Home > Research centre > 2. Causes of climate change
Throughout history the Earth's climate has changed. Usually such changes are slow, but on some occasions it can be disastrously fast. An explanation for the extinction of the dinosaurs is that a large meteorite struck the Earth and changed the climate very quickly.
Another disaster may be looming as human activity increases greenhouse gases at a much faster rate than at any other time in history. Everything known about climate tells us that increasing greenhouse gases will lead to a large change in the Earth's climate.
Questions you may want to consider:
- Why are greenhouse gases increasing?
- What are the major greenhouse gases being released into the atmosphere?
- What makes human release of greenhouse gases different to past climate change?
- How is Australia contributing to climate change?
The fossil fuels
We need energy to make things work: switch on a light and it uses electricity; light a gas heater and it uses natural gas; travel in your family's car and it uses petrol.
Almost all of the energy we use in our daily lives comes from fossil fuels – coal, natural gas and petrol.
Fossil fuels were formed many millions of years ago from the remains of plants and animals placed under extreme heat and pressure in the Earth's crust. They get their name from the fossils that can often be found in coal seams.
Fossil fuels contain carbon. When they burn, heat energy is released and the carbon combines with oxygen to form carbon dioxide. Carbon dioxide is a major contributor to the greenhouse effect.
The burning of fossil fuels also produces a wide range of air pollutants – carbon monoxide, sulfur dioxide, nitrogen oxides and many others.
So we are facing a dilemma with our use of fossil fuels. We depend on them in our daily lives, but it comes at a cost to our environment. What can we do about it?
We can be careful about using fossil fuels, or we can use renewable energy sources such as wind, solar and hydroelectric power that produce no greenhouse gas or air pollutants. Nuclear power is also 'clean' in regards to air pollution, but there are significant difficulties associated with the safe mining of uranium and disposal of nuclear wastes.
Which of the fossil fuels generates the most carbon dioxide? Different fossil fuels have varying numbers of carbon and hydrogen atoms in their molecules. When they burn, they release different amounts of carbon dioxide per unit of heat energy.
Typical values of greenhouse gas emissions of carbon dioxide for each megajoule (MJ) of energy generated by the different fuel types, and their energy contents are shown in the following table.
Typical values of greenhouse gas emissions of carbon dioxide
|Energy type||Amount of energy||Greenhouse gas emissions (kg of CO2 per MJ)|
|electricity from black coal (NSW)||1 MJ||0.294|
|electricity from black coal (Qld)||1 MJ||0.288|
|electricity from black coal (SA)||1 MJ||0.272|
|electricity from brown coal (Vic)||1 MJ||0.363|
|natural gas||1 MJ||0.069|
|unleaded petrol||1 MJ||0.077|
|solar heating||1 MJ||0|
|wind power||1 MJ||0|
|biofuels methane generation||1 MJ||0.005|
As you can see in the table on the previous page, the burning of coal to generate electricity produces the greatest amount of carbon dioxide and natural gas, the lowest (of the fossil fuels). The generation of electricity in Victoria produces the largest greenhouse gas emissions of the four states because of the use of brown coal.
Also, the greenhouse factor for wood burnt in a combustion heater is very low because the carbon that is released by burning has only been removed from the atmosphere quite recently (as the tree grew).
Other factors in determining greenhouse emissions include electricity, oil, LPG, petrol, diesel and natural gas.
It is possible to reduce the greenhouse factor of electricity use by up to two-thirds: by more efficient electricity generation, the use of gas instead of coal, the cogeneration of heat and electricity, or the use of renewable electricity sources.
The transmission of electricity over long distances can increase the greenhouse factor because of losses in the transmission and distribution network. Brown coal fired electricity generation (used in Victoria) is less efficient than black coal fired electricity generation (used in other states), resulting in a greenhouse factor up to 30% higher.
Oil, LPG, petrol, diesel
Wasteful flaring (burning) of some oil during extraction and long-distance transport may increase the greenhouse factor. More efficient refining may reduce it by up to 5%. Mixing renewable fuels such as ethanol from sugarcane with petrol or diesel fuel, reduces the greenhouse factor: for example, adding 15% ethanol (if made from renewable energy sources) could reduce the greenhouse factor of the fuel by up to 15%.
