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Draft Report: Chapter 3

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Chapter 3: The science of climate change

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Key points
3.1 The earth's atmosphere and the natural greenhouse effect
3.2 Understanding climate change
3.3 Linking emissions and climate change
3.4 Adressing the extremes: severe weather events, low likelihood outcomes, and thresholds
3.5 Uncertainty in the climate science
3.6 The science behind global mitigation

Key points

The Review takes as its starting point, on the balance of probabilities and not as a matter of belief, the majority opinion of the Australian and international scientific communities that human activities resulted in substantial global warming from the mid 20th century, and that continued growth in greenhouse gas concentrations caused by human-induced emissions would generate high risks of dangerous climate change.

A natural carbon cycle converts the sun’s energy and atmospheric carbon into organic matter through plants and algae, and stores it in the earth’s crust and oceans. Stabilisation of carbon dioxide concentrations in the atmosphere requires the rate of greenhouse gas emissions to fall to the rate of natural sequestration.

There are many uncertainties around the mean expectations from the science, with the possibility of outcomes that are either more benign—or catastrophic.


Climate change policy must begin with the science. When people who have no background in climate science seek to apply scientific perspectives to policy, they are struck by the qualified and contested nature of the material with which they have to work. Part of the uncertainty derives from the complexity of the scientific issues. In the public discussion of the science, additional complexity derives from the enormity of the possible consequences, which calls for a millennial perspective. Part derives from the large effects of possible policy responses on levels and distributions of income, inviting intense and focused involvement in the discussion by those with vested interests.

The Review is not in a position to independently evaluate the considerable body of scientific knowledge, and it is not the intent of this chapter to debate the existence or extent of human-induced climate change.

In the terms introduced in Chapter 2, there is a great deal of uncertainty about the magnitude of the effects of increased greenhouse gas emissions on science. The scientifically reputed ‘sceptics’, to the Review’s understanding without exception, accept that an increase in carbon dioxide concentrations in itself leads to warming (Lindzen 2008). The ‘sceptics’ variously contest the relationship between human-induced anthropogenic emissions and atmospheric concentrations, or the relative importance of the enhanced greenhouse effect and other factors that influence climate.

It is the nature of uncertainty that new information and analysis can fundamentally change the odds about most particular statements being true. The Review takes as a starting point, on the balance of probabilities and not as a matter of belief, the majority opinion of the Australian and international scientific communities that human-induced climate change is happening, will intensify if greenhouse gas emissions continue to increase, and could impose large costs on human civilisation.

This chapter draws extensively on the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, and on detailed reports prepared by Australian scientists, research published since the IPCC Fourth Assessment Report and work commissioned specifically for the Review. It aims to build an understanding of the way humans can influence the climate and the limitations in our current understanding of the climate system, and introduces key terminology and concepts relevant to policy makers.

The final section considers the science that underlies climate change mitigation, and considers how the complexities and uncertainties can be addressed in long-term policy decisions.

The Review therefore draws on what it describes as mainstream or majority science. In drawing on the work of the IPCC, and the large majority of Australian scientists who are comfortable working within that tradition, we are still faced with immense uncertainties. These are the uncertainties that are carried through in the Review’s analysis. These perspectives may cease to be the mainstream or majority science as the development of climate science proceeds. At this time, the Review believes it is appropriate to give the main weight to them.

3.1 The earth's atmosphere and the natural greenhouse effectKey points

    3.1.1 The changing composition of the atmosphere

The earth is surrounded by an atmosphere that protects it from high-energy radiation and absorbs heat to provide a moderate climate that supports life.

The earth’s atmosphere has not always been the same as it is today. Billions of years ago, the atmosphere was composed mainly of ammonia, water vapour and methane, but over time release of gases from within the planet through volcanic eruptions and discharge of gases from ocean vents changed conditions so that carbon monoxide, carbon dioxide and nitrogen became dominant.

Around 3.5 billion years ago, algae-like organisms first began to use the energy from the sun to convert carbon dioxide from the air into carbohydrates, with a by-product of oxygen. Over time, oxygen levels rose so that when dinosaurs flourished between 230 and 65 million years ago, oxygen levels were around half of what they are today. Between 40 and 50 million years ago levels rose rapidly, after which there was a slight decline to current levels of 21 per cent of atmospheric volume. The rapid rise in oxygen content is linked to the evolution of large mammals (Falkowski et al. 2005).
    3.1.2 The natural greenhouse effect

The earth’s atmosphere behaves like the roof of a greenhouse, allowing short-wavelength (visible) solar radiation from the sun to reach the surface, but absorbing the long-wavelength heat that is emitted back. This process is also referred to as ‘the greenhouse effect’, and the gases that absorb the emitted heat are known as greenhouse gases. ‘Global warming’ refers to the expected increase in average surface temperature due to increasing concentrations of greenhouse gases in the atmosphere. The main naturally occurring greenhouse gases are water vapour, carbon dioxide, methane, nitrous oxide and ozone. These and other greenhouse gases are discussed in detail in section 3.3.

Compared to nitrogen and oxygen, which collectively comprise 99 per cent of the volume of the atmosphere, greenhouse gases occur only at trace levels, making up just 0.1 per cent of the atmosphere by volume (IPCC 2001a).

Despite the low concentration of greenhouse gases in the earth’s atmosphere, their presence means that the earth has an average global surface temperature of about 14C—about 33C warmer than if there were no greenhouse gases at all (IPCC 2007a: 946).

The importance in the earth’s atmosphere in creating the conditions for life is demonstrated by the very different surface conditions on the earth’s moon. On the moon there is no atmosphere at all, and the temperature fluctuates dramatically as the level of sunlight reaching the surface changes. The moon’s surface temperature fluctuates between -233C to 123C (NASA 2008).
    3.1.3 Changes in greenhouse gases and temperature over time

Records of carbon dioxide concentrations taken from ‘proxy’ measures such as fossil plants and algae are available for the last 400 million years. These records indicate that atmospheric concentrations of carbon dioxide have fluctuated between levels similar to pre-industrial concentrations of 280 ppm (parts per million by volume), and levels higher than 4000 ppm (Royer 2006).

During the last 2.5 million years, climate records document a ‘saw-tooth’ pattern of changes in temperature and ice volume. In the last 600 000 years the fluctuations show a periodicity of around 100 000 years (Ruddiman 2008). The periods when polar ice caps were greatly expanded, which resulted in large ice sheets covering large parts of the northern continents, are known as glacial periods or ice ages, while those without extended polar ice caps are known as interglacials. The last glacial maximum occurred 21 000 years ago; for the last 10 000 years the earth has been in an interglacial period (IPCC 2007a: 447).

Glacial periods occur when summer solar radiation in the northern hemisphere is reduced, and interglacials when solar radiation is more intense. Fluctuations in carbon dioxide and methane concentrations have also occurred in the 600 000-year period before the present, but the role of greenhouse gases in contributing or responding to the glacial–interglacial fluctuations is complex and still unclear.

Consistent changes in the intensity of solar radiation from regular variations in the shape of the earth’s orbit and the tilt of its axis, as well as the sunspot cycle, are seen as drivers of these cyclical climate changes (Ruddiman 2008).

There is a high degree of uncertainty in historical measurements of temperature before modern times, which must be estimated from a range of indirect sources such as annual growth rings in trees and corals, and small fossils in ocean and lake sediments.

There is high natural variability in global temperatures in recent millennia. The current high temperatures, though unusual over the last 1000 years, are not unusual on longer time frames. However, the rapid rate of the current warming is highly unusual in the context of the past millennium (CASPI 2007).
How are the recent changes different?

Why are we so concerned about the current changes in climate and greenhouse gas concentrations if they have fluctuated so much over the earth’s history?

Apart from the earliest identified hominids, which existed as early as seven million years ago, the history of our species has been within the period of relatively low carbon dioxide concentrations. Our direct ancestors, Homo erectus, appeared around 1.6 million years ago, and modern forms of our species, Homo sapiens, first appeared only around 200 000 years ago. The last 10 000 years have seen the development of agriculture, large-scale social organisation, writing, cities and the behaviours we associate with modern civilisation. The period in which human civilisation has developed, located within an interglacial period known as the holocene, has been one of equable and reasonably stable temperatures.

[Figure 3.1 Trends in atmospheric concentrations of carbon dioxide, methane and nitrous oxide since 1750]* Note: Measurements are shown from ice cores (symbols with different colours for different studies) and atmospheric samples (red lines). Source: IPCC (2007a: 3), formatted for this publication.

Current concentrations of carbon dioxide are very unusual for the last two million years, and particularly in the last 800 000 years, including the period over which human civilisation has emerged. Concentrations now exceed the natural range of the last two million years by 25 per cent for carbon dioxide, 120 per cent for methane, and 9 per cent for nitrous oxide (IPCC 2007c: 447). In fact, the anthropogenically driven rise in carbon dioxide since the beginning of the industrial revolution (around 100 ppm) is about double the normal ‘operating range’ of carbon dioxide during glacial–interglacial cycling (180–280 ppm) (Steffen et al. 2004). Trends in atmospheric concentration of carbon dioxide, methane and nitrous oxide for the last 250 years are shown in Figure 3.1, which demonstrates the accelerated growth in recent years. It is not just the magnitude of the post-industrial increase in greenhouse gas concentrations that is unusual, but also the rate at which it has occurred.
    3.1.4 Are humans causing the earth to warm?

The rapid development of climate-related research and modelling has allowed increasingly more definitive assessments of the human impacts on climate. The IPCC Fourth Assessment Report (2007) noted an improvement in the scientific understanding of the influence of human activity on climate change. The report concluded that the warming of the climate system is ‘unequivocal’ (IPCC 2007a: 5), that there is a greater than 90 per cent chance that ‘the global average net effect of human activities since 1750 has been one of warming’ (IPCC 2007a: 3). Confidence in the influence of humans on other elements of climate change, such as droughts and severe weather events, is not as high, but is increasing as modelling techniques and observation databases improve.

Some people with relevant scientific credentials (and many who lack them) argue that the warming trend may be mainly the result of factors independent of human activity—the same factors that have been responsible for a continuously changing global climate throughout the earth’s history. However, those who argue against a strong human influence in the contemporary warming trend are a small minority of scientists with competence in relevant fields.

If there are natural as well as anthropogenic causes of recent warming, it is not obvious that this would reduce the urgency or importance of reducing anthropogenic greenhouse gas emissions. It could be argued that the presence of additional sources of warming actually increases the importance of early and strong action to moderate the contributions over which humans have some control. This perspective argues most strongly for more scientific research on natural sources of climate change and their interaction with anthropogenic global warming.

Chapter 5 contains a more detailed discussion of human attribution of other elements of observed climate change.

3.2 Understanding climate change

    3.2.1 Definitions of climate change

The IPCC (2007a: 943) defines climate change as ‘a change in the state of the climate that can be identified (for example, by using statistical tests) by changes in the mean and/or variability of its properties, and that persists for an extended period, typically decades or longer’. Climate change may be due to natural internal processes or external influences, or to persistent anthropogenic changes in the composition of the atmosphere or land use.

By contrast, the United Nations Framework Convention on Climate Change (UNFCCC) defines climate change as ‘change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods’ (UN: 1992).

This report uses the IPCC’s definition, so the discussion of climate change includes changes to the climate caused by natural phenomena, such as volcanic eruptions.
    3.2.2 The climate system

In a narrow sense, climate can be defined as the ‘average weather’ and described in terms of the mean and range of variability of factors such as temperature, rainfall and wind speed (IPCC 2007a: 96).

More broadly, the climate can be described as a system involving highly complex interactions between the atmosphere, the oceans, the water cycle, ice, snow and frozen ground, the land surface and living organisms. This climate system changes over time in response to internal dynamics and variations in external influences such as volcanic eruptions and solar radiation (IPCC 2007a: 943). Examples of the internal interactions in the climate system and key external influences are shown in Figure 3.2.

Larger increases in temperatures occurring on land compared to ocean surfaces, or to land or ocean at higher latitudes than low latitudes, are likely to lead to changes in the patterns of winds and storm tracks. As a result, climate change can cause some areas to receive more rain, or stronger winds from colder areas, even though the climate as a whole is warming (CASPI 2007).

In any location weather patterns may shift on a daily and even hourly basis. Climate is weather averaged over a longer timescale, but it still possesses some degree of natural variability.

