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

“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-

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

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: