Home > Research centre > 6. Understanding climate
The climate in a region controls what we can do and how we live. Plants and animals are even more dependent on their local climate.
Questions you may want to consider:
- What is climate and weather?
- How does the Earth's atmosphere make it possible for complex life to exist?
- How has the carbon cycle stabilised climate?
- What is the role of greenhouse gases in the atmosphere?
- Why is there confusion about greenhouse gases in the lower atmosphere and ozone in the upper atmosphere?
Climate and weather
The weather seems to be different every day. But there are patterns to the weather if you look at it over 12 months. By collecting the information about weather over many years, we get to understand the climate. A change in the Earth's climate has a dramatic effect on the daily weather. There can be more or less rain, and events such as cyclones happen more often.
Understanding cycles in the weather has always been important. For nomadic hunter gatherers, the cycles influenced what was available to eat. They used signs in the environment to know when to locate and use resources. Farmers have always needed to know when to plant different crops.
Every day, meteorologists collect information about the weather from different locations. They measure many things including the highest and lowest temperatures and rainfall. They take measurements of changing air pressure and the strength and direction of the wind. They may use balloons to take weather measurements high up into the atmosphere.
Climate is the averaging of all the weather over many decades for a specific region. A region's climate is influenced by its proximity to the equator or poles. It is affected by altitude, how close it is to the sea or other large bodies of water, the winds, the amount of rainfall and its spread of rainfall over the year, and even by the type of vegetation coverage.
One or two degrees increase in the climate does not sound much. However, in southern Australia this would triple the number of days of extreme bushfire risk and in Europe it would triple the number of heatwave days that have previously caused many deaths. A decrease of two degrees could plunge the Earth into another ice age.
Geologists have collected a lot of information about past changes in climate. They believe these changes take thousands of years. The climate change we are experiencing now is happening much, much faster than in the past.
A thin layer of gases called 'the atmosphere' surrounds our Earth. These gases are held close to the Earth's surface by gravity. As a result, air is heavier near the Earth's surface and most of the atmosphere is found in the lowest 20 km. This is called the troposphere. This is the part of the atmosphere that is responsible for climate and is where the greenhouse effect occurs.
The major gases found in the atmosphere are nitrogen, oxygen and water vapour.
Major gases in the lower atmosphere, by weight (excluding water vapour)
Profile of the atmosphere
Three-quarters of the mass of the atmosphere is located below the height of Mount Everest (8,840 metres). No wonder climbers have trouble breathing at that altitude!
Ninety-nine per cent of Earth's atmosphere lies within only about 80 kilometres of the Earth's surface but some traces of the atmosphere are still found at 1,000 kilometres above the surface.
The atmosphere consists of a number of layers – these are defined by the temperature ranges within them.
|troposphere||Where weather and life occurs. Average surface temperature 15 °C, decreases about 6.5 °C per km to about –55 °C at the tropopause. This layer contains about 75% of the total mass of the atmosphere.|
|tropopause||A boundary between the layers. About 8–18 km from Earth's surface, depending on latitude. The troposphere and stratosphere are distinct layers of the atmosphere, very little mixing occurs between them, except during extreme thunderstorms.|
|stratosphere||The temperature increases in this layer from –55 °C to 0 °C. There is a strong, steady wind and the air is very dry. The ozone layer is part of the stratophere.|
|stratopause||A boundary between the layers. About 50 km from the Earth's surface.|
|mesosphere||Here the temperature decreases with altitude again, and can drop below –100 °C. This is where we see 'falling stars', which are meteors that are burning up in the atmosphere.|
|mesopause||A boundary between the layers. About 80 km from Earth's surface.|
|thermosphere||The temperatures in this layer are quite variable. Energy from the sun is absorbed and bounced back into space. Using a strict definition of temperature (in terms of molecular energy), this layer can be extremely hot (hundreds to thousands of °C). In practice, there are so few molecules that you would not feel any of this heat; in fact you would probably freeze.|
Watch Chapter 4 of the animation, Greenhouse for a visual description of the atmosphere.
The carbon cycle – the many faces of carbon
Carbon is central to life on Earth. It is a building block of many chemicals including carbon dioxide, glucose and other sugars and all fossil fuels.
