The Global Carbon Cycle
Among the most important research initiatives of the 1990s is the study of
global change, including the relationship between the biogeochemical cycles and
the Earth's climate. Of particular interest is the global carbon cycle and its
alteration by human activities.
The burning of fossil fuel releases carbon dioxide into the atmosphere. As a
result, carbon dioxide has been steadily increasing in the atmosphere and oceans
since the beginning of the Industrial Revolution. Over the same period of time,
deforestation has eliminated a significant fraction of the terrestrial plant
life, affecting the rate at which land vegetation can remove carbon dioxide from
the atmosphere. Current climate models predict that the increasing
concentrations of carbon dioxide and other "greenhouse gasses" in the Earth's
atmosphere will produce an increase in average global temperature of some 1-5
degrees Celsius in the next half-century. The impact of a global warming is
difficult to predict but would be accompanied by major changes in precipitation
patterns and land use throughout much of the world.
Approximately half of the carbon dioxide newly released by the burning of fossil
fuels is believed to be absorbed by the ocean. However, the oceanic sinks for
this newly injected carbon have not yet been clearly identified, nor has the
apparent rate of carbon absorption been satisfactorily explained. These
uncertainties focus attention on the role of phytoplankton in the global carbon
cycle.
Most of the annual phytoplankton primary production is eaten by zooplankton
(shrimp-like crustaceans and other miniscule marine animals) or small fish.
The fraction of phytoplankton that is not eaten zooplankton sinks below the
well-lit euphotic zone. Sinking phytoplankton are a major component of the rain
of particulate matter reaching the sea floor (See figure above). Nearly all the
dead phytoplankton reaching the sea floor are eventually consumed by animals or
decomposed by bacteria. However, a small fraction remains intact, so that the
associated organic carbon is not respired to carbon dioxide. This steady
addition of residual carbon to marine sedimants represents an important sink
within the global carbon cycle. Phytoplankton carbon and associated nitrogen,
phosphorus, iron, and other elements thus added to marine sediments are retained
there for millions of years. By contrast, terrestrial plant carbon typically
decomposes within decades. This difference in storage time implies that 99%
of the carbon now stored in the biosphere resides in marine sediments. The
processes governing the formation and dissolution of this huge carbon reservoir
are believed to be highly significant to climate change over geological
timescales.
These observations raise important questions. For example: What is the rate of
carbon flux from surface waters to the deep sea? How is the sedimentation rate
related to the rate of primary production? Providing answers to these and
related questions is a primary aim of the Global Ocean Flux Study (GOFS), the
United States component of an international initiative, the Joint Global Ocean
Flux Study (JGOFS), that will sponsor major investigations into the transport
and transformations of carbon and other biogeochemical elements in the ocean
during the 1990s. Key JGOFS strategies include the use of satellite ocean-color
measurements to obtain improved estimates of
primary productivity, including
the first estimates of interannual variability on ocean-basin scales.
|
Atmos Composition
