Greenhouse: questions and answers
The greenhouse effect
Past climate and sea level
Future changes to climate and sea level
The latest research
The greenhouse effect
The greenhouse effect is a natural process. Sunlight passes through the atmosphere, warming the Earths 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.
This absorption of heat, which keeps the surface of our planet warm enough to sustain us, is called the greenhouse effect. Without heat-trapping greenhouse gases, average global surface temperature would be -18°C rather than the current average of 15°C.
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.)
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 suns 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 peoples 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.
Yes and no! The way in which greenhouse gases affect climate is based on observations and scientific interpretations, as is the evidence that human activities have increased concentrations of greenhouse gases.
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.
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.
Natural climatic cycles are well known, for example the 4-7 year El Nino Southern Oscillation, the Interdecadal Pacific Oscillation, and Milankovitch cycles. The latter are driven by wobbles in the Earths orbit every 20,000 (precession), 40,000 (obliquity) and 96,000 (eccentricity) years. The 96,000-year cycle accounts well for the timing of the last six Ice Ages, but the associated changes in solar radiation contribute only 1-2oC of the 5-7oC of cooling experienced in Ice Ages. Therefore changes in other factors amplify the effect of orbital variations. The main amplifiers are natural changes in greenhouse gases and changes in the extent of polar ice-sheets. During the past four glacial cycles, fluctuations in carbon dioxide closely matched the global temperature variations, with carbon dioxide concentrations peaking at about 280 parts per million (ppm) during warm periods and falling to about 180 ppm during cold periods. However, since the 19th century, concentrations have risen to 370 ppm a level unprecedented in at least the past 420,000 years. Other greenhouse gases have also increased rapidly, due to human activities.
Atmospheric trace gases that keep the Earths 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.
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.
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 didnt 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.
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.
Carbon dioxide persists for more than a century in the air. Methanes average lifetime is about 11 years.
Nitrous oxide and some of the CFCs stay in the air for more than a century.
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:
In 2001, Australia produced 528.1 million tonnes of carbon dioxide equivalent -- this was mainly carbon dioxide (69.9%) as well as methane (22.9%), nitrous oxide (6.3%) and other gases. Of Australia's total net emissions in 2001, the production of energy accounted for 68.0%, 19.5% came from agriculture, emissions from industrial processes contributed 4.6%, and waste emissions contributed 3.1%. More details about Australia's emissions is available from the Australian Greenhouse Office's National Greenhouse Gas Inventory
The average surface temperature of the world is now 0.4 to 0.8°C higher than it was late in the 19th century. Most of the warming occurred over two periods in the 20th century: from 1910 to 1945 and from 1976 to 2002. Evidence for global warming is multi-faceted. In addition to the global average surface warming of about 0.6oC since 1900, there has been an increase in heatwaves, fewer frosts, warming of the lower atmosphere and deep oceans, retreat of glaciers and sea-ice, a rise in sea-level of 10-20 cm and increased heavy rainfall in many regions. Many species of plants and animals have changed their location or the timing of their seasonal responses in ways that provide indirect evidence of global warming. The latest research by Mann and Jones in 2003 confirms that the 20th century Northern Hemisphere warming is greater than any time in the past 1800 years.
Both air over land and over the oceans has warmed. The most recent period of warming has been almost global, although the largest temperature increases have occurred over northern hemisphere continents in the mid- to high- latitudes. Parts of the north-western North Atlantic and the central North Pacific Oceans have cooled in recent decades.
1998 was the warmest year and the 1990s the warmest decade globally since the record began in 1861. Nine out of the ten warmest years on record occurred in the 1990s and 2000s.
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 Meteorologys 1961 to 1990 reference period.
