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THE
CARBON CYCLE
  
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THE
CARBON RECORD, PAST AND PRESENT
Quantifying
carbon as it moves through the biosphere has been a goal for
a long time. For years, scientists have at least known that
there’s an annual rhythm to the cycle of carbon uptake and
release. As the seasons change like the beat of a silent,
age old drum, carbon finds its way both into and out of living
things around the world.
Using
a variety of analytic techniques, researchers have been able
to show that average levels of atmospheric carbon didn’t significantly
change for thousands of years. But with the global phenomenon
of urbanization in the latter half of the second millennium,
particularly marked by the industrial revolution in the nineteenth
century, ambient carbon levels in the atmosphere started rising.
Of
late they’re rising faster than ever.
In
this visualization, we see a graph showing ambient atmospheric
carbon going back roughly a thousand years. Data for the graph
prior to the sharp rise comes from ice core samples collected
in Antarctica. This is considered the preferred method for
determining historic atmospheric carbon dioxide levels.
But
since 1958, researchers working at a field station near the
Mauna Loa caldera in Hawaii have collected data about ambient
carbon dioxide levels once an hour. Their findings,
daily averages constituting the longest continuous record
of atmospheric carbon dioxide in the world, are powerful.
Since
they began tracking it, the record of ambient atmospheric
carbon dioxide shows a steady increase, year after year. In
fact, the Mauna Loa record shows a 16.6% increase in the mean
annual concentration, rising from 315.83 ppmv (parts per million
by volume) of dry air in 1959 to 368.37 ppmv in 1999. Between
1997 and 1998, the record shows the single largest one-year
increase: 2.9 ppmv.
But
the Mauna Loa record also validates something else significant,
shown here in the visualization. There’s an annual pulse to
the presence of carbon, coinciding with seasonal variations.
That oscillation not only marks the heartbeat of the cycle,
but also gives researchers a point of reference for future
study into how the cycle may be changing. As we look at the
last three years of data gathered by SeaWiFS, we clearly see
not only that seasonal variation, but also a steady, inexorable
rise in ambient atmospheric carbon.
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CARBON
AND THE LAND – THE FAST CYCLE
Many of us actually
experience a side effect of the carbon cycle in our lower
backs every autumn. Leaves fall from the trees, and out we
go with rakes and wheelbarrows to whisk them away.
Those thousands
of curled brown bits of trees that flutter to the ground are
packets of carbon, recycling naturally if we didn’t sweep
them up. During the spring and summer, terrestrial plant life
drinks carbon dioxide in from the atmosphere and, combined
with water and nutrients from the soil, grows. This is called
carbon sequestration—carbon being taken in from the surrounding
world and literally trapped for a period of time by the very
body that it enables to grow.
The growing
season pulls carbon out of the air and converts its into building
materials. But in the fall and winter, significant parts of
that growth die off, and that carbon goes back into system,
churned up by the fast (by planetary standards) process of
decomposition.
Another way that
carbon recycles following the terrestrial growth process begins
when the natural life of a plant ends. When a tree, for example,
ultimately dies and begins to decompose, all of the carbon
sequestered in its trunk and branches and roots begins the
cyclic process of passing back into the environment.
Fire can accelerate
this process, sending plumes of carbon-laden aerosols into
the atmosphere, as well as leaving carbon-rich ash deposits
on the ground for further decomposition and recycling.
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CARBON
AND THE OCEAN – THE SLOW CYCLE
The oceans are
vast, and their processes as complex as their waters are deep.
In terms of the carbon cycle, the process moves at a much
slower pace than on land, but its implications are just as
significant, if not more so.
Tiny single celled
plant life called phytoplankton absorb carbon dioxide from
the atmosphere and nutrient rich waters and grow in wide colonies
called blooms. These blooms are highly variable in their nature;
their size and intensity are significantly dependent on surrounding
environmental conditions.
Nutrient rich
waters can come from a variety of places. Two of the most
easily identified are deep-water upwellings and outflows from
rivers. As phytoplankton grows, it forms the foundation for
the food chain, thus passing carbon up to higher life forms.
But just as
on land, links in the ocean’s chain of life also break, and
stored carbon settles out of the top layers of water. A portion
of it gets swept back to the surface as upwellings, only to
begin again, but a major portion sinks to the bottom, becoming
what oceanographers call "marine snow". This decomposing
biological matter literally precipitates through the water
and builds up on the ocean bottom, essentially sequestered
from the rest of the Earth for geologically long periods of
time.
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DEEP
WATER FEAST: UPWELLINGS BRING NUTRIENTS TO THE SURFACE
Like any other
life form, phytoplankton requires nutrients to grow. In the
ocean, those nutrients are often found in cold, deep water.
Large phytoplankton blooms tend to coincide with natural phenomena
that drive that nutrient rich water to the surface. The process
is called upwelling, and it happens in a couple of different
ways.
The principle
mechanism by which deep, cold, and nutrient rich water rises
up through the water column can be found along the western
coasts of the continents. Or, said more specifically, upwellings
often take place along the eastern margins of oceanic basins.
Here’s what’s happening: winds coming off principal land masses
push surface layers of water away from the shore. Into the
resulting wind-driven void deeper water underneath the surface
layers rushes in toward the coast, bringing with it nutrients
for life to bloom.
The process
is largely governed by the rotation of the planet, essentially
sloshing the waters and winds of the world in one dominant
direction.
Exceptions to
this, however, can be found in the Indian Ocean, where unique
monsoon related weather patterns churn the ocean currents
enough to create coastal upwelling patterns along other coasts.
Also, in the equatorial Pacific Ocean, one of the largest
examples of deep water upwelling can be seen. Although primarily
due to process of surface water being pushed out of the way,
allowing deeper water to rush in like we see along coasts,
the process of surface water removal in the Pacific is somewhat
different.
On the equator,
water currents on either side of the hemispheric dividing
line are generally moving in opposite directions—again due
to planetary rotation and the Coriolis effect. As those currents
rush past each other they ostensibly "peel back"
the surface of the ocean, creating a void for deeper water
to rush in and take its place.
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