The atmosphere component is the most unstable and rapidly changing part of the climate system (IPCC 2001a). The atmosphere is divided into five layers with different temperature characteristics, with the lower two having the most influence on the climate system.

The troposphere extends from the surface to an altitude of between 10 and 16 km. Clouds and weather phenomena occur in the troposphere, and greenhouse gases absorb heat radiated from the earth so that temperature decreases with altitude.

The stratosphere, which extends from the boundary of the troposphere to an altitude of around 50 km, is the second layer of the atmosphere. The stratosphere holds a natural layer of high ozone concentrations, which absorb ultraviolet radiation from the sun so that temperature increases with altitude (Figure 3.3). The balance of energy between the layers of the atmosphere is a major driver of atmospheric and ocean circulation that leads to weather and climate patterns (IPCC 2007a: 610).
    3.2.3 The energy balance of the climate system

Energy enters the system as visible and ultraviolet radiation from the sun, and is absorbed, scattered or reflected, re-emitted as heat, transferred between different elements of the system, used by organisms and emitted back into space (Figure 3.3).

[Figure 3.3 A stylised model of the natural greenhouse effect and other influences on the energy balance of the climate system.]*

The balance between the energy entering and leaving the system is what determines whether the earth gets warmer, cooler or stays the same. Changes in the strength of radiation from the sun change the energy that enters the system, while the flow of energy within the system and the amount that is released can be modified by a wide range of factors.
As discussed earlier, the composition of gases in the atmosphere plays a big part in the amount of heat that is retained in the climate system, but there are many other influences. Particles and droplets in the air known as aerosols can scatter the incoming radiation so that it never reaches the surface, and can also cause changes in cloud cover. More clouds will reflect more sunlight from their upper surfaces, but also absorb more heat radiating from the earth. Variations in land cover affect the amount of sunlight that is reflected from the surface (the ‘albedo’ effect), and how much is absorbed and re-emitted as heat (IPCC 2007a: 94).

Human activities can affect the energy balance of the climate system in a number of ways. Examples include changes in land use; emissions of aerosols and other pollutants; emissions of greenhouse gases from activities such as agriculture, energy production, industry and land clearance; emissions of other pollutants that react in the atmosphere to form greenhouse gases; and influences on cloud formation through aviation activities.
    3.2.4 Factors leading to warming of the climate system

The warming of the climate system that is evident in the last half century is a result of the cumulative effect of all the natural and human drivers that influence the amount of warming or cooling in the system. In the context of understanding and mitigating climate change, then, which factors are causing the largest amount of warming?

‘Radiative forcing’ is a measure of the induced change to the energy balance of the atmosphere at the junction of the troposphere and stratosphere. It is a useful way to compare the influences on the energy balance from external, natural and human factors.

The contribution of different factors leading to an overall warming of the atmosphere since 1750 is shown in Figure 3.4.

The dominant influence since 1750 has been an increase in concentrations of carbon dioxide. Aerosols have had a net cooling influence, although this effect is poorly understood. Natural variability in solar radiation has had a small warming influence, but the interactions are complex and there is a high level of uncertainty in the magnitude of this influence (IPCC 2007a: 192).

Even if there were no further induced changes in aerosols and greenhouse gases, the long-lived greenhouse gases would remain for hundreds and even thousands of years, leading to continued warming. Aerosols would be removed from the atmosphere over weeks to months, so their cooling effect would no longer be present. Therefore, in the long term, the major influence of humans on the climate will be through activities that lead to increased concentrations of long-lived greenhouse gases in the atmosphere.

[Figure 3.4 Contribution of human and natural factors to warming since 1750]*

Note: Warming and cooling influences are indicated by positive and negative values, respectively. When elements are grouped, uncertainty bands are approximated from the highest uncertainty in an individual element.

(1) Includes both the direct effect and the cloud albedo effect.
(2) Includes the cooling effect of changes to land use and the warming effect of black carbon on snow.
(3) Includes tropospheric and stratospheric ozone and stratospheric water vapour.
(4) Includes methane, nitrous oxide, HFCs, PFCs and SF6.

Source: Based on IPCC (2007a: 204).

3.3 Linking emissions and climate change

The high natural variability and complex internal interactions create uncertainty in the way the climate will respond to increased emissions. Figure 3.5 illustrates the relationship between emissions from human activities and climate change as a causal chain. A summary of each step in the chain is provided in this chapter, with a particular focus on the associated uncertainties. This causal chain does not explicitly include feedbacks and non-linearities in the climate system that are important in its response to human forcings. Some of these steps are discussed in more detail in other chapters of this draft report.

The potential impacts of climate change in Australia, and the associated uncertainty, are discussed in detail in Chapter 7.

[Figure 3.5 Steps in the causal chain of greenhouse gas emissions leading to climate change]*
    3.3.1 Emissions of greenhouse gases from human activities
The greenhouse gases with the greatest influence on warming of the atmosphere are water vapour (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) and ozone (O3). In addition, there is a range of human-made halocarbons (such as perfluorocarbons (PFCs), hydrofluorocarbons (HFCs), chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6) that exist in small amounts but are very potent and contribute to the total warming.

Only some of these gases are directly emitted by human activities. Humans have less direct control over gases such as water vapour and ozone, although concentrations of these gases can be affected by human emissions of other reactive gases.

Projections of future climate change are highly dependent on the pathway of global greenhouse gas emissions. If emissions continue at the current rate, concentrations in the atmosphere will continue to increase and the magnitude and rate of climate change could be considerable. If emissions are reduced, the climate change outcomes will also be reduced.

Chapters 4 and 5 look at past trends in greenhouse gas emissions, their drivers, and future trends, and how these are used in making climate change projections.
    3.3.2 Accumulation of greenhouse gases in the atmosphere

The accumulation of greenhouse gases in the atmosphere that leads to warming is a function of both the rate of emissions and rate of natural removal from the atmosphere.

Each greenhouse gas has specific characteristics that affect how long it stays in the atmosphere. The ‘lifetime’ of a gas in the atmosphere is a general term used for various timescales that characterise the rate of processes affecting the destruction or removal of trace gases.

With the exception of carbon dioxide, physical and chemical processes generally remove a specific fraction of the amount of a gas in the atmosphere each year. In some cases, the removal rate may vary with atmospheric properties such as temperature or background chemical conditions (IPCC 2007a: 23).