Carbon, in its many different forms, can be found in all parts of the biosphere – the land, atmosphere and water. Importantly, through chemical and physical reactions, carbon atoms are continuously changing form. At one point in time, a carbon atom may be part of a carbon dioxide molecule floating in the atmosphere. At another time it may have been incorporated into a living organism, and yet still another time it may have become part of a coal seam.
The animation, The carbon cycle provides an entertaining introduction to the carbon cycle.
The carbon cycle consists of a number of pathways. In some, carbon dioxide is removed from the atmosphere, in others it is added. By exploring them you will be able to understand the different processes that influence the carbon dioxide levels in our atmosphere.
Carbon is found in the atmosphere in many forms, but mainly as carbon dioxide (CO2). This means that a carbon atom has two oxygen atoms attached to it. Carbon dioxide is a colourless, odourless and tasteless gas which is denser than air. It makes up 0.038% of the atmosphere (as of 2006). It is slightly soluble in water forming a weak acidic solution when it dissolves.
Carbon dioxide is an important greenhouse gas and it is crucial in keeping the world warm enough for life to exist. However, it is a balancing act as most scientists agree that increasing the amount of carbon dioxide in the atmosphere will lead to global warming.
It is estimated that there are 800 gigatonnes (or 800 X 109 tonnes) of carbon stored as carbon dioxide in the atmosphere (as of 2006). There is, however, continuous movement of carbon between the atmosphere, the land and the oceans.
Overall, the amount of carbon dioxide in the atmosphere is increasing. From the ice record and atmospheric readings, it has been estimated that there has been a 25% increase in the concentration of carbon dioxide over the past 200 years.
Read more about the atmosphere.
An important property of carbon dioxide is that it can dissolve in water. Open a can of soft drink and you can see the escaping carbon dioxide bubbles. It is a natural process, and any open body of water will absorb some carbon dioxide.
The oceans of the world contain large amounts of dissolved carbon dioxide – and they can absorb more under the right conditions. The oceans are called 'sinks' because they can remove some of the carbon dioxide that is added to the atmosphere by human activity. Scientists estimate that the oceans can remove up to 40% of this excess carbon dioxide. This is certainly an important feature of the carbon cycle!
Importantly, cold water dissolves more carbon dioxide than warm water. So the cold oceans in the arctic regions tend to absorb carbon dioxide whereas the warm waters of the tropics release it. Scientists have estimated that the oceans absorb around 92 gigatonnes and release back 90 gigatonnes of carbon every year. This means that the oceans are absorbing slightly more carbon than they are releasing. This helps to offset the extra carbon dioxide given off each year by the combustion of fossil fuels and land use changes.
Plants take up carbon dioxide from the atmosphere during a process called photosynthesis. During the daytime, green plants take in water and carbon dioxide from the environment and, with the help of solar energy and chlorophyll, convert them to sugar (glucose) and oxygen:
6 CO2 + 6 H2O + solar energy → C6H12O6 + 6 O2
Carbon dioxide and water use solar energy to combine to produce glucose and oxygen.
Plants use some of the sugar as energy and the rest is used to provide the raw materials needed for the plant to grow. The carbon, now within the structure of the plants, can become food for animals.
At night time, plants take in oxygen and give out carbon dioxide, just like we do. However, the net balance (day and night) is that plants are a 'sink' for carbon dioxide.
Scientists have estimated that 61.4 gigatonnes of carbon is removed from the atmosphere by photosynthesis each year.
Every time we breathe, carbon dioxide is produced.
Nearly every living thing needs oxygen to help convert food into energy. In the most common process, glucose is converted to carbon dioxide and water, with energy also released. Some of this energy is used to help the organism live, the rest is lost as heat.
This process is called respiration and takes place throughout the life of the organism.
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy
Glucose and oxygen produces carbon dioxide and water, and energy is released.
Scientists have estimated that 60 gigatonnes of carbon is added to the atmosphere by respiration each year.
Combustion of fossil fuels
Small amounts of carbon are temporarily stored within the Earth's crust as fossil fuels. Fossil fuels (coal, natural gas and petrol) 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.
