The greenhouse effect is a natural process. Sunlight passes through the atmosphere, warming the Earth’s surface. In turn, the land and oceans release heat, or infrared radiation, into the atmosphere, balancing the incoming energy. Water vapour, carbon dioxide and some other naturally occurring gases can absorb part of this radiation, allowing it to warm the lower atmosphere.

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Since the industrial revolution and expansion of agriculture around 200 years ago, we have been raising the concentration of carbon dioxide gas in the global atmosphere. Levels of other greenhouse gases have also increased because of human activities.

Higher concentrations of greenhouse gases in the Earth's atmosphere will lead to increased trapping of infrared radiation. The lower atmosphere is likely to warm, changing weather and climate.

Thus, the enhanced greenhouse effect is additional to the natural greenhouse effect and is due to human activity changing the make-up of the atmosphere. (The enhanced greenhouse effect is often referred to as global warming.

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What’s the difference between the enhanced greenhouse effect and ozone depletion?

Ozone depletion is a different environmental problem from the enhanced greenhouse effect. However, ozone depletion is also caused by changes to the atmosphere caused by humans.

Ozone depletion has been happening since the late 1970s. It is caused by CFCs and halons, industrially produced chemicals used in the past for refrigeration, plastic making and fire fighting. Once in the atmosphere, these chemicals destroy ozone in the stratosphere, 20 to 30 kilometres above the ground. This is the ozone layer, which stops much of the sun’s harmful ultraviolet radiation reaching us.

Damage to the ozone layer means that over much of the planet, more ultraviolet radiation reaches the ground than in the past.

Both the greenhouse effect and ozone depletion are due to chemicals released into the air by people’s activities. Another similarity is that CFCs are ozone destroyers and greenhouse gases.

In a curious turn of events, the warming effect of CFCs is offset by the fact that they destroy ozone, also a greenhouse gas, in the lower stratosphere.

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Is greenhouse just a theory?

The way in which these increases will affect our future climate is, and can only be, the result of theoretical calculations.

However, there is unequivocal evidence that greenhouse gases are increasing in the atmosphere. Since the industrial revolution the level of carbon dioxide alone has risen from approximately 280 ppm (parts per million) to approximately 360 ppm. This will have an effect on the world's climate. What is not clear is the exact magnitude of that effect.

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Isn’t greenhouse just part of a natural cycle?

The greenhouse effect is a natural phenomenon, but the extra gases produced by human activity are making it stronger.

We are now adding to these gases faster than oceans and plants can absorb them — the greenhouse effect is being ‘enhanced’ by humans. There is strong evidence that recent changes are unprecedented and not due to natural causes.

When considering how climate will be affected, we need to be mindful that global warming due to the enhanced greenhouse effect will be in addition to the natural fluctuations of climate.

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Greenhouse gases

What are greenhouse gases?

Atmospheric trace gases that keep the Earth’s surface warm are known as greenhouse gases. About three-quarters of the natural greenhouse effect is due to water vapour. The next most significant greenhouse gas is carbon dioxide. Methane, nitrous oxide, ozone in the lower atmosphere, and CFCs are also greenhouse gases.

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How do we know what is happening to concentrations of greenhouse gases in the air?

For many years, researchers have been measuring the make-up of air so they can monitor changes.

CSIRO collects extensive data on atmospheric composition from the remote Cape Grim Baseline Air Pollution Station in Tasmania as well as from observatories around the world. The Station is the foremost facility of its type for monitoring pollutant levels in southern hemispheric air. It is operated jointly by the Australian Bureau of Meteorology and CSIRO.

For a far longer record of atmospheric make-up, CSIRO researchers extract air from ice cores supplied by the Australian Antarctic Division. Analysis of the air reveals changes to the composition of the atmosphere dating back thousands of years.

At CSIRO Atmospheric Research, air samples are analysed in the Global Atmospheric Sampling Laboratory (GASLAB). Results from GASLAB help us determine levels of greenhouse gases, where they are coming from and what happens to them once they are in the atmosphere.

How much have greenhouse gas concentrations increased?

The concentration of carbon dioxide is approximately 30 per cent greater than it was in the 18th century, before the industrial revolution. It has increased from around 280 parts per million (ppm) to approximately 360 ppm today. Although carbon dioxide comprises only 0.036 per cent of the air, its warming effect is significant.

Methane levels have risen from a pre-industrial concentration of about 700 parts per billion (ppb) to 1700 ppb. However, the rapid growth of methane has slowed considerably since the 1980s.

Nitrous oxide concentrations have increased from approximately 275 ppb to 315 ppb.

There is strong evidence that ozone concentrations in the lower atmosphere are greater than in pre-industrial times, especially in the northern hemisphere.