The lifetime of a gas affects the speed at which concentrations of that gas change following changes in the level of emissions. Gases with short lifetimes such as methane respond quickly to changes in emissions. Other gases with longer lifetimes respond very slowly. Over the course of a century, half of the carbon dioxide emitted in any one year will be removed, but around 20 per cent will remain in the atmosphere for millennia (IPCC 2007a: 824).

Carbon dioxide

After water vapour, carbon dioxide is the most abundant greenhouse gas in the atmosphere. Most gases are removed from the atmosphere by chemical reaction or are destroyed by ultraviolet radiation. Carbon dioxide, however, is very stable in the atmosphere. When it enters the atmosphere, carbon dioxide exchanges rapidly with plants and the surface ocean, and is then redistributed on timescales of hundreds to thousands of years through various forms of carbon storage, or ‘sinks’, as part of the carbon cycle, discussed in detail in section 3.3.3.

As discussed in section 3.1.3, the concentration of carbon dioxide in the atmosphere increased from about 280 ppm in 1750 to 383 ppm in 2007. Over the last 10 years, carbon dioxide in the atmosphere has increased at an average rate of 2 ppm per year (Tans 2008).

Key natural sources of atmospheric carbon dioxide are respiration from living organisms, volcanic eruptions, forest fires, decomposition of dead animals and plants, and outgassing from the ocean. The dominant human activities leading to carbon dioxide emissions are the combustion of fossil fuels and cement manufacture, which account for more than 75 per cent of the increase in concentration since pre-industrial times (IPCC 2007a: 512). Other contributors include land-use changes, dominated by deforestation and changing agricultural practices.


Methane is a reactive compound of carbon and hydrogen that is short lived in the atmosphere, where it reacts to form water vapour and carbon dioxide. The main natural sources of methane include forests, oceans and termites; the largest source is natural wetlands. Anthropogenic sources include fossil fuel mining, vegetation burning, waste treatment, rice agriculture, ruminant livestock and landfill, which account for 60 per cent of present-day emissions. The main route for methane removal is oxidation in the atmosphere. Considerable methane is stored in frozen soils and as methane hydrates in ocean sediments.

The methane concentration in the atmosphere has doubled to about 1774 ppb since pre-industrial times, after a slow fluctuation, between about 580 ppb and 730 ppb over the last 10 000 years. Since the early 1990s growth rates have declined, and virtually no growth occurred between 1999 and 2005 (IPCC 2007a: 27), which implies that emissions over that time matched the rate of natural removal.

Nitrous oxide

Nitrous oxide is relatively stable in the atmosphere, but is eventually destroyed in the stratosphere when it reacts with ultraviolet light and charged oxygen molecules. The main natural sources are processes in soils and oceans and oxidation of ammonia in the atmosphere. Tropical soils are a particularly important source (IPCC 2007a: 513).

Anthropogenic sources include fertiliser use, biomass burning, cattle production, fossil fuel combustion and industrial activities such as nylon manufacture.

Nitrous oxide concentrations have increased by about 18 per cent to 319 ppb since pre-industrial times.Fluorinated

Generally, fluorinated gases (HFCs, PFCs, sulphur hexafluoride) are non-flammable and non-toxic and have a low boiling point, which makes them useful for a number of manufacturing processes. HFCs and PFCs have been developed as replacements for ozone-depleting gases being phased out under the Montreal Protocol on Substances that Deplete the Ozone Layer, which entered into force in 1989.

Fluorinated gases are purely human in origin. HFCs are used in refrigeration, air conditioning, solvents, fire retardants, foam manufacture and aerosol propellants. The major source of PFCs is aluminium production, while sulphur hexafluoride is used in the electricity supply industry in switches and high-voltage systems.

The main paths of removal of these gases from the atmosphere are destruction through reaction with ultraviolet light and other agents in the atmosphere such as chlorine, uptake in oceanic surface waters, and chemical and biological degradation processes. Concentrations are relatively small but increasing rapidly (IPCC 2007a: 28).

Water vapour

Water vapour is the most abundant and important greenhouse gas in the atmosphere, accounting for around 60 per cent of the natural greenhouse effect for clear skies (IPCC 2007a: 271). The amount of water vapour in the atmosphere is a function of temperature and tends to fluctuate regionally and on short timescales. Humans have a limited ability to directly influence its concentration (IPCC 2007a: 135).

Stratospheric water vapour has shown significant long-term variability (IPCC 2007a: 275). Stratospheric water vapour is produced by the oxidation of methane; the rate of reaction increases as methane concentrations rise (IPCC 2007a: 274). However, this effect is estimated to be equivalent to only about 1 to 4 per cent of the total change caused by long-lived greenhouse gases.

Humans generate some direct emissions of water vapour—largely from irrigation and fossil fuel production—but these emissions are equivalent to less than 1 per cent of emissions from natural sources (IPCC 2007a: 28).

Human activities that contribute to warming indirectly affect the amount of water vapour in the atmosphere, because a warmer atmosphere can hold more water vapour than a cooler one. Increases in global temperature as a result of climate change are likely to affect water vapour concentrations substantially (IPCC 2007a: 135).

The lack of understanding in the way water vapour will respond to climate change, specifically its role in cloud formation, is a key factor in the uncertainty surrounding the response of the climate to increased temperatures.

Chlorofluorocarbons and hydrochlorofluorocarbons

Chlorofluorocarbons and hydrochlorofluorocarbons (CFCs and HCFCs), like fluorinated gases, are human-made gases that are odourless, non-toxic, non-flammable and non-reactive. This makes them popular for a variety of uses such as propellants in aerosol cans, as refrigerants in refrigerators and air conditioners and in the manufacture of foam packaging.

There are a number of different CFCs and HCFCs with different industrial applications. Each has a different lifetime in the atmosphere and different effects on warming. These gases are at low concentrations but have a strong warming effect: as a group they contribute to about 12 per cent of the warming from long-lived greenhouse gases.

The low reactivity of CFCs and HCFCs allows them to remain in the atmosphere for long periods and cycle up to the stratosphere. Through a series of chemical reactions, CFCs destroy ozone and other similar compounds in the stratosphere.

The Montreal Protocol has caused a substantial reduction in emissions of these gases. Emissions of two major CFCs (CFC-11 and CFC-13) decreased substantially between 1990 and 2002 (IPCC 2007a: 513), but with lifetimes of 45 and 85 years, respectively, their concentrations in the atmosphere are reducing much more slowly.

