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Climate change

Enhanced greenhouse effect, Earth's atmosphere

Alice Thompson


The evolution of the Earth’s atmosphere
What is the greenhouse effect?
History of concern
Recent debates
Climate predictions
Australian emissions

The evolution of the Earth’s atmosphere

The atmosphere - When our planet was very young it did not have an atmosphere. But as time went by gases emitted from the Earth’s crust and from volcanoes accumulated at the surface. These gases were probably similar to those emitted from volcanoes today, and when the earliest forms of life came into existence the atmosphere probably consisted mainly of nitrogen and carbon dioxide, but without any oxygen.

The first living things were single-celled bacterium-like organisms. Except for sub-cellular viruses, these were the only form of life on the planet for a thousand million years. Their immediate sources of energy are believed to have been energy-containing chemical compounds formed through the action of UV radiation and of electrical discharges in storms.

Photosynthesis - It was the emergence, in evolution, of photosynthesis that resulted in the eventual accumulation of oxygen in the atmosphere, and that made possible the evolution of more complex forms of life, including the plants and animals of the present day. Single-celled organisms capable of photosynthesis, cyanobacteria, are thought to have been in existence by around 2 800 million years ago.

Some of the oxygen released into the atmosphere in photosynthesis was converted to ozone, and accumulated in the layer of the upper atmosphere known as the stratosphere, where it acted as a filter, absorbing much of the ultraviolet radiation from the Sun. As a result, by the time that humans appeared on Earth, and probably by two thousand million years before that, only about half of the total solar ultraviolet radiation, and a much smaller fraction of the short-wave UV-B rays, penetrated to the surface of the planet. Life as it exists on land today would not have been possible had it not been for this development.

Atmosphere changes - Evidence from ancient rocks indicates that there have been big changes in the composition of the atmosphere over time. Early in the history of the planet there was a very high concentration of carbon dioxide, and other gases like hydrogen, sulphur dioxide, carbon monoxide, ammonia, methane and hydrogen sulphide were also present. Later, the concentration of carbon dioxide was greatly reduced, and oxygen became abundant. Nitrogen has probably always been present in a concentration similar to that in today’s atmosphere.

What is the greenhouse effect?

The atmosphere allows light energy from the sun to penetrate to the Earth’s surface, where it is converted to heat energy. This causes the surface of the Earth to warm up, but much of this heat is re-radiated towards the highest parts of the atmosphere and back into space. Some of this outgoing radiation is absorbed by certain gases in the atmosphere, and then re-radiated in all directions. Thus some of it is directed back to the surface of the Earth, and because of this effect the average temperature at the surface of the planet is some 34° C higher than it would otherwise be. The gases which cause this so-called greenhouse effect are called greenhouse gases.

The natural greenhouse gases include: water vapour (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone (O3). Other gases, like carbon monoxide (CO), sulphur dioxide (SO2) and nitrogen oxides (NOx), play an indirect role by influencing the levels of other greenhouse gases.

Figure 1 The Greenhouse Effect

Greenhouse fig2 1

That the greenhouse effect exists has been well accepted in scientific circles for over a century. It is a natural phenomenon, and without it our planet would be much too cold to support life as we know it.


History of concern

The first scientific papers suggesting that continued combustion of fossil fuels could lead to global warming were published in the 1890s. In the late 1950s it was calculated that about half the carbon dioxide from the burning of fossil fuels was remaining in the atmosphere, and during the 1960s and 1970s, many studies confirmed the potential for climate change due to rising carbon dioxide levels.  Various other trace gases were also identified as greenhouse gases. The first multi-dimensional climate models were being developed at this time.
It was not until the late 1980s that the issue of long-term global warming due to increasing emissions of greenhouse gases came to widespread public notice, leading eventually to the development of various international initiatives, including the establishment of the Intergovernmental Panel on Climate Change (IPCC) in 1988, and the Kyoto conference in 1997. These developments are discussed below.

Recent controversies

Human activities  The main debate over recent years has focused on the degree to which the release of greenhouse gases as a result of human activities, are causing, or are likely to cause, global climate change.

At present the great majority of atmospheric scientists agree that we are already beginning to live with the effects of an enhanced greenhouse effect resulting from human activities, experiencing warmer temperatures, rising sea levels and significant changes in weather patterns. There are still uncertainties, however, about the precise nature of these changes, especially at regional and local levels.

