long-term erosion. For example Hurricane Fran (Fig. 2) devastated parts of several eastern states and was especially destructive in North Carolina where winds of over 100 mph generated more than $5 billion in damages.

Figure 2. Damage from Hurricane Fran (1996) along coast of North Carolina. Note change in position of house indicated by arrow. Images courtesy of USGS Recent Highlights-Hazards.

The evolution of landforms along a coastline depends upon the interaction of wave action with the shore with processes in the continental interior and human actions. The coastline is a dynamic environment that advances or retreats depending upon the balance between the supply of sediment and the material removed by erosion. The section on shorelines and the sediment budget examines this balance.

The National Park Service completed the tricky task of relocating the historical Cape Hatteras lighthouse to a site further inland in July 1999, to protect the light from erosion that threatened to topple the structure. Twenty-six of the thirty states bordering an ocean or Great Lake are presently experiencing net loss of their shorelines. Well-intentioned efforts at shoreline protection often resulted in the construction of coastal structures such as jetties, groins, or breakwaters that altered the natural movement of sediment along the coastline and simply exacerbated existing erosion problems. We finish by examining the interaction of shoreline erosion and shoreline protection efforts along the southern shore of Lake Erie.