Due to their coverage under the Montreal Protocol, which aims to phase out emissions of these gases by 2030, CFCs and HCFCs are not considered in detail in this report.


Aerosols are tiny particles or droplets in the atmosphere, including sulphates, ash, soot, dust and sea salt, that can be natural or anthropogenic in origin. The major anthropogenic source is fossil fuel combustion.

A major natural source of aerosols is volcanic eruptions. The eruption of Mt Pinatubo in the Philippines in 1991 produced an aerosol cloud that spread around the globe in the year after the eruption, contributing to a reduction in global average temperature of around 0.5C between 1991 and 1993 (Newhall et al. 1997).

Aerosols generally create a cooling effect, but there is great uncertainty about the magnitude of this effect. Black carbon, or soot, has a warming effect because it absorbs solar radiation. Recent research suggests this effect may be considerably higher than previously estimated (Ramanathan & Carmichael 2008). Because the lifetime of aerosols in the atmosphere is much shorter than those of greenhouse gases, the effects are more likely to be felt in the region in which the aerosol is produced.

Aerosols are associated with poor air quality and have negative effects on human health. A decrease in aerosol emissions is expected to result from measures to improve air quality (IPCC 2007a: 78).Tropospheric ozone and precursor species

Tropospheric ozone is a short-lived greenhouse gas produced by chemical reaction between other gases known as ‘precursor species’, which include carbon monoxide, chemicals from industrial uses, methane, and nitrogen oxides. Increases in tropospheric ozone have accounted for 10–15 per cent of the positive change in the earth’s energy balance since pre-industrial times (IPCC 2007a: 204).

While humans have limited direct influence over tropospheric ozone concentrations, they can affect the concentrations of precursor species. Methane has a range of natural and anthropogenic sources as described above, while sources of nitrogen oxides from human activity include industry, power generation and transport.

As with aerosols, ozone precursor gases are known air pollutants associated with the formation of photochemical smog, and recently emissions of these gases have decreased as a result of air quality policy implementation.

How is the warming from different gases compared?

Global warming potential is an index that compares the radiative forcing from a given mass of that of a greenhouse gas to the radiative forcing caused by the same mass of carbon dioxide (CASPI 2007). Global warming potential depends both on intrinsic capability of a molecule to absorb heat, and the lifetime of the gas in the atmosphere. Thus, global warming potential values will vary depending on the time period used in the calculation.

The global warming potential values take into account the lifetime, existing concentration and warming potential of gases. Sulphur hexafluoride has the highest global warming potential of all gases at 22 800 times that of carbon dioxide, but has a low impact on overall warming due to its low concentrations.

Global warming potential is used under the Kyoto Protocol to compare the magnitude of emissions and removals of different greenhouse gases from the atmosphere. It is also the framework being used in the design and implementation of multi-gas emissions trading schemes for calculating the value of a trade between the reductions in emissions of different greenhouse gases.

While the global warming potential framework is useful for comparing the relative impact of different gases, there are limits to its use in modelling and analysis.
    3.3.3 The carbon cycle

The ‘carbon cycle’ refers to the transfer of carbon, in various forms, through the atmosphere, oceans, plants, animals, soils and sediments. As part of the carbon cycle, plants and algae convert carbon dioxide and water into biomass using energy from the sun (photosynthesis). Living organisms return carbon to the atmosphere when they respire, decompose or burn. Methane is released through decomposition of plants, animals and waste when no oxygen is present.

Carbon dioxide dissolves in the ocean and is returned to the atmosphere through dissolution in a continuous exchange. Dissolved carbon dioxide is carried deep into the oceans through the sinking of colder water and waste and debris from dead organisms, where it is either buried or re-dissolves. The transfer of carbon to the deep ocean is very slow. Water at intermediate depths mixes with the surface water over decades or centuries, but deep waters mix only on millennial timescales and thus provides a long-term carbon sink.

Carbon sinks

The parts of the carbon cycle that store carbon in various forms are referred to as ‘carbon sinks’. The majority of carbon that was present in the early atmosphere is now stored in sedimentary rocks and marine sediments. Other carbon sinks are the atmosphere, oceans, fossil fuels such as coal, petroleum and natural gas, living plants and organic matter in the soil.

Table 3.1 provides estimates of the amount of carbon stored in different sinks in 1750 and how they have changed up to the end of the 20th century.

Table 3.1 Estimates of the amount of carbon stored in different sinks in 1750 and how they have changed

Carbon sink
Giga tonnes carbon stored
Percentage of total cycling carbon
Net change in sink between 1750 and 1994
Percentage change
Vegetation, soil and detritus
2 300
Fossil fuels
3 700
Surface ocean
Marine biota
Deeper ocean
37 100
Surface sediments
Sedimentary rocks^
> 66 000 000

Note: Due to the very slow exchange with other carbon sinks, percentages of total carbon do not include storage in sedimentary rocks
^UNEP/GRID–Arendal (2005).
Source: IPCC (2007a: Figure 7.3).

The high solubility of carbon dioxide in sea water makes the ocean a key reservoir for carbon, with the deep and surface ocean accounting for more than 85 per cent of the carbon being cycled more actively. Although the atmosphere accounts for just over 1 per cent of carbon storage, it shows the largest percentage increase since pre-industrial times. Vegetation and soil have had a net decrease in carbon stored—a considerable loss from land-use change has been partially offset by carbon uptake by living organisms.

The major change to the carbon cycle from human activity is increased emissions of carbon dioxide to the atmosphere from the burning of fossil fuels. This returns carbon to the air that was captured by plants earlier in the earth’s geological history and stored away from the atmospheric–terrestrial–ocean cycle. The rate of exchange between the ocean and the atmosphere has increased in both directions, but with a net movement to the ocean. Terrestrial ecosystems are also a significant sink of carbon dioxide from the atmosphere. However, absorption by both the ocean and land cannot keep pace with emissions from fossil fuels. Almost 45 per cent of human emissions since 1750 have remained in the atmosphere.

Carbon-climate feedbacks

There is a high level of uncertainty in how the carbon cycle will respond to climate change, and how this will affect the amount of carbon dioxide removed from the atmosphere through absorption by carbon sinks.