According to the prevailing view, whatever actions might be taken in the future to reduce greenhouse gas emissions, we are already committed to significant climate change due to the greenhouse gases that have already been released into the atmosphere. However, there is some disagreement about the degree to which climate changes that have occurred in recent decades are attributable to the increasing concentrations of greenhouse gases in the atmosphere. This uncertainty has led to confusion among the general public about the urgency of the problem, and has impeded the introduction of policies aimed at effectively reducing greenhouse gas emissions.

The information presented here is based largely on reports of the IPCC and of Australia’s CSIRO Division of Atmospheric Science. As in most areas of environmental concern, there are other viewpoints, although there is no disagreement about the fact carbon dioxide concentrations in the atmosphere are rising significantly as a result of human activities.

Greenhouse gases and growth - The greenhouse gases produced by human society include the naturally occurring gases carbon dioxide and methane, as well as some synthetic compounds like CFCs and HFCs. The increasing production of greenhouse gases is a function not only of population growth, but also, and more importantly, of our current economic arrangements and lifestyles associated with exceptionally high rates of resource and energy use and technological waste production. Thus the quantities of these gases released by different countries are closely correlated with levels of consumerism and economic growth.


The concentration of carbon dioxide in the atmosphere has increased over 30% since the beginning of the industrial revolution phase of human history. Towards the end of the last century the concentration increased on average by 0.4% per year (Table 1 - source).


Table 1
Carbon dioxide
Nitrous oxide
Current concentration
370 ppmv
1720 ppbv
312 ppbv
260 pptv

Pre-industrial concentration (~1700s)

288 ppmv
850 ppbv
285 ppbv
Annual rate of increase
Atmospheric lifetime
50-200 years
12 years
120 years
50 years

According to CSIRO Marine and Atmospheric Research scientist, Dr Mike Raupach, who is the co-Chair of the Global Carbon Project, the growth rate of carbon dioxide emissions is accelerating. He writes:

‘From 2000 to 2005, the growth rate of carbon dioxide emissions was more than 2.5 per cent per year, whereas in the 1990s it was less than one per cent per year’.

The atmospheric concentrations of carbon dioxide, methane and nitrous oxide over the past 1000 years are shown in Figure.2, which nicely illustrates the increasing impact of human activities since the industrial revolution.

Figure 2

fig2 climate change

The global average surface temperature has increased during the 20th century by approximately 0.6°C. Temperatures in Australia have risen over this period by 0.5ºC to 0.9ºC which, on average, is higher than mean global trends. The 1990s have been the warmest decade globally, and 2005 has been the warmest year on record. Longer term observations from tree rings, corals and ice cores suggest that the rate of warming during the 20th century was greater than that seen in any century of the past 1000 years, and the trend is continuing (Figure.3).

Figure .3

Greenhouse fig2 3
Ice and snow - Corresponding with the increase in average surface temperatures globally, there has been a general decrease in snow cover and ice extent around the world. Observations in the Northern hemisphere suggest that there has been a diminution of 10% in the extent of snow cover since the 1960s. The extent of seasonal sea-ice has also decreased by between 10-15% over the past 100 years and mountain glaciers in non-polar regions have retreated considerably.

Sea level - The past century also experienced an increase in average sea levels around the globe in the order of 0.1 to 0.2 metres. Sea level measurements in the Australasian region reflect this general mean trend, and appear to have risen about 2mm per year over the past 50 years.

Climate predictions

A range of climate models and climate change scenarios have been developed by different scientific organisations. The scenarios project future climatic conditions based on different estimations of greenhouse gas emissions, taking account of a range of assumptions regarding economic growth, emissions controls, the availability and use of energy, patterns of agriculture and land use, the use of halocarbons and CFCs and levels of population.

According to the IPCC, the average surface temperature globally is projected to increase by 1.4 to 5.8ºC by the year 2100. This rate of increase in temperature is greater than any seen in the last 10 000 years.

In regional scenarios developed by CSIRO, it is projected that by 2030 Australia will experience warming in the order of 0.4 to 1.4ºC in inland areas, 0.3 to 1.0ºC in northern coastal areas, and 0.3 to 1.3ºC in southern coastal areas.

Extreme weather - All global models predict an increase in the frequency of high temperatures coupled with a decrease in the frequency of extremely low temperatures. In addition, precipitation will be affected, with projected increases in high rainfall events over the next century. Some regions may experience more severe floods and droughts, while in other areas such extreme weather events may be reduced. There is greater uncertainty about the extent to which other extreme weather events – like cyclones and tornadoes – will be affected, although they are likely to increase.