Flaring or venting of gas during extraction; release of carbon dioxide which occurs with gas in some gas fields; leakage of gas from gas pipes; and long-distance transport may increase the greenhouse factor by 5 to 50%.
While carbon dioxide is the most important gas responsible for climate change, there are other gases that also affect the amount of heat retained by the atmosphere. Increases in methane, nitrous oxide, chlorofluorocarbons, and similar gases are able to retain even more heat than carbon dioxide. Extra water vapour can also retain heat. Some forms of pollution, including tiny particles in the atmosphere, can have a cooling effect.
When averaged over a 100-year period, methane is 21 times better at trapping heat than carbon dioxide. The main sources are from agriculture (cattle and sheep farming and rice fields), leaking gas pipes and old rubbish tips. In the past 200 years, methane has increased threefold. However the methane eventually breaks down.
Atmospheric nitrous oxide is a result of air pollution from industry, and fertilisers and other agricultural activities.
Chlorofluorocarbons and other manufactured halocarbons
These are among the most warming gases. The gases are used in fridges, spray cans and some fire extinguishers. The amount of these gases escaping is being reduced. CFCs are being phased out so that the ozone in the upper atmosphere is not destroyed. The replacement gases are also powerful greenhouse gases.
A warmer planet will have more water vapour. Depending on the type and extent of clouds, this might add to the warming.
Ozone pollution in the lower atmosphere
The natural ozone in the upper atmosphere protects the Earth from ultraviolet radiation. Ozone in the lower atmosphere exists naturally at around 25 parts per billion, but higher levels can be found in summer smog. Ozone in summer smog has a warming effect.
Air pollution and small particles
Some forms of air pollution and, in particular, tiny floating particles have a cooling effect on the atmosphere. It has been suggested that, without the current amount of air pollution, the change in the Earth's temperature would be even greater.
We have all heard of the Ice Age. Over hundreds of thousands of years the Earth goes through a cycle of getting hotter and cooler. It has been shown that these changes in climate are linked with the amount of greenhouse gases in the atmosphere. There are many theories about what drives these changes.
There is a lot of evidence that the Earth's climate changes over long periods of time.
Geologists have found a great amount of evidence about the changes in climate over the past 2.5 million years. The Earth has gone through several very cold ice ages. Large parts of North America, Europe and Northern Asia were covered in thick ice during these times. Places like Australia had a much wetter interior.
When the ice on the northern continents melted, the oceans rose about 60 metres. The only thick ice sheets left were on Antarctica and Greenland. Over the past 10,000 years the climate has been very stable with only tiny changes in climate up until late in the 20th century.
Scientists have many ideas about why the Earth has cycles going in and out of ice ages and believe there is more than one reason. Theories include that the amount of heat radiated by the sun might change, the Earth's orbit might change, and the atmosphere is affected by volcanoes.
Scientists do know that the Earth's temperature during these natural changes is linked to the amount of greenhouse gases in the atmosphere. They are not certain why the greenhouse gases have changed. It is believed that the changes occur over thousands of years. But the climate change caused by humans may only take 100 years.
Greenhouse gas emissions in Australia
Australia is a relatively small producer of greenhouse gases, accounting for about 1.4% of total global emission. However, our population is small and when emissions are considered per person or by wealth (GDP), our emissions are the highest in the world.
This is because we rely on coal to generate electricity and have energy- and emission-intensive industries such as processing of aluminium, iron and steel, and petroleum products.
Most of Australia's greenhouse gas emissions are produced by the electricity, gas and water sector, followed by the agriculture, forestry and fishing sector.
Percentage of Australia's greenhouse gas emissions by sector 2008
|agriculture, forestry, fishing||20.9|
|electricity, gas, water||36.6|
|transport and storage||7.1|
In 2008, Australia's net greenhouse gas emissions totalled 5.49 X 108 tonnes. Between 1991 and 2008 there was an increase in greenhouse gas emissions by energy (43% increase) and industrial processes (32% increase). The forestry sector is a store or 'sink' of carbon as trees absorb carbon dioxide while they grow. Between 1991 and 2008, the amount of carbon dioxide absorbed by forests remained about the same.
National greenhouse gas emissions – carbon dioxide equivalent – Gg (1,000 tonnes)*