Carbon–climate feedbacks occur when changes in climate affect the rate of absorption or release of carbon dioxide from land and ocean sinks. In general, higher atmospheric concentrations of greenhouse gases, and the resulting changes to the climate system, reduce the absorptive capacity of the carbon cycle so that a larger fraction of emissions remain in the atmosphere compared to current levels (IPCC 2007a: 750). Examples of climate–carbon feedbacks include the decrease in the ability of the oceans to remove carbon dioxide from the atmosphere with increasing water temperature, reduced circulation and increased acidity (IPCC 2007a: 531); and the weakening of the uptake of carbon in terrestrial sinks due to vegetation dieback and reduced growth from reduced water availability, increased soil respiration at higher temperatures and increased fire occurrence (IPCC 2007a: 527; Canadell et al. 2007).

Large positive climate–carbon feedbacks could also result from the release of carbon from long-term sinks such as methane stored deep in ocean sediments and in frozen soils as temperatures increase (IPCC 2007a). Positive climate–carbon feedbacks are discussed further in Chapter 5.

    3.3.4 The relationship between greenhouse gas concentrations and temperature rise

How do different greenhouse gases change the energy balance?

The way the climate responds to changes in the energy balance is highly complex. In section 3.2.4 the concept of radiative forcing was introduced to demonstrate the relative influence of different human and natural ‘forcings’ on the energy balance of the atmosphere. This section looks at radiative forcing in more detail, specifically in relation to the different greenhouse gases.

The radiative forcing of a greenhouse gas represents the change in the effect of that gas on the energy balance of the atmosphere, and takes into account its concentration in the atmosphere at the start of the period (in this report pre-industrial), the amount the concentration has changed due to human activities, and the way a molecule of that gas absorbs heat. Radiative forcing is a measure of change between two specific points in time, so the lifetime of a gas in the atmosphere has a considerable influence on future forcing.

For most gases, removal from the atmosphere is minimally influenced by changes in the climate. However, for the calculation of carbon dioxide concentrations, the complexity of the carbon cycle and the uncertainty over how it will change over time as the climate changes must be taken into account.

Carbon dioxide molecules absorb heat in a particular range of wavelengths, and as concentrations increase additional heat of those wavelengths gets absorbed. If concentrations keep growing, carbon dioxide added later will cause proportionately less warming than carbon dioxide added now. The relationship is approximately logarithmic. The same amount of warming will occur from a doubling from 280 ppm (pre-industrial levels) to 560 ppm as from another doubling from 560 ppm to 1120 ppm.

In contrast, one of the key features of gases such as HFCs, PFCs and CFCs is that they did not exist in the atmosphere before humans. Some of these gases absorb heat at wavelengths that are not absorbed by naturally occurring gases, so they have a larger impact on the energy balance. For HFCs and PFCs, the relationship between their concentration and the warming they cause is approximately linear (IPCC/TEAP 2005: 21).

The forcing due to carbon dioxide, methane, nitrous oxide and halocarbons is relatively well understood. However, the contribution of factors such as ozone at different levels in the atmosphere, aerosols and linear clouds from aviation is poorly understood (CASPI 2007), as demonstrated by the uncertainty bands in Figure 3.4.

A measure used commonly in the literature and policy discussions is the concept of carbon dioxide equivalent (CO2-e) of a gas concentration, measured in parts per million—a different but related measure to carbon dioxide equivalent emissions calculated using the global warming potential index. This is the concentration of carbon dioxide that would cause the same amount of radiative forcing as a particular concentration of a greenhouse gas. This term is often used in discussions of global stabilisation or concentration targets, discussed further in section 3.6.

Table 3.2 summarises the characteristics of the long-lived greenhouse gases, in terms of their current concentration, lifetime and global warming potential. Carbon dioxide is by far the most abundant greenhouse gas in the atmosphere and despite having a lower warming effect for a given amount, it still dominates the overall warming influence.

Table 3.2 Long-lived greenhouse gas concentrations and radiative forcing

Greenhouse gas

Lifetime in the atmosphere (years)

2005 concentration (units vary)*

Current radiative forcing

Warming potential (radiative efficiency)
(W/sq m/ppb)

100 year global warming potential

Carbon dioxide


379 ( 0.65) ppm

( 0.17)





1,774 ( 1.8 ppb)

( 0.05)



Nitrous oxide


319 ( 0.12 ppb)

( 0.02)




Range: 740
to 50 000

Range: 3 to 74 ppt


0.1 to 0.57

Range: 7400 to 17 700


1.4 to 270

Range: 3 to 35 ppt

0.09 to 0.4

Range: 124 to 14 800

Sulphur hexafluoride


5.6 ( 0.038) ppt


22 800

Montreal gases

1 to 17 000

15 to 538 ppt

0.06 to 0.33

Range: 5 to 14 400

Note: The ranges are indicative only. The 90% confidence interval is shown in brackets where it is available.
* ppm = parts per million, ppb = parts per billion, ppt = parts per trillion.
Source: IPCC (2007a).

The total radiative forcing of the long-lived greenhouse gases is 2.63

( 0.26). In terms of carbon dioxide equivilance, this equates to a concentration around 455 ppm CO2-e (range: 433–477 ppm CO2-e).

However, the warming that would result from this is offset by the cooling effects of aerosol and land-use changes, which reduce the concentration to a range of 311 to 435 ppm CO2-e, with a central estimate of about 375 ppm CO2-e (IPCC 2007c: 102). It is the combined effect of all the influences on radiative forcing that is most relevant to the consideration of changes to the climate system.

Climate sensitivity

The relationship between warming and the change in the energy balance of the climate system is complex, largely due to the presence of internal feedbacks within the system that amplify or dampen the effect. Climate models provide a wide range of estimates of climate response. The variation in estimates results from limitations both in scientific understanding and in the ability of climate models to reflect the complexity. Key feedbacks that add to this complexity include:

‘Water vapour feedback’—Warmer air holds more water vapour, and as a strong greenhouse gas it adds to the warming of the atmosphere, which in turn leads to further warming. The feedback mechanisms in this relationship are not well understood.

Cloud formation—Changes in temperature, water vapour content and aerosols will influence cloud formation. By reflecting sunlight from their upper surface and absorbing radiated heat, clouds have competing effects on the climate response. The balance depends on many factors and understanding and agreement is low—this is the largest source of uncertainty in current estimates of climate response.

