The current unprecedented
increase in atmospheric carbon dioxide is a result of the burning of fossil
fuels since the advent of the industrial revolution round about 1700 AD.
This extra release of carbon dioxide into the atmosphere that was previously
locked away in a carbon sink is not a part of any natural cycle and is the
reason for the current worries about climate change.
The carbon cycle of the planet maintains the
amount of carbon in the various carbon sinks
and causes carbon to be transferred between them. In most cases carbon is
transferred as carbon dioxide CO2 or as methane CH4.
This cycle has been turning since when the
earth cooled from a lump of glowing rock at its birth, although it is more
accurate to see it as something much more complicated than the simple single
wheel that "cycle" implies.
At various times in the history of the earth,
different parts of the carbon cycle have had different levels of importance
and the amount held in sinks and movement between them has varied enormously.
For instance in the Carboniferous period 360MYA to 299MYA (MYA=millions
of years ago), carbon dioxide was being captured by trees and tree-like plants
and laid down in conditions without oxygen that would eventually become the
coal deposits that the earth has today. In the very early atmosphere of the
earth over 4 billion years ago there were very high levels of CO2
and temperatures that may have been as high as 70C, as the earth cooled and
the oceans formed, so this carbon was firstly dissolved in the oceans and then
laid down as carbonate rocks.
In between the early atmosphere and the present
day, atmospheric carbon dioxide has fluctuated considerably and so has the temperature
of the earth. At times the equator has been almost uninhabitable for most
plants and animals on land due to to high temperatures while the poles have
been very productive. At other times, the equator has been tropical and lush
as it is now, while the poles are ice-bound and barren. It is not just carbon
dioxide that has had this effect on the climate, though it has a significant
part to play.
Sun spots and the position of the earth in space relative to the sun (the
Milankovitch cycles) occur in cycles and affect the amount of solar radiation
reaching the earth and so the temperature and climate. Other factors such as
volcanic activity and the reflectivity of the surface due to snow/ice cover
have their effects too so making the understanding of why the temperature at
any particular time in the past not straightforward, particularly in estimating
the point in the various cycles.
However - if we could have been there in the
historical or pre-historical past and measured the various factors in place,
the relative part played by each of them would be far easier to explain which
factor is having what effect.
If there wasn't a natural greenhouse effect,
then the average surface temperature of the earth would be about -18C rather
than the current +15C (ref)
so some degree of greenhouse effect is vital for life on earth as we know it
to be viable at all.
About a half of all the extra "industrial" carbon
dioxide released has been produced in the last 20 years or so.
The involvement of Antarctica in the carbon cycle
Antarctica is involved in the carbon cycle in
the ways that other ecosystems in the world are, notably in the absorption of
carbon dioxide by plants (in Antarctica, these are overwhelmingly phytoplankton
in the oceans), the production of carbon dioxide by the respiration of plants
and animals and also the decay of dead plants and animals.
As the Antarctic ecosystem is largely a marine
system the involvement of Antarctica in the carbon cycle is the story of the
role of the Southern Ocean.
The Southern Ocean has a major role to play in
locking away (sequestering) anthropogenic (due to the actions of man) carbon
dioxide. To start with gases dissolve more readily in cold water than they do
in warm water, so cold Antarctic waters can hold more of the dissolved gas.
the Antarctic continent oceanic water upwells bringing with it large amounts
of dissolved minerals which along with long hours of daylight in the austral
summer leads to the huge blooms of phytoplankton that drive the very rich marine
ecosystem. There is also a huge amount of water around Antarctica that cools
and sinks, some of it is the same water that has upwelled when it had less carbon
dioxide dissolved in it. Upon exposure to modern higher levels of atmospheric
carbon dioxide this water takes in more than it previously held and carries
it to the ocean depths, so removing anthropogenic carbon dioxide as it
form "Antarctic Bottom Water".
The archetypal Antarctic animal (at least to
biologists) is krill Euphausia superba, this is the source of food for
all sorts of animals such as whales, seals, penguins and a whole host of other
birds. Krill feed on phytoplankton by filter feeding at the surface where the
phytoplankton are found, but this puts them at danger from predation. It has
long been known that krill migrate to the surface and then to deeper levels
in the ocean when not feeding to put themselves out of the line of danger unnecessarily.
Recent research has shown that rather than making the surface / deep cycle once
every 24 hours, the krill are doing so up to three times a day. They swim to the
surface (krill naturally sink unless swimming) to feed and then when they are full,
allow themselves to slowly sink out of the danger zone (they actually "parachute"
down as they spread out their swimming and feeding appendages).
This is significant as when the krill sink they
pass below the level of the mixed surface layer of the sea to the region where
little or no mixing takes place with the surface layers. As they pass to this
layer they release faeces which rather than becoming recycled by decomposers
and being part of the surface carbon cycle, become sequested in the ocean depths
and so less likely to be released quickly back into the atmosphere.
It is estimated that 2.3 x 107 tonnes
of carbon (23 million tonnes) are locked away below the mixed layer by krill
in this way every year. This is an extra 8% of sequestration on a global scale
of all sources. This is the annual equivalent carbon output of about 35 million
Global Warming |
GW Antarctica |
Carbon sinks | Carbon
cycle | Prevention |
Carbon Offsetting |