Thursday, March 09, 2006


Global Dimming is the concept of the Earths atmosphere not receiving as much irradiance as in previous years. In other words, there is less light reaching the planets surface. This is evident from lower evaporation rates recorded from evaporation pans on farms. Evaporation occurs when photons of light hit the surface of the water, increasing its energy and causing the water molecule to rise into the air. Light is the key driver, and outweighs temperature, humidity and other factors that influence evaporation.


So if less light is penetrating the Earths atmosphere, it means that less energy is reaching the Earths surface. It could be the result of greenhouse gasses and other aerosols reflecting the light fro the sun, or change in cloud chemistry due to air pollution.


A concern is whether this reduction in light affects the planet. At present, Carbon Dioxide is a major contributor to global warming. It’s source being anthropogenic. With global dimming, less energy is reaching the planet, so less energy in the form of heat is contributing to global temperature. It is therefore considered to balance out global warming. So is global temperature change really such a major issue.


Change in light intensity however could be of major concern. Records of light intensity have only been dated by man, which means the records are fairly recent. This makes it difficult to create models and therefore uncertainty of what future events it may lead to.


Light is the source of all life on the Earth. Primary producers use it to produce simple sugars, which put them at the base of all food chains. With lower or no light energy, the process of photosynthesis would occur at such a slow rate, primary consumers would not be able to survive. So as one link in the food chain is broken, an entire ecosystem is destroyed.


With humans already exploiting resources at current rates, it would only be a matter of time till an issue like global dimming will be fully recognized as a worldwide crisis.


Reviewed by Eugene M Marinus

Anzio Abels
Biodiversity & Conservation Biology Department
University of the Western Cape


Briefly describe what a Global Climate Model (GCM) is?

A GCM predicts weather conditions that prevail for years. They show characteristics weather characteristics that occur over years. They cannot give accurate weather accounts for everyday but give indications of climatic indicators.

They work with the physics equations of motion. There are four different types of climate models:
1. Energy Balance Models (EBM)
2. 1*Dimensional Radiative *Convective Models (RCM)
3. 2 - Dimensional Statistical * Dynamical Models (SDM)
4. 3- Dimensional General Circulation Models (GCM)

The EBM are simple models that deal with the radiation balance, between incoming solar radiation and heat loss, and longitudinal energy transfer. Latitudinal Characteristics are ignored when EBMs are dimensionless. If the model is one dimension then the latitude is included. The RCM can be either one or two-dimensional and the height is also considered. The SDM are two dimensional with both the horizontal and vertical dimension. The horizontal energy transfer of the energy balance model is combined with the radiative- convective functions of the radiative convective models.

The GCMs are the best tool for predicting the response of the global climate system to increasing concentrations of greenhouse gases. They are three dimensional, and involve very complicated calculations of atmospheric functions. The most complex is the coupled atmospheric ocean general circulation models (AOGCMs), because they have ten to twenty layers in the atmosphere and approximately thirty in the oceans. The data collected is taken for the whole globes therefore there are lot calculations that are carried out. This means that it is computationally intensive.

The GCM cannot give a stable equilibrium condition under the current climatic conditions. Therefore flux adjustments have been provided to make sure that can do so. These are really just non-physical correlation constants that are used to make sure models stay in track.
There are many different groups that have been developed and refined GCMs over the years, and the IPCC Third Assessment Report uses 34 AOGCMs. They are developed and operated by 18 different climatological centres

GCMs are used for a better understanding of the climatic processes, and for indicating how the physical characteristics of the earth are interacting. The allow climatologists to make informed and scientifically relevant predictions based on the their understanding and knowledge of climate. They can also help in establishing the age where the human race began when palaeoclimatological data to see the long-term climate effects. They are best tools for climate science and aid conservationists, planners and politicians to test the different response scenarios.

Environmental and Water Sciences


A: Firstly climate is the averaging of the mean day to day effects of natural conditions in an area and changes or is predicted conditions for a long period of time e.g. year to year. Climate may be influenced on a short-term basis by the solar radiation levels, the earths orbit, atmospheres gaseous composition and the vegetation in the area and these short-term changes within the climate is called climatic variation or variability. When short term changes averages and becomes long-term transformation it is than called climate change. Because of changes in the gaseous composition and solar radiation level within the atmosphere long term changes within the climate has taken place, this is called climate change.

