Geoscience Reference
In-Depth Information
sea ice is responding to the global warming signal. In the Antarctic, sea ice levels
are maintained and even growing in association with enhanced westerly winds.
2.5 THE BIOSPHERE
Life on earth has played a crucial role in the development of the atmosphere
and the physical climate system, and it plays a continuing role in maintaining
and changing climate. While this aspect of climate dynamics is not a focus for
this topic, a few examples are provided to demonstrate interactions between
the physical climate system and the planet's biology.
THE OCEAN BIOLOGICAL PUMP
One example of the influence of biological activity on climate is the role of
marine life in determining the chemical composition of the atmosphere. The
ocean biological pump
is the mechanism by which CO
2
is absorbed during
photosynthesis by marine organisms in the ocean's
euphotic zone
, the region
into which sunlight penetrates, and then transferred deeper into the ocean's
interior when the organism dies. Some of this CO
2
sequestration is temporary,
and the CO
2
is returned to the ocean waters to increase the partial pressure of
CO
2
when organisms decompose within the euphotic zone. But some of the
CO
2
is removed from contact with the atmosphere for very long periods. For
example, when phytoplankton is ingested by a clam the CO
2
can be bound
into the clam's calcareous shell. When the clam dies, the shell can settle un-
der gravity into the deep ocean. This settling of decaying phytoplankton and
zooplankton, fecal pellets, shells, and various other particles is referred to as
the
detritus rain
, and it transports significant amounts of CO
2
into the deep
ocean.
The overall effect of the ocean biological pump is to reduce the partial pres-
sure of dissolved CO
2
in the surface waters, allowing the additional uptake of
CO
2
from the atmosphere. If the ocean biological pump were not operating,
that is, with a “dead” ocean, it is estimated that the current atmospheric CO
2
concentration would be 25% greater.
EVOLUTION OF THE ATMOSPHERE
The presence of molecular oxygen (O
2
) in the earth's atmosphere at 21% by
volume is the result of biological activity. When photosynthesis began on earth,
roughly 3.5 billion years ago, CO
2
was consumed from the atmosphere—
atmospheric CO
2
levels decreased and O
2
levels increased. Very early photosyn-
thetic activity was carried on by
prokaryotes
, which are single-cell organisms,
including various bacteria species. Not all types of prokaryotes are able to
undertake photosynthesis, and for those that can the process is inefficient for
increasing atmospheric O
2
levels dramatically. Nevertheless, atmospheric O
2
levels increased slowly to a concentration of about 0.2% by about 2 billion
