28 
The Global Carbon Cycle 
Over the last few decades, our knowledge of 
the Earth's oceans, atmosphere, continents, and 
ice cover has increased dramatically. The inter- 
action and balance among these elements of the 
biosphere are being increasingly appreciated, as 
is their influence on man and human society. 
Most change in the global system is natural, due 
to such causes as volcanic activity and changes 
in the Earth-to-Sun distance, but evidence is ac- 
cumulating that human activity also plays a major 
role in aspects of global ecology that directly af- 
fect the Earth as a unique home for life. 
The steady increase in the carbon dioxide 
content of the atmosphere associated with the 
burning of fossil fuels is a well-documented exam- 
ple, but the cycles of other biogenic gases, such 
as nitrous oxide, methane, and carbon monoxide 
are also affected by anthropogenic activity. 
These gases also contribute to the heating of our 
atmosphere through their property of absorbing 
infrared radiation (i.e., the "greenhouse effect"). 
With respect to carbon dioxide, at the present 
rate of increase of about 1.5 ppm per year, the 
concentration of carbon dioxide in the atmos- 
phere is expected to double relative to pre- 
Industrial-Age levels sometime in the next 
century. The other biogenic gases mentioned 
above are also increasing in the atmosphere and 
contributing to the greenhouse effect. Thus, 
some atmospheric models predict a gradual 
warming of the Earth's climate with as-yet- 
unknown consequences. 
The pathways and rates of removal of carbon 
dioxide from the atmosphere have not been de- 
finitively established, but it is known that about 
50% of the carbon dioxide released from burning 
fossil fuels has accumulated in the atmosphere, 
and most of the remainder is in the ocean. To 
provide quantitative answers to questions con- 
cerning the ocean's role in the global carbon 
cycle and to predict the fate of anthropogenically 
derived carbon dioxide that reaches the ocean, 
observations and models must focus on key as- 
pects of the ocean's biogeochemistry. 
The Role of Phytoplankton in the 
Global Carbon Cycle 
The role of ocean biota in the global carbon 
cycle is understood qualitatively. Marine phyto- 
plankton carry out photosynthesis, converting in- 
organic carbon dissolved in the water to organic 
particles and dissolved organic materials. This 
process is known as primary production. The 
rate of primary production varies by as much as 
a factor of ten from ocean region to ocean re- 
gion, and, thus, some parts of the ocean are re- 
ferred to as being more productive than others. 
Much of the organic carbon produced from pho- 
tosynthesis is eaten and recycled back to inor- 
ganic carbon in the surface waters by animals 
and bacteria. The residual organic matter, along 
with associated inorganic skeletal components, 
such as calcite, aragonite, and opal, settle out of 
the surface waters. A small fraction of this flux is 
ultimately buried in the sediments and potentially 
represents an important mechanism for removing 
carbon from the global cycle. 
Ocean Productivity Measurements 
on a Global Scale 
The magnitude and variability of annual pri- 
mary production by marine phytoplankton is poor- 
ly specified on a global scale, largely due to the 
high degree of spatial and temporal variability in 
the distribution of phytoplankton in the sea. As 
noted previously, phytoplankton primary produc- 
tion accounts for at least 30% of the total annual 
global photosynthesis, but this percentage is not 
known with the level of accuracy required to ac- 
curately model the role of phytoplankton in the 
global carbon cycle. Recent studies suggest that 
phytoplankton photosynthesis may be underesti- 
mated by a factor of two; this uncertainty partially 
accounts for the difficulties encountered when 
attempting global-scale analyses using traditional 
oceanographic sampling methods. 
Only through satellite remote sensing of 
ocean color can information on marine primary 
production be obtained on a global scale, given 