CFCs didn’t exist 200 years ago. However, the concentrations of many of them are now starting to fall, thanks to international agreements to protect the ozone layer.

Human activities do not directly change atmospheric water vapour concentrations. However, changes to water vapour concentrations may occur in response to increases in concentrations of carbon dioxide and other greenhouse gases.

Carbon dioxide concentrations in the atmosphere during the past thousand years, from measurements of air trapped in Antarctic ice (supplied by the Australian Antarctic Division) and, since the late 1970s, from analysis by the Cape Grim Baseline Air Pollution Station.

Methane concentrations in the atmosphere during the past thousand years, from measurements of air trapped in Antarctic ice (supplied by the Australian Antarctic Division) and, since the late 1970s, from analysis by the Cape Grim Baseline Air Pollution Station.

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Where do greenhouse gases come from?

Most of the increase in carbon dioxide comes from burning of fossil fuels such as oil, coal and natural gas for energy, and from deforestation.

Cows, sheep and other ruminant animals ‘burp’ methane into the air. Rice paddies also generate methane. Other sources of methane are landfills, burning vegetation, coal mines and natural gas fields.

Nitrous oxide concentrations are increasing because of changes to the way in which we use land, from fertiliser use, from some industrial processes, and from burning vegetation.

Ozone is a component of photochemical smog, which, in turn, is the result of emissions of hydrocarbons and nitrogen oxides from motor vehicles and industry.

CFCs were made in the past for refrigerants, spray pack propellants, producing foam plastics and as solvents for electronic components. All developed countries, including Australia, have stopped producing CFCs.

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How long do the greenhouse gases last in the atmosphere?

Carbon dioxide persists for more than a century in the air. Methane’s average lifetime is about 11 years.

Nitrous oxide and some of the CFCs stay in the air for more than a century.

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Are all greenhouse gases equally as effective at trapping heat?

No. Greenhouse gases differ in their ability to trap heat. A kilogram of methane released into the air today, for example, will lead to about 20 times more atmospheric warming over the next century than a kilogram of carbon dioxide.

Molecule for molecule, methane, CFCs and nitrous oxide are more potent greenhouse gases than carbon dioxide.

In order to compare the heating effect of different greenhouse gases, scientists have calculated a global warming potential for each one. The global warming potential takes into account:

•  the amount of radiation that the gas absorbs and the wavelength at which it absorbs

• the time that the gas stays in the atmosphere before reacting or being washed out by rainwater.

• the current concentration of the gas in the atmosphere

• any indirect effects of the gas. For example, methane will produce ozone gas in the lower atmosphere and water vapour in the stratosphere.  

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How much greenhouse gas does Australia produce?

In 1996, Australia produced the equivalent of 419 Mt of carbon dioxide. (The actual release of carbon dioxide wasn’t this amount, but total greenhouse gas emissions in 1996 would make the same contribution to heat trapping as 419 Mt of carbon dioxide.)

Energy and transport sectors combined produced 86 per cent of carbon dioxide emissions in 1996.

For more information, visit Highlights from the National Greenhouse Gas Inventory 1996

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Past climate and sea level

Has climate changed in the past 100 years?

The average surface temperature of the world is now somewhere between 0.3° and 0.6°C higher than it was late in the 19th century. The past 40 years is the period with the most reliable temperature measurements. During this time, average world temperature has risen by 0.2°C to 0.3°C.

Both air over land and over the oceans has warmed, although warming has not been the same everywhere. Northern hemisphere continents between 40°N and 70°N have warmed more than elsewhere. A few areas, such as parts of the North Atlantic Ocean and some surrounding land areas, have cooled in recent decades.

1998 was the warmest year on record globally. Ten of the past 12 years have been warmer than any previous years on record.

In 1998 Australia recorded its highest ever annual mean temperature since high-quality data records began in 1910. The Australian mean temperature for 1998 was 22.54°C, 0.73°C higher than the average for the Australian Bureau of Meteorology’s 1961 to 1990 reference period.

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What information are satellites giving us about temperature changes?

Scientists have studied the air temperature recordings made by satellite instruments since 1979, looking for any trends. The satellite measurements appear to show a slight cooling of the world’s atmosphere — about -0.06°C per decade. This is despite the global warming trend shown by surface measurements of temperature.

However, if short-term events such as El Niño and volcanic eruptions are taken into account, both satellite measurements of the lower atmosphere (the troposphere) and recordings at the surface show slight warming since 1979.

In addition, both weather balloons and satellites show that the stratosphere (the layer of the atmosphere from about 12 to 50 kilometres above the ground) is cooling. This is a change that scientists expect to happen as levels of greenhouse gases increase and the ozone layer thins.