Climatologists have used proxy data to determine various factors affecting climate systems in the past. Instrumental measurements of wind, rainfall, humidity, aerosol (atmospheric), and gaseous composition can be used to understand how these factors all contributed to the climate in the past. Temperature tells us exactly how much energy a system has and this is an important factor telling us or giving us information about the past climatic conditions. Proxy data such as Geological, Glacial assessments and sediments be it terrestrial or marine, organic or inorganic are all good resources of information of the past climatic conditions.

The evidence that we have that climate change has taken place from now compared to the past is the change in temperature, glaciers retreating, an increase in sea levels and dissociation and integration of the icesheet. The temperature has increased climatically in some areas and is said to be due to the industrial revolution resulting in global warming owing to the increase in green house gasses namely methane, carbon dioxide, ozone and nitrous oxides. Permafrost which is water which is in the soil water (H²O) that is frozen all year long now has formed large potholes and the frozen bottom of the rivers now flow away, is also another event that shows us air climate has changed.

Glaciers are said to have decreased in surface area now, because when glaciers are measured then we look at the total or net loss/gain divided by the surface area of the glacier. Size and mass are two separate means of measurements used to determine if climate change did occur. When the Larsen-B ice shelf disintegrated, it lost 3250km² when looking now at the shelf it has lost 5700 squared kilometers in total. Four other glaciers have said to melt as well and now the total amount of area of sea ice lost is 13500km squared. The amount of melting taking place or which has taken place over the years can be seen through satellite images. This melting is due to temperature changes owing to global warming. The sea level has increased by 15cm.

Sea ice as well has also started to melt but this does not contribute to sea levels. The melting of sea ice does contribute for the salinity status or profile of water columb (ocean) and also the frequency interactions of glaciers to the open ocean.
Low laying areas such as Bangladesh affected by melting owing to them having flat planes and could suffer because of the increasing sea levels. This melting is influenced by temperature changes.

The increase of the composition of the gases within the atmosphere contributes to the changes in the amount of solar radiation levels that will penetrate our earth. It is said that because of out burning of fossil fuels we are increasing the greenhouse gases within air atmosphere. These greenhouse gases actually absorb all the long wavelengths that were produces by ozone which converts the short wavelengths radiated by the sun. The greenhouse gases then absorbs these long wavelengths and causes an increase in temperature. This increase in temperature may allow certain areas to become hotter and other areas to become colder. Because of us burning fossil fuels we are therefore responsible for this climate changes that has taken place.

Some activities or occurrences were natural but we by out activities are speeding up these processes and are therefore partially to blame.

Reviewed By
Simone Neetling

Angela Campher
Biodiversity and Conservation Biology Department
University of the Western Cape


Climate change is a reality and it has dominated the earths processes
since its creation. When referring to past climate changes it can be
seen that rapid climate changes such as that in Greenland, when vikings
roamed the terrain, there was no anthropogenic factors which caused
cooling and froze the once green terrain. Natural factors such as
emmision of volcanic carbon dioxide can cause a massive effect in terms
of dissallowing solar radiation to over heat the earth. Gases such as
methane, carbon dioxide, nitrous oxides, sulphates and nitrates are all
greenhouse gases which in effect heat the earth. Aerosols such as soot
and dust are also cuased by natural processes. Evidence of the much
feared ''ice age'' has been seen to have occured about about 10 000
years ago, and there was no anthropogenic disturbances then. An example
of climate change in the Western Cape is that of the reduction of fynbos
species, giving way to succulent karro. This is because of the drying
out and the rise of temperature which is sufficient for succulent

However, as far as humans are concerned, their contribution to the
climate change is much greater. Evidence is this is the burning of
foosil fuels which emits larger amounts of gases such as carbon dioxiode
which has a negative effect. An example of this is that about during the
''ice age'' trees grew much faster, but until about 500 years ago, there
has been a reduction in tree growth rate. One hypothesis is that the
cause of drought in Ethiopa was caused by large countries such as the
U.S.A and Britain, emitting dangerous, polluted aerosols. These areas
around which droplets of water arrange themselves did'nt allow the rain
belt from producing rain in the Sahel. Another example is the aeroplane
contrails which can cause a ''pseudo'' cooling effect on the earth.

The crux of the matter is that humans and natural effects can cause
climate change. However, humans have raped the earths resources (fossil
fuels) and in so doing caused an unnatural increase in climate change.
Climate is always changing though but it is up to us to take control and
not exaggerate this deadly change. Things such as global climate models
helps us identify the causes and effects of factors which causes climate
change.One of the major problems the earth is facing today is El nino
bwhich is a disruption in the ocean-atmosphere currents which which
could have detrimental effects on the rest of the world even though it
is situated in the Pacific ocean. It is still unknown whether it is
natural or caused by anthropogenic effects.



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