Almost all of the energy we use in our daily lives comes from the burning of fossil fuels. Fossil fuels contain carbon. When they burn, heat energy is released and the carbon combines with oxygen to form carbon dioxide and other products including water.
Scientists have estimated that 5.5 gigatonnes of carbon is added to the atmosphere by combustion each year.
Read more about fossil fuels.
Land use change
The cutting down of trees and the clearing of land for agriculture has two impacts on carbon dioxide levels in the atmosphere. Firstly, it removes trees and vegetation that act as 'sinks' for absorbing carbon dioxide. Secondly, the burning of the timber releases carbon dioxide back into the atmosphere.
A surge of land clearing took place in Australia from the mid-1960s to the mid-1970s. Since then, land clearing has declined, but it is still occurring at a rate of half a million hectares per year (Climate Change: Australia's Second National Report under the United Nations Framework Convention on Climate Change, 1997). Land use change is responsible for the emission of 63,000 kilotonnes of greenhouse gases each year from Australia, about 11% of our total greenhouse emissions (2006 data, National Greenhouse Gas Inventory).
Scientist have estimated that 1.1 gigatonnes of carbon is added to the atmosphere by land use change each year.
What are the greenhouse gases?
The main greenhouse gases are carbon dioxide, methane and nitrous oxide. Although these gases play a big part in keeping the Earth warm, they make up less than 0.05% of the atmosphere.
Carbon dioxide, the most important greenhouse gas, is colourless and odourless. So it can't be seen or smelt. However, like all gases, it can be weighed. A kilogram of carbon dioxide would fill a large family refrigerator. A tonne of carbon dioxide would fill a family home.
The concentration of another greenhouse gas, water vapour, can change very dramatically from place to place and even during one day. It can vary from 0 to 4% of the atmosphere. However, scientists believe that human activity does not significantly effect its distribution in the atmosphere.
The greenhouse gases trap different amounts of heat. For example, a kilogram of methane traps as much heat as 21 kilograms of carbon dioxide, while a kilogram of nitrous oxide traps as much heat as 310 kilograms of carbon dioxide. In calculations of greenhouse gas emissions throughout this resource, different greenhouse gases are expressed in terms of the amount of carbon dioxide which would warm the Earth to the same extent.
Greenhouse gases on the increase
Scientists now know that the concentration of greenhouse gases, especially carbon dioxide and methane, has rapidly increased over the past 100 years.
By taking core samples of ice from the Antarctic and Greenland, scientists have been able to study the small pockets of air trapped in the ice that has been laid down over long periods of time. As well, at Mauna Loa, a remote island in Hawaii, and at Cape Grim in northwest Tasmania, scientists have been taking regular air sample measurements since 1957. As a result they have been able to develop the following graphs that show the increasing concentrations of the greenhouse gases, carbon dioxide and methane.
Global average atmospheric concentrations of five major greenhouse gases – 1978 to 2008
The most important greenhouse gases
Carbon dioxide (chemical symbol CO2) is given off by the burning of fossil fuels such as oil, gas and coal. It is also given off when waste materials and wood are burnt. We burn fuels to release heat energy that can then be transformed for use in activities such as heating, manufacturing and transport. Often the heat energy is used to generate electricity. Carbon dioxide is generated when energy is used to manufacture everything we use. For example, the manufacture of one tonne of aluminium produces up to ten times more carbon dioxide than the manufacture of 1 tonne of steel. Some chemical processes, such as occur during the manufacture of cement (used in concrete), also produce carbon dioxide.
Another important source of carbon dioxide is from plants and animals. Nearly every living thing takes in oxygen and releases carbon dioxide as a waste in the process of respiration. For example, in three years you would breathe out about one tonne of carbon dioxide.
Fortunately for us, this production of carbon dioxide is balanced by plants which take in carbon dioxide and water as the raw materials they need to make their food and energy (sugars) using the sun's energy. This process is called 'photosynthesis', and it acts as a 'sink' to remove carbon dioxide from the atmosphere.
Like most gases, carbon dioxide is invisible, but it still has weight. One kilogram takes up the same space as a family refrigerator (about 500 litres). A tonne of carbon dioxide would completely fill a family home from floor to rooftop (about 500 cubic metres).
