Watching Earth Breathe
|The SeaStar spacecraft, developed by OSC, carries the SeaWiFS instrument
The first continuous global observations of the biological engine that drives life on Earth – the countless forms of plants that cover the land and oceans – were published in the March 30 issue of the journal Science.
This study is based on the first three years of daily observations of ocean algae and land plants from the Sea-viewing Wide Field-of-View Sensor, or SeaWiFS, mission, creating the most comprehensive global biological record ever assembled. Scientists will use the new record of the Earth’s surface to study the fate of carbon in the atmosphere, the length of terrestrial growing seasons and the vitality of the ocean’s food web.
Launched to Earth orbit on August 1, 1997, the satellite’s daily observations of the ocean are helping scientists understand how to look at life on a planetary scale and how remotely to track the biological productivity of Earth’s vast oceans, which cover seventy percent of the planet’s surface.
Researchers expect the detailed new record, which NASA plans to continue for a decade or longer, will reveal as much about how our living planet functions today as the fossil and geologic records have revealed about its past.
"We’ve never been able to see the Earth this way before," said lead author Michael Behrenfeld, an oceanographer at NASA’s Goddard Space Flight Center, Greenbelt, MD.
"These [SeaWiFS] images reveal the pulse of the planet," said Behrenfeld. "They show how the oceans drive plant life and in turn, our climate. This gives us one of the first global views of three major cycles: seasonal variations from winter to summer, nearly decadal variations from the ocean currents – like the cold El Nino and warming La Nina winds – and finally the very long period changes, like small amplitude human contributions and temperature changes."
The new study presents a global assessment of the fundamental work that plants perform to make life possible – producing food, fiber, and oxygen – and how their productivity changes from season to season and year to year in response to our changing environment.
"With this record we have more biological data today than has been collected by all previous field surveys and ship cruises," says Gene Carl Feldman, SeaWiFS project manager at Goddard. "It would take a ship steaming at 6 knots over 4,000 years to provide the same coverage as a single global SeaWiFS image." Fourteen times every day, SeaWiFS orbits the Earth from pole to pole, providing a complete global view every two days. From orbit, SeaWiFS can pick out terrestrial features as small as 1 kilometer (0.6 miles) across.
|The SeaWiFS instrument.
The biological record from SeaWiFS indicates that global plant photosynthesis increased between September 1997 and August 2000. Photosynthesis by land plants and algae absorbs carbon dioxide from the atmosphere and ocean and thus plays a critical role in regulating atmospheric carbon levels. The initial increase in carbon fixation was largely due to the response of marine plants to a strong El Niño to La Niña transition, but the cause of the continued increase during the later portion of the record is not yet clear.
"With three years of observations we can see seasonal changes in plant and algae chlorophyll levels very well, but we don’t yet have a long enough record to distinguish multi-year cycles, like El Niño, from fundamental long-term changes caused by such things as higher carbon dioxide levels in the atmosphere," Behrenfeld added.
"The SeaWiFS record provides a baseline against which future estimates of Earth system carbon cycling can be compared," said Feldman.
The new biological record benefits ongoing studies of desertification and changes in growing-season lengths by joining an existing 20-year record of land plant productivity based on observations from meteorological satellites with the new generation of spacecraft instruments. These records will complement ongoing observations obtained on land and at sea.
"SeaWiFS not only adds finer detail to our observing capability, it supplies essential continuity between data records that is critical to long-term monitoring of changes in the biosphere," says biogeochemist James Randerson of the California Institute of Technology, a co-author of the study.
|Scientists are using the biological records from SeaWiFS to monitor the health of coral reefs, track harmful "red tides" and algae blooms, and improve global climate models.
Scientists are using the biological records from SeaWiFS to monitor the health of coral reefs, track harmful "red tides" and algae blooms, and improve global climate models. More han 1600 scientists representing 35 countries have registered to use the data.
This research was conducted by NASA’s Earth Science Enterprise, a long-term research effort dedicated to studying how human-induced and natural change affects our global environment.
NASA plans to produce a five-year record using SeaWiFS observations and to extend the continuous biological record with two Earth Observing System (EOS) spacecraft, Terra, launched in December 1999, and Aqua, scheduled for launch later this year. This constellation of EOS satellites allows U.S. scientists to examine many different aspects of Earth’s atmosphere, oceans and continents.
These missions will help scientists answer questions such as how human activity contributes to changes in the Earth’s environment, and how Earth’s carbon cycles through the land, ocean and atmosphere.