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Unit 3: Oceans // Section 1: Introduction

Although it may seem counterintuitive, the oceans play a major part in creating conditions for life on land. Together with the atmosphere, oceans regulate global temperatures, shape weather and climate patterns, and cycle elements through the biosphere. They also contain nearly all of the water on Earth's surface and are an important food source. Life on Earth originated in the oceans, and they are home to many unique ecosystems that are important sources of biodiversity, from coral reefs to polar sea ice communities (Fig. 1).

Coral reef in the Hawaiian islands

Figure 1. Coral reef in the Hawaiian islands
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Source: © United States Geological Survey.

Scientific understanding of the oceans made great advances during the 20th century, but we still know relatively little about such central issues as the abundance and diversity of marine species, the declining health of coral reefs, and how future changes in global climate might affect ocean circulation. Technologies such as remote sensing from satellites are making it possible to collect and analyze more of the physical, chemical, and biological data that researchers need to address these questions.

However, the sheer size of the oceans and the difficulties involved in exploring them—especially at depths where no light penetrates and overlying water generates crushing pressures—make it an ongoing challenge to understand how the seas work. In an effort to fill this gap, several U.S. expert panels recently have recommended investing in a national ocean exploration program and a broad-scale system like the National Weather Service to observe and forecast ocean conditions (footnote 1).

"[T]he ocean remains one of the least explored and understood environments on the planet—a frontier for discoveries that could provide important benefits . . . . Ocean science and technology will play an increasingly central role in the multidisciplinary study and management of the whole-Earth system."

U.S. Commission on Ocean Policy,
An Ocean Blueprint for the 21st Century

This unit explores the working of ocean currents and circulation patterns and their influence on global climate cycles. It then turns to biological activity in the oceans, focusing on microscopic plankton that form the base of ocean food webs, and the influence of physical conditions like temperatures and currents on ocean food production.

Finally, this unit looks at one of the most important global regulating mechanisms: the so-called "biological pump" in which plankton take up carbon from the atmosphere, then carry it to the deep ocean where it can remain for thousands of years. This process is centrally important to life on Earth for several reasons. First, as discussed in Unit 2, "Atmosphere," the process of photosynthesis releases one molecule of oxygen to the atmosphere for every carbon atom that is packaged in organic carbon. Phytoplankton photosynthesis produces about half of the world's oxygen supply. And as we will see, by exporting carbon to deep waters, the biological pump helps to regulate Earth's energy balance and partially offsets rising CO2 concentrations in the atmosphere.

Despite their global scope, the oceans are highly vulnerable to human impacts, including marine pollution (discussed in Unit 8, "Water Resources") and over-fishing (addressed in Unit 9, "Biodiversity Decline"). Most profoundly, scientists increasingly agree that human-induced climate change could affect ocean temperatures, circulation, and carbon cycling over the next century. Scientists widely agree today that global climate change is causing ocean temperatures to rise and is changing fresh and salt water balances in many areas of the seas. Before we consider how ocean systems may be affected by global climate change, however, it is essential to understand how biological and physical processes interact normally in the oceans and how the oceans help to shape conditions for life on Earth. (For more details on how global climate change affects the oceans, see Unit 12, "Earth's Changing Climate," and Unit 13, "Looking Forward: Our Global Experiment.")

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