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Are humans responsible for changes to our climate?

It is difficult to distinguish natural variability in climate from human-induced climate change. However, the pattern of temperature changes observed over land and sea and through the atmosphere are consistent with predictions from global climate models of the combined effects of increasing concentrations of greenhouse gases and of other human activities.

Scientists are becoming more and more convinced that human actions are in part responsible for changes to global climate in recent decades.

However, whether or not greenhouse-induced warming has been detected, the scientific prognosis is that such warming will occur.

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What about the ‘heat island’ effect?

Some people have claimed that measurements of global temperatures have been distorted because a number were made in cities where local temperature rises have been caused by urban development.

Climatologists have long recognised the urban heat island effect, and have allowed for it in their assessments. Ocean temperatures, which cannot be affected by the heat island effect, also show global warming, even to depths of 1000 metres. Other evidence of warming is available from tree rings, ice cores, boreholes and glacial retreat.

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Has sea level changed since 1900?

During the past 100 years, global average sea level has risen by between 10 and 25 cm. However, we have no evidence to associate this increase with global warming.

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Future changes to climate and sea level

What impact will rising greenhouse gases have on climate?

Increasing levels of greenhouse gases are likely to produce a warming at the Earth’s surface. This warming is likely to lead to world-wide changes in weather and climate. Some places may get more rain and storms while others may get less. Not all changes will be bad for everybody. However, almost everywhere the weather and climate will be different from what it used to be.

By the end of the 21st century, according to the Intergovernmental Panel on Climate Change, average world temperatures are likely to be between 1.0°C and 3.5°C higher than they were in the year 1990. The mid-range estimate for the year 2100 is a warming of about 2°C.

Average rainfall across the globe is likely to increase, particularly during winter in high latitudes.

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How will Australia’s climate change in future?

By the year 2030, our average temperatures are likely to be higher than today. Northern coastal regions may experience an increase of from 0.3°C to 1°C. Southern coastal regions may warm by between 0.3°C and 1.3°C. Inland areas are likely to warm slightly more than coastal regions.

(We present a range of likely temperature increases in an attempt to show both the uncertainty of future greenhouse gas emissions and hence the growth of atmospheric concentrations, and the uncertainty of how much warming will be associated with particular concentration changes.)

Changes to rainfall are much harder to predict than temperature. By the year 2030, winter rainfall in southern Australia may decrease by up to 10% (or increase by up to 5%). Summer rainfall over Australia may increase or decrease by up to 10%.

More hot days are expected, and fewer frosts and less snow. In regions where rainfall increases, more floods are anticipated. In parts of the country where rainfall remains the same or decreases, increased evaporation may lead to drier conditions.

Several climate models suggest that where rainfall increases there will be more extreme rainfall events. At this stage it is unclear whether the distribution and frequency of severe storms such as tropical cyclones will change.  

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CSIRO’s climate change scenario

Most climate models indicate that in many places global warming is likely to increase the frequency and duration of extreme events such as heavy rains, droughts and floods.

We don’t know what impact global warming will have on the frequency and severity of El Niño events. It is these events that are so often responsible for devastating droughts in Australia.

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What will happen to sea level?

By the year 2030, the average world sea level is likely to be between 5 and 25 cm higher than now. By 2100, sea level is projected to rise by approximately 15 to 100 cm, compared with today, with a ‘best estimate’ of about 50 cm.

The rate and magnitude of sea-level change will vary from place to place in response to coastline features, changes in ocean currents, differences in tidal patterns and seawater density, and vertical movements of the land itself. In some areas, sea level may actually fall. For much of the planet though, sea levels are expected to continue rising for hundreds of years after atmospheric temperatures stabilise.

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Why will sea level rise?

If the Earth's atmosphere warms, the upper layers of the oceans will also warm. Like most substances, water expands when heated. Expansion will raise sea level.

Land-based ice in temperate regions such as South America and North America and Greenland will melt more rapidly. Glaciers may retreat. Melting also contributes to increased sea level.

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What’s happening to Antarctica?

Overall, Antarctica is not warming significantly. Only the Antarctic Peninsula is warming throughout the year at a rate that statisticians call ‘significant’.

Ice shelves, such as those in the Antarctic Peninsula, float and will not change sea level if they disintegrate or melt. (You can check this by adding an ice block to water in a glass. Mark the height of the water on the glass and then see what happens to the height after the ice melts.)

Global warming may even lead to increased precipitation over Antarctica and Greenland, which would lock water away in the ice caps. This may offset some of the sea-level rise caused by thermal expansion of water.

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International agreements

Are there any moves to limit global warming?

