Resource: Global View of the Seasons

Print Background Essay

When studying Earth, it is often useful to observe it from a distance. A large field of view that makes it possible to observe the planet as a whole can be very useful for detecting global patterns and changes. The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Project does just that — onboard a satellite, the SeaWiFS instrument monitors Earth from space. With a sensor designed to study chlorophyll concentrations, SeaWiFS — launched in 1997 — has collected years of continuous data about vegetation in the oceans and on land, allowing researchers to see patterns such as seasonal changes in the distribution of plant life.

Light is absorbed and reflected differently by different materials, depending on the properties of the material; SeaWiFs monitors the way light interacts with chlorophyll, the green pigment in plants that converts sunlight into chemical energy. While the human eye can only see a small range of wavelengths, SeaWiFS is much more sensitive and can detect and gather data over a wider range of visible and infrared wavelengths. To help scientists visualize the information that is gathered, false colors are assigned to the data to create images.

Ocean color is greatly affected by phytoplankton — the microscopic plant life that forms the base of ocean food chains. Using SeaWiFS, scientists can measure the color of the ocean and thereby determine the amount and type of phytoplankton in an area. Because phytoplankton are sensitive to conditions such as the amount of sunlight available, temperature, ocean currents, and wind, the abundance and distribution of phytoplankton offers clues about the environment. For example, phytoplankton thrive in nutrient-rich waters. Satellite images show higher concentrations of chlorophyll near the mouths of rivers, where the flow of nutrients from land enters the ocean, as well as in cold waters, which are more nutrient-rich because of upwelling — the upward movement of cold, deeper water that brings nutrients from decomposing organisms on the sea floor to the surface.

Periodic changes in ocean chlorophyll concentrations, such as the annual Spring Bloom — a sudden rise of phytoplankton abundance in the North Atlantic Ocean — are also correlated with the seasons. Each spring, when the waters are cold and the amount of sunlight increases, the phytoplankton population flourishes for a limited time until the supply of nutrients diminishes. Similarly, the distribution of vegetation on land is affected by the changing conditions of the seasons. Vegetation thrives during summer, when the temperature and sunlight conditions are more favorable to plant growth. For example, satellite images show a high density of plant life in the United States during July and August while Australia shows little vegetation during those months. These differences are explained by the fact that the Northern and Southern Hemispheres experience opposite seasons — it is winter in Australia when it is summer in the United States.

To learn more about seasons, check out Earth in Motion: Seasons.

To learn more about satellites observing Earth, check out Satellites Orbiting Earth.

To learn more about different climates, check out Observe Images of Different Climate Zones and Ocean Temperatures and Climate Patterns.

To learn more about the effects of changes in ocean temperature, check out Earth System: El Niño.

Print Questions for Discussion

• What interacting factors contribute to the observed patterns of chlorophyll distribution in the oceans?
• What seasonal variations do you see in the distribution of plant life on land? What about in the oceans?
• What do these seasonal changes in vegetation indicate about the Sun's effect on Earth?
• By what steps do scientists obtain the finished SeaWIFS images that you are viewing here?