Methane (chemical symbol CH4) is the main constituent of natural gas, which we use in our homes for heating and cooking. Methane is produced when plant and animal remains or wastes break down under special conditions. This happens at rubbish tips and landfill sites, and even under water like in rice paddies and swamps. Methane can also be produced when wood is only partly burnt. This often happens in bushfires and open fireplaces.
Another source of methane is from grazing animals (known as ruminants) such as cattle and sheep. Grazing animals have micro-organisms in their digestive systems which break down the grass that they eat. These micro-organisms break down the grasses so the animal gets the energy and nutrients from the grass and the micro-organisms continue to get their food source as well. A side-product of this process is the production of methane released to the atmosphere as they burp and break wind a lot!
Small amounts of methane also leak from faulty gas appliances and damaged pipes, the natural gas extraction process, and from black coal mines.
Nitrous oxide (chemical symbol N2O) is produced in small amounts by burning coal, gas, oil or the incomplete burning of plants. The largest amounts of nitrous oxide are thought to result from our agricultural methods. Soil cultivation and spreading fertilisers can result in chemical reactions that produce nitrous oxide.
Other greenhouse gases
Chlorofluorocarbons (CFCs) have been used in air conditioners and refrigerators; as solvents; and in making foam products, especially packaging. Halons, which are chemically similar, are used in some types of fire extinguishers.
When we think of these gases we usually think of their effect on the ozone layer. CFCs and halons break down the ozone layer and, for this reason, their use is being phased out. Replacements for CFCs, such as hydrochlorofluorocarbons (HCFCs) cause much less damage to the ozone layer, and contribute about a tenth as much to the greenhouse effect as CFCs.
CFCs are excluded from the Australian Greenhouse Gas Inventory and Greenhouse Strategy because they are already being phased out under the Montreal Protocol on ozone-depleting substances.
Water vapour is a very significant greenhouse gas. There is a lot of water vapour in the atmosphere and it is good at trapping heat. However scientists believe that human activity does not significantly affect the distribution of water vapour in the atmosphere.
A number of other gases also make significant contributions to the greenhouse effect. They include:
- carbon monoxide from farming, forestry and energy use, mostly in cars
- oxides of nitrogen from fuel burning, especially car engines, and agriculture
- perfluorocarbons, mainly released during aluminium production
- ozone gas in the lower atmosphere.
The amounts of these minor gases are now being measured by scientists in order to determine how much impact they are actually having. In any case, since these gases are major contributors to air pollution – particularly in cities – efforts are being made to reduce the amounts produced. Measurements for perfluorocarbons have recently been included in inventories, and are believed to make up about one per cent of Australia's contribution to the greenhouse effect.
Comparing greenhouse gases
One way of comparing these gases is by looking at their concentrations. Most of these gases are only found in very small amounts compared to the important gases like nitrogen, oxygen and even helium. As a result, their concentrations in the atmosphere are often given in parts per million (ppm) or even parts per billion (ppb).
Greenhouse gas concentrations
|Greenhouse gas||Chemical symbol||ppm|
|carbon dioxide||CO2||388 ppm|
|carbon monoxide||CO||0.2 ppm|
|nitrous oxide||N2O||0.3 ppm|
Carbon dioxide (CO2) is the major contributor to the greenhouse effect. The concentration of carbon dioxide in the atmosphere has increased from 280 ppm before the industrial revolution, to about 388 ppm in 2008.
However, if we only looked at the concentrations or compared the amounts of the greenhouse gases we would not get a true picture of each gas's contribution to the greenhouse effect. The contribution a gas makes to the greenhouse effect depends upon:
- the ability of the gas to absorb heat
- the length of time it persists in the atmosphere
- its concentration in the atmosphere.
A common measure must be used for comparisons. Greenhouse gases are usually expressed as the equivalent amount of carbon dioxide that would cause the same amount of warming of the Earth. For example, the release of one kilogram of CFC-12 (a specific type of CFC) has the same greenhouse warming effect as the release of 8,500 kilograms of carbon dioxide, while a kilogram of methane has the same warming effect as 21 kilograms of carbon dioxide. A kilogram of nitrous oxide has the same effect as 310 kilograms of carbon dioxide.
