Changing Oceans
Physical and chemical effects of climate change
A rise in emissions of greenhouse gases have increased
the earth's temperature by almost 1oC in the last 50 years.
Increased air temperatures are quickly translated into warmer oceans.
However, changes are not expected to be uniform around
the globe; a few areas may actually experience cooling. The direct impact
of temperature change on marine life in the open ocean is difficult to
assess and secondary effects of changes in other ocean conditions further
compound this problem. Increases in water temperature have an effect on:
Deep-ocean circulation patterns
Ocean water is moved around the earth by wind generated
surface currents, which cause upwelling and downwelling, as well as by
thermohaline circulation. The latter results from cold, hypersaline and,
therefore, dense water in Polar Regions sinking and moving to the equator
across the ocean floor. Warmer surface waters from lower latitudes then
flow towards the poles. This continuous circulation oxygenates the deep
oceans and redistributes heat from equator to the poles. An increase in
temperature would lead to melting sea ice reducing the salinity and, therefore,
density of polar waters, and could in turn reduce the strength of thermohaline
circulation. Not only would this limit transport of oceanic heat to various
regions of the world but warmer waters in the tropics would hold less
dissolved carbon dioxide further enhancing its build up in the atmosphere.
Wind patterns
Sea surface temperatures affect the patterns in atmospheric
pressure, which in turn are responsible for wind generation. Accelerated
warming of the oceans may produce stronger winds in certain areas, and
increase the frequency of extreme events such as storms and hurricanes.
 Tropical
storms are formed when the sea surface temperature is more than 270C
and heated air rises to create a low pressure area with dense cloud formation
and rainfall. Air from high pressure areas rushes in at high speed to
replace this warm air, and large amounts of moisture are absorbed in a
spiralling effect eventually falling as heavy rain. The threshold temperature
for tropical storms could be reached more readily if the climate continues
to change and such storms could spread to higher latitudes.
Changes in wind generated surface currents would not
only modify the weather conditions for many continents but an alteration
of the upwelling process could have serious effects on the marine ecosystem.
Upwelling is the result of water being pulled away from an area by the
surface current and replaced by water from greater depths. This nutrient-rich
deeper water is vital for primary production and if reduced in certain
areas could seriously affect species distribution and abundance.
Ocean stratification
The oceanic water column is stratified with warmer water
on the surface. In warm, calm conditions this stratification is intensified
and becomes more resistant to mixing by surface winds. This mixing is
actually critical for bringing more nutrients from deeper water to replenish
those being used in the upper layers. In more temperate areas stratification
of the water column occurs during the warmer summer months. When winter
comes the water cools, and together with the action of winter winds, stratification
is broken down. This seasonal cycle is important as winter mixing brings
nutrients from deeper waters to the surface. Phytoplankton can then utilise
the nutrients in spring and summer, once stratification forms to trap
them in the warm waters near the surface. Abnormally elevated summer temperatures
will create greater stratification and winter winds will be less able
to mix these two layers.
Chemical composition
The complexity of factors controlling composition of
oceanic seawater make any predictions with climate change difficult. An
increase in water temperature will have the direct effect of reducing
concentrations of dissolved gases that the oceans can hold. Both dissolved
oxygen and carbon dioxide, vital for all stages of food production and
breakdown of organic matter, could be in shorter supply. The reduced ability
of seawater to hold carbon dioxide would mean more of the gas present
in the atmosphere further exacerbating the temperature rise. Such an impact
may be ameliorated by an opposing effect. Seawater of lower salinity holds
higher concentrations of carbon dioxide than more saline water and, therefore,
a greater input of freshwater from the melting of polar ice, could allow
more carbon dioxide to be dissolved at higher latitudes.
The ability of the oceans to hold dissolved carbon dioxide
modifies the chemical composition of surface waters. Increases in carbon
dioxide are thought to reduce concentrations of aragonite, a form of calcium
carbonate and an important component of the coral skeleton. Growth rates
and skeletal strength may be reduced on coral reefs, and other marine
organisms that incorporate calcium carbonate into their skeletal structures
may be similarly affected.
Seawater is, therefore, a complex, dynamic fluid. It
is made up of pure water, dissolved inorganic salts and gases, dissolved
organic substances, and various microscopic living organisms. Circulation
patterns are responsible for taking all these components to areas where
they are required for maintenance of life systems. A change or cessation
in this circulation, as a result of climate
change, could drastically alter the distribution of these elements, which
could have knock-on effects for food production in the sea.
El Niño Southern Oscillation
(ENSO) and North Atlantic Oscillation (NAO)
In addition to the continuous or seasonal patterns in
ocean-atmosphere dynamics there are a number of more complex processes
which drive natural variation in climate over longer and less predictable
time-scales. Best known among these is the El Niño Southern Oscillation
(ENSO). ENSO is caused by a naturally occurring oscillation of atmospheric
pressures in the Pacific Ocean that weakens the trade winds. These trade
winds normally move warm water away from the eastern Pacific, creating
an upwelling, but in their reduced state, warm water remains in that region
along the equator and nutrient concentrations are lowered. ENSO events
have occurred on average every 2-8 years for the past several thousand
years and some of the effects on the oceans are thought to be similar
to those predicted for climate change. The frequency and duration of these
events appear to have increased over the last few decades and computer
models suggest that this trend will continue with year-to-year variations
becoming more extreme. An additional factor to consider is that the "natural"
fluctuations associated with ENSO events may appear to worsen against
a background of rising base-line temperature.
Climate variability over the Atlantic basin is associated
with the North Atlantic Oscillation (NAO). The two phases, negative and
positive, are reliant on the relative strengths of the subtropical high
to the Icelandic low pressure system. The negative phase shows a weak
subtropical high and weak Icelandic low and the reduced pressure gradient
results in fewer and weaker storms crossing the Atlantic. Over the last
30 years there has been a trend towards a more positive phase which exhibits
a stronger than usual high pressure and deeper than normal Icelandic low.
These positive phases have brought with them stronger winter storms across
the Atlantic. Although these storm increases have been linked to climate
change there needs to be a greater understanding of the physical mechanisms
that govern the NAO before any real conclusions can be drawn.
Changing Oceans introduction
Physical effects of ozone depletion and enhanced ultraviolet
radiation
Effects on Biodiversity
Effects on important ecosystems
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