Geography Reference
In-Depth Information
concentrations in the atmosphere were greater during interglacials
and lower during glacials. As a greenhouse gas the changes in
carbon dioxide concentrations in the atmosphere will have signifi-
cantly affected the Earth's temperature. Carbon dioxide is
exchanged fairly easily between the oceans and the atmosphere. If
there are changes to carbon dioxide concentrations in the ocean
surface waters this may affect atmospheric concentrations which
may then cause the Earth's climate to warm or cool. Plankton (tiny
green plants) in the upper ocean take up carbon dioxide from the
water as part of photosynthesis and this is converted into the plant
material composed of carbohydrates. Dead plankton and other
marine waste may sink to the ocean floor moving the carbon into
the deep. The thermohaline circulation system is driven by tem-
perature and salt concentration gradients and acts as a strong, deep
current that pumps carbon dioxide and nutrients from the surface
of the oceans to the deeper waters and returns them to the surface
again. There are sensitive zones where such downwelling and
upwelling occurs (see Chapter 1). If the thermohaline circulation
acts in its current fashion then the carbon dioxide on the ocean
floors would be stirred up and taken back to the surface. Deep
carbon stores will not be returned to the surface as quickly if the
thermohaline circulation was to slow down. The surface of the
oceans would therefore be depleted of carbon dioxide and less will
be returned to the atmosphere. Overall this process would result in
decline of atmospheric carbon dioxide concentrations as the ocean
plants continue photosynthesis. The energy transfer rates between
the equator and poles would also be affected by changes in the
ocean circulation system.
There is evidence that the thermohaline circulation system slows
down during glacials. One reason suggested for this is the change
in local air currents around thick ice sheets (of several kilometres).
Reduced evaporation in sensitive areas of deep water formation in
the North Atlantic (i.e. wind enhances evaporation, which makes
the water more saline which then makes it more dense and so it
sinks) could slow down the rate of sinking or switch it off and so
the whole deep ocean circulation system becomes more sluggish.
Return of carbon dioxide from the deep to the atmosphere would
slow, since the upwelling that occurs elsewhere would be reduced.
The above theory is not entirely accepted but it does elucidate the