Scientific explanation of the greenhouse effect
In this resource, the principles underlying the greenhouse effect are explored in more detail.
The animation Greenhouse provides an entertaining introduction to this phenomenon.
- Radiation from the sun mostly passes through the Earth's atmosphere to the Earth's surface, where it is absorbed and then re-emitted as heat (infrared) energy.
- Greenhouse gases do not absorb most wavelengths of the sun's radiation, but do absorb some of the infrared energy emitted from the Earth. After absorbing this energy, the greenhouse gas molecules re-emit it as infrared energy.
- As an outcome of (1.) and (2.), most heating of the atmosphere occurs as re-radiated energy moves outwards from the Earth, rather than from solar radiation passing through the atmosphere on its way to the Earth's surface.
- As a body's temperature increases relative to its surroundings, it radiates more energy to its surroundings.
- The law of conservation of energy dictates that the energy flowing into a system equals the sum of the energy stored and the energy leaving the system. In this case, the amount of solar energy received by the Earth must equal the sum of the energy stored by changing the Earth's temperature and the amount of energy returned to space.
The following discussion considers the roles of one unit of solar energy and one molecule of a perfect greenhouse gas (which absorbs all the heat energy that hits it) in global warming.
No greenhouse gas present
If no greenhouse gases were present in the Earth's atmosphere, solar energy would reach the Earth's surface, be absorbed and converted into heat energy. The energy absorbed from the sun would raise the Earth's surface temperature above that of the surrounding space. As the surface temperature rose relative to that of its surroundings, it would radiate more heat (following principle 4 above), until the rate at which heat left the Earth equalled that at which it arrived from the sun (principle 5 above). So, as one unit of solar energy reached the Earth, a unit of heat energy would leave it.
Calculations have found that if there were no greenhouse gases, the Earth's average surface temperature would be –18 °C, a lot warmer than the temperature of space, but a bit cool for most living things!
Some greenhouse gas present
If a molecule of a greenhouse gas is added to the Earth's atmosphere, it will absorb heat energy leaving the Earth's surface, then re-emit it equally in all directions.
In this situation, up to half of the heat energy leaving the Earth's surface is returned to it by the greenhouse gas. So, one unit of solar energy and half a unit of energy from the greenhouse gas would be received by the Earth, while only half a unit of energy would escape to space. The balance between energy flowing in and out would be upset: the Earth's surface is receiving more energy than it is loses.
So the Earth's surface temperature must rise if the law of conservation of energy (principle 5) is to be obeyed. In turn (see principle 4), the Earth will radiate increasing amounts of energy to space as it heats up, until the balance between energy flows is re-established. This is the greenhouse effect.
More greenhouse gas present
If more molecules of greenhouse gas are present in the atmosphere, less heat energy from the Earth's surface can escape to space, so its temperature will increase until the heat-flows to and from the Earth are again balanced. So, human activities which increase the quantities of greenhouse gases in the atmosphere lead to an increase in the Earth's temperature – an enhanced greenhouse effect.
The real situation is somewhat more complicated because:
- greenhouse gases are not perfect and different greenhouse gases have different warming effects. Global warming may increase the amount of water vapour (a greenhouse gas) in the atmosphere, which would amplify the warming effect. However, this may also create more clouds, which may have both a cooling effect as re-radiated energy moves outwards from the Earth (they reflect more solar radiation), and a warming effect (they also absorb outgoing infrared radiation).Volcanic eruptions produce large quantities of small particles which cause temporary cooling.
- particles emitted by human activity may either have a warming effect or a cooling effect, depending on their chemical composition. Particles containing a lot of 'black carbon' (typically from diesel exhaust) tend to have a warming effect, whereas particles containing sulphate (typically arising from the burning of high-sulphur fuels) have a cooling effect. Variations in the Earth's orbit and cycles of the sun's energy output lead to variations in the amount of solar energy reaching the Earth.
