Satellites scan the ocean: modern technology in the service of oceanography

Satellites scan the ocean

WHEN I entered oceanography in the early 1960s, rapid strides were being made towards describing and predicting ocean behaviour. The first expedition in which I participated spent six months in the Indian Ocean helping to measure the currents and winds in the equatorial regions there. But the technology and the research ships available to us at that time had serious limitations. The ship we used was the Argo, a former submarine rescue ship deeded from the United States Navy to the Scripps Institution of Oceanography and our equipment was largely devoid of electronics. Our officers used celestial navigation, dead-reckoning and local measurements. The difficulty of obtaining any kind of global view or photograph of conditions at any one time over a large area of the Indian Ocean was painfully obvious.

Today, our instruments are much more sophisticated, largely because of the revolution in micro-electronics and computers, but the limitations of measurements from ships are still with us. Measurement of the ocean from satellites promises to liberate us from many of these shipboard limitations. Navigation by satellite, a technique that uses satellites and on-board receivers and computers, is commonplace for research ships and commercial vessels. Accurate positioning is available at two hourly intervals in most regions of the world and in any weather conditions. We are also now beginning to use other satellite data to get photographs of the behaviour of the oceans--its temperature, the shape of its surface, its waves and its chemical and biological properties.

The ocean is a subject of intense interest to the applied scientist for a variety of reasons. Whereas in the past increasing energy and food requirements could be met by expansion of frontiers, use of chemicals for fertilizers and pesticides, and by using available reserves of gas and oil, we are today being confronted by the finite dimensions of the world. We are at the point where we affect both regional and global climate. We are a factor in the global cycles of carbon, nitrogen, phosphorus and sulphur because we produce significant amounts of these chemicals and dump them into the air and the sea. It is essential that we understand how the overall ocean-atmosphere system works if we are to live successfully with global change. We have to understand the cycling of energy, waste and essential chemicals through the atmosphere, land and oceans.

The oceans regulate the climate and make it more predictable. The slow variations of ocean currents which affect the transfer of heat to the atmosphere could, in the view of many experts, provide a mechanism for predicting the climatic changes in the atmosphere. But in order to understand these variations of ocean currents we need to understand the general circulation of the ocean, how the currents are driven and how and why they change. Another largely unknown factor in the climatic system involves the role of sea-snow and sea-ice. Both of these are quantities that can be measured and monitored by satellite.

The distribution and cycling of nutrients is also important to the "habitability" of our planet. The nutrients are related to the biological productivity cycle through the atmosphere, rivers, coastal zones and the deep ocean. The continental shelf is a critical area. Most of the world's fisheries occur here, much of the world's petroleum production may be in these areas in the future and it is here that most of man's pollution enters the ocean. The shelf is a direct source of food for humanity and the pressures on this food supply are increasing rapidly. We must know what is happening here and we must be able to understand the mechanisms that control the transport of these nutrients from terrestrial sources to oceanic sinks. To do this we must be able to understand the interactions between physical and biological processes in the oceans. Large-scale satellite views of nutrients in the ocean are necessary for this understanding.

Changes in the amount and timing of global rainfall are critical for farmers. The green revolution has shown dramatically how genetic engineering can provide farmers with exactly the right kind of crops for a maximum of productivity if the water resources are known. Thus rainfall is critical, but we must know it globally. The problem is not yet solved; in fact, the natural rainfall patterns over the globe are only marginally predictable by the use of climate models now available. Global rainfall is another parameter that we can measure by satellite.

For the oceanographer, ships are essential. There is no other way to get samples of water, chemicals or biological organisms from the sea. But ships alone cannot give us an overall view of the ocean. A typical research ship travels at about ten knots; at that speed it takes about ten days to make a single crossing of the North Atlantic. It can therefore take months to complete a careful survey of any large region, and in that time the state of the ocean can change drastically.

From ships and from the relatively few buoys that are moored in the ocean for weather measurements and for research purposes, we get an average view of the ocean's properties (for example its temperature and salinity) and of near-surface weather. This average view is important, but we have to recognize that the ocean is continually changing. It does not change as fast as the atmosphere, but it does behave in a similar way. The atmosphere changes its weather in a few days and its climate varies from year to year. The ocean's "weather", that is, the currents and temperatures, changes over periods of a few weeks instead of a few days. The ocean's "climate" also changes from year to year.

But how do we get an overall view of the ocean? Our instruments have to be placed in a position away from the earth in order to see it all at once. Satellites are the only way. Typically satellites receive their power from the sun, and they are classified as either passive or active. The passive satellites receive the radiation that comes from the earth and attempt to interpret it; the active satellites illuminate the sea with specified radiation and observe the reflected pulses. We now have satellites that can measure the shape of the ocean's surface, the surface temperature of the ocean, the wind speed near the surface, the extent of the polar ice, the surface chlorophyll and a variety of properties in the atmosphere.

From these instruments we get a global view--either an image of almost half the earth at a time from a satellite in geostationary orbit, or from a series of tracks of "low earth orbiter satellites" which travel closer to the earth and repeat their orbits about every ninety minutes. Thus we have at our disposal a new and remarkable series of pictures of the ocean and the land, and this new imagery has dramatically changed the way in which we view the ocean, allowing us to plan for new research programmes. We now see the possibility of describing and understanding some of the important cycles of climate, of nutrients and of other environmental factors that affect global habitability.

Enormous quantities of data have been collected since the early 1960s, when satellites first came into use, but it is only in recent years that satellite pictures that can be used for real scientific understanding of the ocean have become available. The four photographs on page 22 demonstrate the power of these new techniques.

Encouraged by this technological advance, a number of scientists have proposed the establishment of an international Global Habitability Programme aimed at improving our understanding of the climate and the global cycles. The central elements of the programme would be satellite measurements by many nations and shipboard and land-based measurement and modelling of the ocean, the earth and the biosphere. Using satellites, ships and computers, environmental scientists have the technology to carry out the required research. In addition, their theoretical tools have advanced to the point where the integrated study of the global system is becoming feasible. With scientists working together internationally it is possible that rational management of the earth and its resources may be achieved.

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