Ice and snow - It is projected that snow cover and sea-ice extent will continue to decrease over the coming century in northern regions of the world, along with the widespread retreat of glaciers and icecaps. Paradoxically, the influx of cold fresh water into the North Atlantic Ocean could shut down that Ocean’s circulation, cutting off the Gulf Stream and plunging western Europe into an ice age. It is possible that the ice sheet in the Antarctic may actually gain mass, due to an increase in precipitation.

The loss of mass from glaciers and icecaps around the world, as well as the increased thermal expansion of oceans, are predicted to lead to an overall increase in global mean sea level of 0.09 to 0.88 metres between 1990 and 2100.

Temperature rise - Even if concentrations of greenhouse gases could be stabilised by 2100, it is likely that global temperature will keep rising beyond that time due to thermal inertia of oceans and the continued presence of emissions in the atmosphere.

Although the great majority of climate scientists agree that global warming is taking place, there is much uncertainty about the actual degree of temperature change. This is because the situation really is extraordinarily complicated. For example, some recent evidence suggests that an increase in certain human-induced aerosols in the atmosphere may be causing a decrease in solar radiation reaching the Earth’s surface, resulting in a certain cooling effect and a reduction in the rates of evaporation of water.

Biological adaptation - Given the complexity of ecosystem interactions, it is very difficult to predict how different life forms will be affected by long-term climatic variations. However, the ecological changes are likely to be beyond the adaptive capacity of many species, leading to widespread extinctions.

Soil moisture levels are likely to be affected by changes in temperature and precipitation, with disruption of established water and nutrient cycles. Plant productivity and species interactions, such as competition, predation and parasitism are also likely to be affected. Increases in the occurrence of fire and outbreaks of insects would have further impacts.

Human wellbeing - Human health and wellbeing will be significantly affected by climate change.  It is likely that a greater frequency of high temperature events will result in higher levels of heat stress mortality across populations. Warmer temperatures will also extend the range of disease-carrying vectors, like mosquitoes, exposing greater numbers of populations to tropical vector-borne diseases such as malaria and dengue.

Rising sea levels will threaten the homes and livelihoods of millions of people living in low lying, fertile and densely populated coastal areas – for instance, in Bangladesh and the South Pacific Islands.

Significant impacts on agricultural production are predicted.  Any increase in crop production due to higher CO2 levels is likely to be offset by greater variability in precipitation.

Greenhouse or ice age? - An argument put forth by the business-as-usual school of thought is that increasing global temperature will be a good thing, or at least better than global cooling, and that the enhanced greenhouse effect may be protecting us from the onset of another ice age. A contrary viewpoint is that if fossil fuels really have the potential to prevent another ice age – then surely we should be saving them up until there is clear evidence that the planet is, in fact, entering such an ice age.


International climate action - The concern expressed by scientists about the enhanced greenhouse effect over the past couple of decades has resulted in a considerable amount of discussion and debate internationally. One of the most significant developments was the establishment in 1988 by the United Nations Environment Program and the World Meteorological Organisation of the Intergovernmental Panel on Climate Change (IPCC).  The role of the IPCC is to assess information related to climate change issues and to formulate realistic response strategies for the management of climate change issues.

Another important development was the United Nations Framework Convention on Climate Change, which was signed by 155 States at the Rio Earth Summit in 1992, and entered into force in 1994. It provides the overall policy framework for addressing the climate change issue. The ultimate goal of this Convention is to:

‘achieve....stabilisation of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system...Such a level should be achieved within a timeframe enable economic development to proceed in a sustainable manner.’

Signatory parties are committed to taking steps to achieve this objective under the Convention.  Since developed countries have been largely responsible for past and current emissions, some of them have agreed to take the lead in stabilising and reducing emissions. Developing countries are also committed to taking action, but this will depend on the provision of assistance by developed countries. Action to be taken by all parties includes the provision of information and promotion of educational programs about climate change, as well as the adoption of policies and measures that aim to reduce emissions of greenhouse gases.

Kyoto Protocol - The Framework Convention provided for signatories to meet annually to discuss and review the latest in climate change science, and assess the adequacy of the policy response at a series of Conferences of the Parties (COP). These meetings led to negotiations aimed at strengthening commitments to the Framework Convention, resulting in the development of the Kyoto Protocol. The text of the Kyoto Protocol was agreed to at the third COP in 1997. The most important feature of the protocol is the emissions targets for OECD and Eastern European countries. These parties agreed to reduce aggregate CO2 emissions by 5.2% of 1990 levels by 2008-2012. The Protocol allows for specific targets to be set for individual countries that take into account differing economic circumstances and capacity for change, and also includes several flexibility mechanisms along with options for carbon sequestration.