It is easy to see why scientists have to use complicated computer models to simulate the processes that contribute to the warming of the Earth. The complexity of the situation also explains why there is still some uncertainty regarding the amount of warming which may occur as a result of a specific increase in the concentrations of greenhouse gases in the atmosphere. It is even more difficult to predict the regional climate changes which may result. Nevertheless, intensive research is reducing these uncertainties.
Solar radiation, infrared radiation and the greenhouse effect
Most of the sun's radiation passes through the Earth's atmosphere to the surface, where it is absorbed and re-emitted as heat energy of a longer wavelength. This process underpins the greenhouse effect: if the radiation emitted by the Earth was not in the range absorbed by the greenhouse gases, there would not be a greenhouse effect.
The other critical process is that, as the Earth warms up, it radiates more heat until a new balance between energy flows into and away from the Earth is established with the Earth at a higher temperature. If heat losses to space did not increase as the Earth's temperature rose, the greenhouse effect would lead to runaway heating of the Earth to ever-higher temperatures!
The radiation spectrum and the greenhouse effect
The sun's radiation lies mostly within the wavelength range of 0.2 to 2 micrometres. The energy emitted from the Earth's surface falls mostly in the wavelength range of 5 to 25 micrometres.
Why do the sun and the Earth radiate energy at different wavelengths? The hotter an object is, the shorter the wavelength of the radiation it emits to space. The actual distribution of radiation over the range of wavelengths is given by Planck's Equation, while the wavelength (in micrometres) of maximum rate of radiant energy loss was found to be a constant (equal to 2,898) divided by the object's temperature (in degrees Kelvin or K). This is called Wein's Displacement Law. For the Earth (average surface temperature of about 288 K or 25 °C), this equation predicts a peak radiant output at a wavelength of 10 micrometres.
For the sun (surface temperature around 6,000 K), this is a wavelength of 0.5 micrometres. So, the different wavelengths of energy radiated from the sun and the Earth can be accounted for by their different temperatures.
A simple example of this principle is an incandescent globe controlled by a dimmer. With the dimmer turned down low, the globe produces a soft yellow-orange light of relatively long wavelength. As the dimmer is adjusted to increase light output, the temperature of the globe element rises and the light becomes white, as the average wavelength of radiation produced becomes shorter. The amount of energy radiated also increases.
As the radiation leaving the Earth's surface passes back through the atmosphere, greenhouse gases absorb some wavelengths of radiation while allowing others to escape to space. This sets up an imbalance in the energy flows which leads to the warming of the Earth.
Why do hotter bodies radiate more heat?
The Stefan-Boltzmann Law states that the rate at which a body radiates energy to its surroundings is proportional to the fourth power of its temperature relative to its surroundings. So, as greenhouse gases trap more heat and warm the Earth's surface, the Earth radiates more heat until the balance between incoming solar energy and outgoing radiation is regained.
For example, if the Earth's surface temperature rises by 1 Kelvin relative to its existing temperature of about 288 K, its heat loss to space (near 0 Kelvin) will increase by (289)4 ÷ (288)4 which is almost 1.4%.
The greenhouse effect and ozone layer depletion are different
The greenhouse effect and ozone depletion are two different problems occurring at different levels in the atmosphere.
High above the Earth (in the stratosphere) there is a layer of ozone (O3) which blocks most of the sun's harmful ultraviolet radiation. Too much ultraviolet radiation is known to cause skin cancers and genetic damage. This ozone layer is being depleted (or reduced) by CFCs which chemically break down the ozone. As a result, we are not as well-protected from damaging ultraviolet radiation from the sun.
Ozone depletion and the greenhouse effect can sometimes be confused because CFCs are involved in both processes.
The animation Ozone layer provides you with a visual explanation of the differences between these two events.
In the case of the ozone layer, CFCs destroy it by chemical action, and in the case of the greenhouse effect, CFCs help to trap heat by absorbing radiation instead of allowing it to escape into space (see table on the next page).
Summary of differences between the greenhouse effect and ozone layer depletion
|location in atmosphere||low – below 20 km (troposphere)||high – above 20 km (stratosphere)|
Fortunately, CFCs are being successfully phased out worldwide and recent results show that concentrations in the atmosphere have started to taper off or decline.