Ice and snow feedbacks—As ice and snow melt, the darker surface that replaces them (oceans, soil and forest) absorbs more heat. There is reasonable confidence in the impact of this effect on warming.

‘Climate sensitivity’ is the measure of the climate system’s response to sustained radiative forcing. It is strictly defined as the global average surface warming (measured in degrees Celsius) that will occur when the climate reaches equilibrium following a doubling of carbon dioxide concentrations above the pre-industrial value. A doubling of carbon dioxide levels is approximately equivalent to reaching 560 ppm carbon dioxide, which is twice pre-industrial levels of 280 ppm. Models predict a wide range of climate sensitivities due to differing assumptions about the magnitude of feedbacks in the climate system.

Climate sensitivity usually relates to the equilibrium temperature reached when all elements of the climate system have responded to induced changes to the climate system. Due to the long timescale of response, this may not occur for thousands of years. The IPCC estimates that it is likely that a doubling of carbon dioxide will lead to a long-term temperature increase of between 2C and 4.5C (IPCC 2007a: 12). It is considered unlikely that climate sensitivity will be less than 1.5C. For fundamental physical reasons and data limitations, values substantially higher than 4.5C cannot be excluded, but these higher outcomes are less well supported (IPCC 2007a: 799). The best estimate of the IPCC is about 3C (IPCC 2007a: 12).

When considering the impacts of climate change on human society in the coming century, and how to respond to those impacts, temperature change over shorter time frames is more relevant. The effective climate sensitivity reflects the warming occurring in the short term, and takes into account climate feedbacks at a particular time. It can also be better related to observed temperature data. Assumptions on rate of warming of the oceans in different models have a considerable effect on the short-term temperature outcomes.

Climate sensitivity is the largest of the uncertainties affecting the amount of warming when a single future pathway of greenhouse gases is selected (IPCC 2007a: 629). Climate variability

Climate variability refers to the natural variations in climate from the average state, and occurs over both space and time—over years or decades.

Natural climate variability occurs as a result of variations in atmospheric and ocean circulations, larger modes of variability such as the El Nio – Southern Oscillation (see Box 3.3), and events such as volcanic eruptions and changes in incoming solar radiation.

The extent of natural climate variability differs from place to place. In Australia, a notable feature of the climate is the high variability in rainfall from year to year, influenced by the El Nio – Southern Oscillation. This affects the ability of scientists to identify long-term trends in the climate system and establish whether the changes result from human activities.

Sustained changes to the energy balance of the climate system will cause a change in the long-term means of elements of the climate system such as temperature and rainfall, but may also lead to a change in the pattern of variability about a given mean. In Australia, observed changes in the climate suggest that the frequency of extremes in rainfall events is increasing at a faster rate than the mean (CSIRO & BoM 2007). Changes in extremes of climate variability are discussed further in section 3.4.The slow response of the climate system

Figure 3.6 shows estimates of the time it takes for different parts of the climate system to respond to a situation where emissions are reduced to equal the rate of natural removal. While greenhouse gas concentrations stabilise in around a hundred years, the temperature and sea-level rise due to thermal expansion of the oceans takes much longer to stabilise. Sea-level rise due to the melting of ice sheets is still increasing even after a thousand years.

[Figure 3.6 Schematic of inertia in the climate system]*
Source: IPCC (2001b: Figure 5.2), reformatted for this publication.Rate of change in climate response

Much of the analysis undertaken in relation to projections of climate change impacts focuses on the temperature increase under a certain emissions pathway by a point in time, often 2100. However, it is not just the magnitude of temperature rise, but also the rate at which it occurs that determines climate change impacts, as a higher rate of change in temperature affects the adaptive capacity in natural and human systems (Warren 2006; Ambrosi 2007; IPCC 2007a: 774).

3.4 Addressing the extremes: severe weather events, low likelihood outcomes, and thresholds

Due to the high level of uncertainty and the range of possible climate responses, there is a tendency in the policy community to focus on the mean, or median, or best-estimate, outcomes of climate change. An understanding of plausible extremes in the response of the climate system is vital to assessing risk.
    3.4.1 Definitions and terminology

When considering outcomes other than the mean, it is important to make a distinction between a climate event and a climate response or outcome.

An event is an element of climate or weather that can occur repeatedly as a function of climate variability. The occurrence of an event of a given intensity does not exclude the occurrence of a related event at a different intensity—average rain and intense rain, for example, can both occur in a single location, but the average will occur more frequently. The uncertainty relates to when an event will happen, not whether it will happen. The likelihood of these events is often identified by a factor that reflects the magnitude of the outcome as well as the likelihood—such as a ‘one-in-a-hundred-year storm event’.

By contrast, the occurrence of a particular response or outcome excludes the occurrence of a different one—the uncertainty relates to whether it will happen, when it will happen and the magnitude of its impact.

For all climate outcomes, there is a level of change that can be considered as extreme. Which outcomes are considered extreme is a subjective judgment dependent on many factors—including the sensitivity of the rest of the climate system to the change, and the vulnerability or adaptive capacity of the human or natural systems affected. Regardless of the definition, the likelihood of extreme climate responses generally increases with temperature.

Some climate responses involve singular events that have a large impact on the rest of the climate system, such as changes to large-scale patterns of variability or the melting of ice sheets. These outcomes are better assessed in terms of the likelihood of the event occurring for a given temperature.

For some of the better understood elements of the climate system, such as precipitation, different assumptions and model outcomes allow the identification of a range of outcomes for a given temperature increase. To better understand the potential impacts that could result for a given temperature rise, it is appropriate to consider the less likely outcomes—the bounds of the probability distribution—as well as the mean response to a given temperature increase.
    3.4.2 Severe weather events

In this report, a ‘severe weather event’ is defined as an event of an intensity that is rare at a particular place and time of year. Definitions of ‘rare’ vary, but severe weather events are usually as rare as, or rarer than, the 10th or 90th percentile of probability (IPCC 2007a: 945). Examples of severe weather events include:
  • hot days and nights (including heat waves)
  • cold days and nights (including frosts)
  • heavy rainfall events
  • droughts
  • floods
  • hail and thunderstorms
  • tropical cyclones
  • bushfires
  • extreme winds

The characteristics of what is called severe weather may vary from place to place in an absolute sense—for example, the temperature required to define a heatwave in Hobart would be lower than in Darwin.