Australia is a signatory to and has ratified the 1992 United Nations Framework Convention on Climate Change, which is now international law. The objective of this Convention is to stabilise concentrations of greenhouse gases in the atmosphere at a level that would ‘prevent dangerous human interference’ with global climate.

Australia has also signed the 1997 Kyoto Protocol, which will become international law if sufficient countries ratify it over the next couple of years. The Kyoto Protocol will bind many developed nations to greenhouse gas emission targets. The Protocol aims to cut emissions from developed countries by about 5% from 1990 levels by the year 2012.

However, the Kyoto Protocol target will not lead to stabilisation of carbon dioxide in the atmosphere. The target represents only the first step towards meeting the objectives of the Framework Convention on Climate Change.

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The latest research

How do we know what the make-up of air was like in the past?

Scientists have been regularly measuring the amount of carbon dioxide in air since the late 1950s. We have been monitoring air in the southern hemisphere since the early 1970s.

In fact, CSIRO Atmospheric Research is the only laboratory in the world with a collection of ‘vintage’ air. The collection, held in stainless steel flasks, dates back to the first samples of pristine "baseline" air collected at the Cape Grim Baseline Air Pollution Station in Tasmania in 1978.

To go back further in time, scientists study air trapped within Antarctic ice.

Snow falling in polar regions such as Antarctic continuously traps tiny pockets of air. More snow lands on top and after a while the enclosed air forms a bubble in the ice. In this way, air is preserved for thousands of years. Ice deep below the surface has older air trapped in it than ice at the surface. Thanks to polar ice, scientists can analyse air dating back more than 300,000 years.

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How does haze in the air affect global temperatures?

Haze is caused by fine pollutant particles and droplets suspended in air.

The best known impact of these particles, called aerosols, is the white haze of pollution visible over heavily industrialised areas of the northern hemisphere, and to a lesser extent over Melbourne and Sydney on high pollution days. This haze reflects some sunlight back to space, and can have a small, but significant, cooling effect on climate.

Aerosols can also make clouds brighter and last longer, causing them to be more reflective than normal. This is also likely to cool the planet in some regions.

However, the cooling effect of aerosols is largely restricted to the more polluted regions, whereas greenhouse gases are well mixed throughout the entire atmosphere.

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How do scientists work out what the climate is going to be like in future?

Scientists use sophisticated computer models of the world's atmosphere, surface and oceans to examine likely future changes to climate due to global warming.

Climate models are complex, lengthy computer programs based upon the physical laws and equations of motion that govern the Earth’s climate system. The models work by mimicking (or reproducing) the way in which the Earth's climate behaves from day to day, and from season to season. They do this for all parts of the globe: the surface, throughout the atmosphere, and for the depths of the oceans.

Climate models are good at simulating the broad features of our present climate. Simulated distribution of surface temperatures, winds and precipitation over the seasons are very similar to what is observed. This gives us confidence that the models adequately represent the important physical and dynamic processes of climate.

Using these climate models, scientists can simulate present climatic conditions (‘control’ runs). They can also simulate anticipated future conditions, such as increased atmospheric concentrations of greenhouse gases, changes to aerosol levels or different ozone levels (‘climate prediction’ runs). By comparing results from the two (or more) simulations allows scientists to assess likely future climate changes.

Scientists also study changes that have happened throughout history on geological timescales when greenhouse gas concentrations were higher than today to learn about what may happen in future.

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What greenhouse work is CSIRO Atmospheric Research doing?

The Division studies changes to greenhouse gas concentrations in the atmosphere as well as determining past changes to the make-up of air from bubbles trapped in ice cores.

We are also using powerful scientific tools to establish where greenhouse gases are coming from and what happens to them once they reach the atmosphere.

Divisional scientists also study the way in which the atmosphere, land surfaces and the oceans interact to determine our climate. The research involves satellite remote sensing and aircraft measurements, theory and numerical models and underpins development of more advanced climate models.

We are examining clouds and cloud processes and the interaction of clouds and radiation. For this activity, we use data from satellite and ground-based remote sensing instruments.

We have developed powerful computer-based global and regional climate models, linking models of the atmosphere, biosphere, oceans and sea-ice.

By evaluating and applying the latest scientific findings and model results, we also produce scenarios and assessments of likely climatic changes and their impacts for various regions in Australia and overseas. Of particular interest are future changes to rainfall, the incidence of droughts and floods, tropical cyclone behaviour, evaporation rates and sea level.

Research is performed in close collaboration with a number of other CSIRO Divisions, with the Bureau of Meteorology, and with universities, in particular with Monash University via the Co-operative Research Centre for Southern Hemisphere Meteorology.

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