Annual mean temperature anomalies for Australia. (Bureau of Meteorology)
Despite the wide range of indicators of global warming, critics often focus on a 23-year period from 1979-2001 when early studies with satellite data showed little or no warming in the lower atmosphere, whereas thermometer data showed that surface temperatures had increased. However, this disparity has declined in recent years. A recent study by Vinnikov and Grody found good agreement between the satellite and surface data from 1978-2002, with a satellite-based warming of 0.24°C per decade compared with 0.17°C per decade from surface data. Another study by Mears et al (2003) found a satellite-based warming of 0.10°C per decade. Santer and others have concluded that apparent inconsistencies between surface and satellite results may be an artefact of satellite data uncertainties. The satellite record is too short to be certain. The longer record of temperature measurements from weather balloons shows that the lower atmosphere has warmed by about 0.10°C per decade from 1958 to 2000, a similar rate to the surface warming.
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.
It is difficult to distinguish natural variability in climate from human-induced climate change.
Global warming in the early part of the 20th century can be explained
by a combination of natural and human-induced changes, while most of the
warming in the last 50 years was due to human activities, namely increasing
greenhouse gas concentrations. Considering the 20th century as a whole,
it is extremely unlikely that global warming can be explained by natural
variability. Hence, while a variety of factors (increased air-borne particles,
stratospheric ozone depletion, volcanic eruptions and internal climate
variability) influence climate, the most dominant driver of change in
the past few decades has been the increased greenhouse gas concentrations.
Given the projected increases in concentrations, greenhouse warming is
expected to be even more dominant in the 21st century.
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. Sea-surface temperatures and small-island temperatures, which are not affected by the urbanisation, also show global warming, as do ocean temperatures to depths of 1000 metres. Other evidence of warming is available from tree rings, ice cores, boreholes and glacial retreat.
During the past 100 years, global average sea level has risen by between 10 and 20 cm. However, we have no evidence to associate this increase with global warming.
Increasing levels of greenhouse gases are likely to produce a warming at the Earths 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 1.4°C and 5.8°C higher than they were in the year 1990. This is much larger than the changes observed over the 20th century, and the rate of warming is unprecedented in at least the last 10,000 years.
Average rainfall across the globe is likely to increase, particularly
during winter in northern mid- to high latitudes. Precipitation events
are very likely to be more intense over most areas of the globe, as well
as a likely increase in summer risk of drought.
Australia will be hotter and drier in coming decades.
Warmer conditions will produce more extremely hot days and fewer cold days. Over most of the continent, annual average temperatures will be 0.4 to 2 degrees Celsius greater than 1990 by 2030. By 2070, average temperatures are likely to increase by 1 to 6 degrees Celsius. The temperature ranges quoted indicate the scientific uncertainty associated with the projections.
The warming won't be the same everywhere. There will be slightly less warming in some coastal areas and Tasmania, and slightly more warming in the north-west. South-western Australia can expect decreases in rainfall, as can parts of south-eastern Australia and Queensland. Wetter conditions are possible in northern and eastern Australia in summer and inland Australia in autumn. When combined with the increase in potential evaporation, the changes in rainfall will lead to drier conditions in Australia.
In areas that experience little change or an increase in average rainfall, more frequent or heavier downpours are likely. Conversely, there will be more dry spells in regions where average rainfall decreases.
See CSIRO's climate change projections, released May 2001
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 dont 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.
By the year 2030, the global average sea level is likely to be between 3 and 17 cm higher than the 1990 level. By 2100, sea level is projected to rise by approximately 9 to 88 cm, compared with 1990.
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 sea-water 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 even if atmospheric temperatures stabilise.
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 will melt more rapidly. Glaciers may retreat. Melting also contributes to increased sea level. The net effect on sea level rise from ice changes in Greenland and Antarctica is likely to be small.
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, which would lock water away in the ice caps. This may offset some of the sea-level rise caused by thermal expansion of water.
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 (but will not ratify) the 1997 Kyoto Protocol, which would become international law if sufficient countries ratify it (see http://www.greenhouse.gov.au/international/kyoto/index.html). The Kyoto Protocol would 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.
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.
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, 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.
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 Earths 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.
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.
Holper, Torok, Hopkins and Hennessy
Modified: April 3, 2008