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September 16, 2003
Ocean Plant Life Slows Down and Absorbs Less Carbon
Plant life in the world's oceans has become less productive since the
early 1980s, absorbing less carbon, which may in turn impact the Earth's
carbon cycle, according to a study that combines NASA satellite data
with NOAA surface observations of marine plants.
Microscopic ocean plants called phytoplankton account for about half
the transfer of carbon dioxide (CO2) from the environment into plant
cells by photosynthesis. Land plants pull in the other half. In the atmosphere,
CO2 is a heat- trapping greenhouse gas.
Watson Gregg, a NASA GSFC researcher and lead author of the study, finds
that the oceans' net primary productivity (NPP) has declined more than
6 percent globally over the last two decades, possibly as a result of
climatic changes. NPP is the rate at which plant cells take in CO2 during
photosynthesis from sunlight, using the carbon for growth. The NASA funded
study appears in a recent issue of Geophysical Research Letters.
"This research shows ocean primary productivity is declining, and it
may be a result of climate changes such as increased temperatures and
decreased iron deposition into parts of the oceans. This has major implications
for the global carbon cycle," Gregg said. Iron from trans-continental
dust clouds is an important nutrient for phytoplankton, and when lacking
can keep populations from growing.
Gregg and colleagues used two datasets from NASA satellites: one from
the Coastal Zone Color Scanner aboard NASA's Nimbus- 7 satellite (1979-1986);
and another from Sea-viewing Wide Field-of-view Sensor data on the OrbView-2
satellite (1997- 2002).
The satellites monitor the green pigment in plants, or chlorophyll,
which leads to estimates of phytoplankton amounts. The older data was
reanalyzed to conform to modern standards, which helped make the two
data records consistent with each other. The sets were blended with surface
data from NOAA research vessels and buoys to reduce errors in the satellite
records and to create an improved estimate of NPP.
The authors found nearly 70 percent of the NPP global decline per decade
occurred in the high latitudes (above 30 degrees). In the North Pacific
and North Atlantic basins, phytoplankton bloom rapidly in high concentrations
in spring, leading to shorter, more intense lifecycles. In these areas,
plankton quickly dies and can sink to the ocean floor, creating a potential
pathway of carbon from the atmosphere into the deep ocean.
In the high latitudes, rates of plankton growth declined by 7 percent
in the North Atlantic basin, 9 percent in the North Pacific basin, and
10 percent in the Antarctic basin when comparing the 1980s dataset with
the late 1990s observations.
The decline in global ocean NPP corresponds with an increase in global
sea surface temperatures of 0.36 degrees Fahrenheit (F) (0.2 degrees
Celsius (C)) over the last 20 years. Warmer water creates more distinct
ocean layers and limits mixing of deeper nutrient-rich cooler water with
warmer surface water. The lack of rising nutrients keeps phytoplankton
growth in check at the surface.
The North Atlantic and North Pacific experienced major increases in
sea surface temperatures: 0.7 degrees C (1.26 F) and 0.4 degrees C (0.72
F) respectively. In the Antarctic, there was less warming, but lower
NPP was associated with increased surface winds. These winds caused plankton
to mix downward, cutting exposure to sunlight.
Also, the amount of iron deposited from desert dust clouds into the
global oceans decreased by 25 percent over two decades. These dust clouds
blow across the oceans. Reductions in NPP in the South Pacific were associated
with a 35 percent decline in atmospheric iron deposition.
"These results illustrate the complexities of climate change, since
there may be one or more processes, such as changes in temperature and
the intensity of winds, influencing how much carbon dioxide is taken
up by photosynthesis in the oceans," said co-author Margarita Conkright,
a scientist at NOAA's National Oceanographic Data Center, Silver Spring,
Md.
Other recent NASA findings have shown land cover on Earth has actually
been greening. For information and images on the Internet, visit:
http://www.nasa.gov/home/hqnews/2003/jun/HQ_03182_green_garden.html
and
http://www.gsfc.nasa.gov/topstory/2003/0815oceancarbon.html
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Contacts:
David E. Steitz
Headquarters, Washington
(Phone: 202/358-1730)
Krishna Ramanujan
Goddard Space Flight Center (GSFC), Greenbelt, Md
(Phone: 301/286-3026)
Kent LaBorde
NOAA, Washington
(Phone: 202/482-5757) |
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Distributions of Ocean Net Primary Productivity (1997-2002)
The image shows ocean net primary productivity distributions from the
Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data on the OrbView-2
satellite (1997-2002). The units are in grams of Carbon per meter squared
per year. Light gray areas indicate missing data. Credit: Images by Robert
Simmon, NASA GSFC Earth Observatory, based on data provided by Watson
Gregg, NASA GSFC.
High-Resolution
Image
Distributions of Ocean Net Primary Productivity (1979-1986)
The image shows ocean net primary productivity distributions from the
Coastal Zone Color Scanner (CZCS) aboard NASA's Nimbus-7 Satellite (1979-1986).
The units are in grams of Carbon per meter squared per year. Light gray
areas indicate missing data. Credit: Images by Robert Simmon, NASA GSFC
Earth Observatory, based on data provided by Watson Gregg, NASA GSFC.
High-Resolution
Image
Difference in Distributions of Ocean Net Primary Productivity between
1997-2002 and 1979-1986 Data
The image shows the difference in ocean net primary productivity between
the SeaWiFS era (1997-2002) and the CZCS era (1979-1986). To obtain the
differences, the CZCS results were subtracted from the SeaWiFS results.
The units are in grams of Carbon per meter squared per year. Light gray
areas indicate missing data. Credit: Images by Robert Simmon, NASA GSFC
Earth Observatory, based on data provided by Watson Gregg, NASA GSFC.
High-Resolution
Image
Differences between the SeaWIFS (1997-2002) Data and the CZCS (1979-1986)
Data in the 12 Oceanographic Basins
These graphs show differences between the 1980s and 1990s for a number
of ocean variables that impact phytoplankton production as well as annual
primary production of marine plant life. To obtain the differences, the
CZCS results were subtracted from the SeaWiFS results. Image 4a: Change
in annual primary production in petagrams of Carbon per year. Image 4b:
Change in iron deposition in percentages. Image 4c: Change in sea surface
temperature in degrees Celsius. Image 4d: Change in mean wind stress
on the oceans' surfaces in percentages. Credit: Images by Robert Simmon,
NASA GSFC Earth Observatory, based on data provided by Watson Gregg,
NASA GSFC.
High-Resolution
4a
High-Resolution
4b
High-Resolution
4c
High-Resolution
4d
Major Ocean Basins
Credit: Images by Robert Simmon, NASA GSFC Earth Observatory, based on
data provided by Watson Gregg, NASA GSFC.
High-Resolution
Image
Animation of Changes in Ocean Net Primary Productivity (1997-2002)
This animation depicts monthly changes in ocean net primary productivity
from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data on the
OrbView-2 satellite (1997-2002). The units are in grams of Carbon per
meter squared per year. Light gray areas indicate missing data. Credit:
Images by Robert Simmon, NASA GSFC Earth Observatory, based on data provided
by Watson Gregg, NASA GSFC
High-Resolution
Animation
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