The pact could not come into force until it was ratified by countries accounting for at least 55 per cent of the greenhouse gas emissions of developed nations.  It finally became law in February 2005 after eventual ratification by Russia. The Protocol was ratified by Australia in December 2007. The United States of America has so far refused to ratify the treaty.

In July 2005 the United States, China, India, Japan, South Korea and Australia agreed to form an Asia-Pacific Partnership on Clean Development and Climate. These six countries are responsible for about 50 per cent of global greenhouse emissions. The first meeting of ministers met in Sydney in January 2006. The group says it complements, but does not replace the Kyoto Protocol, and that they ‘will work together to develop, deploy and transfer cleaner, more efficient technologies and to meet national pollution reduction, energy security and climate change concerns’. So far there are apparently no commitments among the members of the group to meet any target or to take any specific action.

Contraction and convergence - An alternative plan for reducing carbon emissions has recently been proposed. It is known as the ‘contraction and convergence, or ‘C and C’.  Essentially, this proposal suggests that all countries aim for the same rate of carbon emissions on a per capita basis – say 0.3 tonnes per year (the average today is about one tonne per year per person).  This would mean a very substantial drop in carbon dioxide emissions in the developed countries, but would allow an increase in some developing regions.

Geosequestration - Finally, reference must be made to the proposal that the carbon dioxide produced, for example, in coal or petroleum-powered electricity generating plants, should be sequestered below ground.  The idea is that the CO2 would be separated and then compressed to a dense ‘supercritical state’. If this material is injected underground, at a depth of 800m or more, it will remain in that dense state for thousands of years or longer.  Proponents of this idea believe that the technology is available to do this, although other scientists are doubtful about its practicability.  Much research is underway and the technique is being trialled in some countries.

Australian emissions

Australia has an unusual emissions profile in comparison with other industrialised nations. While this country is a relatively small producer of greenhouse gases, accounting for only 1.4% of total global emissions, our emissions per capita rank third among all nations. This has been attributed to the abundance of fossil fuel resources in Australia which has influenced the Australian economy and trade profile, and to our high dependence on fossil fuel based transport.

Although energy production and use is a major source of greenhouse gases, accounting for almost 65% of our total emissions, non-energy sectors are more significant in the Australian inventory than for most other OECD countries. Emissions from the agricultural sector result in an unusually large proportion of methane in the national emissions profile, although, CO2 still dominates total emissions.

Another distinctive feature of Australia’s emissions profile is that certain activities in the forestry sector, such as clearing, are an important source greenhouse gas. It has been estimated that in 1990 carbon dioxide emissions resulting from forest clearance amounted to 156 million tonnes, or 27.3 per cent of this country’s net emissions: and the rate of clearance has increase considerably since that time.

Despite all current measures, and being signatory to the Framework Convention on Climate Change, Australia’s total emissions are projected to increase by 18% from 1990 to 2010.



Globally, the situation can be summarised as follows:

1.  Human technological activities involving the combustion of fossil fuels are resulting in progressive increase in the concentration of carbon dioxide in the atmosphere of our planet.  The concentration is still increasing, at a rate of 0.4% per year.The concentrations of two other greenhouse gases, methane and the CFCs, which were increasing until recently, seem to have stabilised.

2.  Most atmospheric scientists, including members of Australia’s CSIRO Division of Atmospheric Science, predict that this increase in carbon dioxide in the atmosphere will result in progressive global warming, with uncertain consequences for humankind.

3.  Climate change due to the release of greenhouse gases by human society is an issue that calls for governmental decision-making on a scale and of a kind not seen before in human history. Given the progressive and cumulative nature of the problem, the longer effective action is postponed, the greater the problem will become, and the more difficult it will be to address.

Further reading

For further information see:

(1) the various reports of the Intergovernmental Panel on Climate Change (IPCC)

(2) The Nova website of the Australian Academy of Science

(3)  The United Nations Environment Program


The stratosphere - The stratosphere is the zone in the atmosphere that exists between an average of 15 and 50 km above the surface of the planet, lying between the lowest layer (the troposphere), and the mesosphere. In the stratosphere temperature increases with altitude.


Born and raised in Canberra, Alice Thompson was brought up with an appreciation of, and interest in the environment, leading her to study at the Australian National University, majoring in Geography/Human Ecology and Population Studies, and her involvement in the Nature and Society Forum (NSF). She now lives in Sydney where she currently pursues a career in Government working for the NSW Office of the Australian Bureau of Statistics (ABS). Before joining ABS Alice was employed by NSF as a Research Officer to prepare reports on important ecological issues in Australia.

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