In some cases, an increase in the frequency of one extreme will be associated with a decrease in the opposite extreme—an example is an increase in record hot days and a decrease in frost events with an increase in mean temperature (see Figure 3.7). However, if the variability (or variance) were to change rather than the mean, hot days and frosts would both increase. If both the variability and the mean were to change, there would be fewer frost events but a more significant increase in hot days.

Weather events may also be considered severe if they cause extensive damage due to timing or location, even if they are not considered rare in relation to their likelihood.
    3.4.3 High-consequence climate events and outcomes

Understanding the potential risk from climate change requires consideration of both the likelihood and the consequence of an event or response occurring. The extent of the impact of a change to the climate system will depend on various factors related to the change, such as its magnitude and timing, but also on the way natural or human systems respond to that change.

Severe weather events, or climate change outcomes or responses could be considered of high consequence, or ‘catastrophic’ due to a range of factors including:
  • the magnitude, timing, persistence or irreversibility of the changes to the climate system or impacts on natural and human systems
  • the importance of the systems at risk;
  • the potential for adaptation
  • the distributional aspects of impacts and vulnerabilities (IPCC 2007b: 781).

The uncertainty behind the likelihood, timing and extent of extreme climate responses is considerable. A selection of high-consequence climate events and outcomes are considered in more detail in Chapter 5.
    3.4.4 Tipping points or thresholds
The threshold at which a system is pushed into irreversible or abrupt climate change occurs is often referred to as the ‘tipping point’.

Many processes within the climate and other earth systems (such as the carbon cycle) are well buffered and appear to be unresponsive to changes until a threshold is crossed. Once the threshold has been crossed, the response can be sudden and severe and lead to a change of state or equilibrium in the system—this is often referred to as rapid or abrupt climate change (see Figure 3.8). Under abrupt climate change, a small change can have long-term consequences for a system (Lenton et al. 2008).

[Figure 3.7 Effect on extremes of temperature from an increase in mean temperature, an increase in variance, and an increase in both mean temperature and variance.]*
Source: IPCC (2001b: Figure 4.1), reformatted for this publication.

[Figure 3.8 Abrupt or rapid climate change showing the lack of response until a threshold is reached]*

In other cases, the crossing of a threshold may lead to a gradual response that is difficult or even impossible to reverse. An example is the melting of the Greenland ice sheet, which may occur over a period of 300 to 1000 years, but once started would be difficult to reverse.

3.5 Uncertainty in the climate science

    3.5.1 Addressing and communicating scientific uncertainty

The fact that many key aspects of the science of climate change are now well understood and agreed has not eliminated all uncertainties in relation to how the climate has and will respond to increases in greenhouse gas concentrations.

An understanding of the uncertainties in the climate science is essential to the assessment of climate risk. Uncertainty is not the same as low likelihood, but if it is not communicated well it can be difficult to incorporate into the decision-making process. It is therefore important that the approach taken to establishing an outcome be clearly communicated, transparent and reproducible (CSIRO & BoM 2007).
There is uncertainty at each step involved in assessing the past and future effects of human activity on climate change. A key point when considering and communicating uncertainty is how it ‘propagates’ when all the unknowns are taken into account. This is illustrated in Figure 3.9 (the blue bands demonstrate that as each layer of uncertainty is considered, the total range of uncertainty gets larger).

[Figure 3.9 Cumulative nature of uncertainties in the climate change science for a given pathway of future emissions]*
    3.5.2 Techniques for quantifying uncertainity

Uncertainty can be treated and quantified in different ways. Preferred approaches depend on the type of uncertainty and the time and modelling resources available.

The range of possible outcomes for a given set of assumptions can be assessed with multimodel simulations, which use a number of models with differences in their underlying mechanisms. This approach provides a useful assessment of the level and sources of uncertainty, but is both resource and time intensive.

To test the effect a given input assumption may have on a result, a sensitivity analysis is sometimes undertaken. This kind of analysis involves varying certain inputs in both plausible and implausible ways in order to explore how they change the outputs. If the outputs of interest are found to be insensitive to a change in a particular input, uncertainty in that input can be overlooked (CSIRO & BoM 2007).

The uncertainties in a range of different input assumptions can be tested through techniques such as the Monte Carlo analysis, which involves thousands of simulations being run which draw randomly from a set of input values. Where computing power is more limited, an analysis of the high, medium and low ends of probability of a certain outcome can be used to explore the potential range of impacts.

Additional information on the extent of uncertainty can be obtained through the consideration of expert judgment and other analytic techniques. Inclusion of these additional techniques in the consideration of total uncertainty recognises that models may underestimate uncertainties if they only include those aspects of the climate system in which scientists have confidence (Hansen 2007).

3.6 The science behind global mitigation

Goals for mitigation have typically been cast in terms of stabilisation of greenhouse gas concentrations in the atmosphere. Article 2 of the UNFCCC, which provides the international framework for climate change mitigation, states as the ultimate objective of the Convention:
    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 time frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner (UN 1992).

However, the UNFCCC does not define the point at which ‘dangerous anthropogenic interference with the climate system’ or ‘dangerous climate change’ might occur. Even if the climate change resulting from a given pathway of future emissions were known with certainty, there would be different approaches to defining ‘danger’, and interpretation of the UNFCCC goal will not be defined only by the science. Ethical, economic and political judgments will also be required (IPCC 2007c).

The Review’s terms of reference require it to analyse two specific stabilisation goals: one at which greenhouse gases are stabilised at 550 ppm CO2-e (strong global mitigation) and one at which they are stabilised at 450 ppm CO2-e (ambitious global mitigation). A stabilisation target of 450 ppm CO2-e gives about a 50 per cent chance of limiting the global mean temperature increase to 2C above pre-industrial levels (Meinshausen 2006), a goal endorsed by the European Union (Council of the European Union 2005) among others. Stabilisation at 500 ppm or 550 ppm CO2-e would be less costly than a more ambitious target, but is associated with higher risks of dangerous climate change.

Based on a ‘best estimate’ climate sensitivity of 3oC, stabilisation at 550 ppm CO2-e is likely to lead to an equilibrium global mean temperature increase of 3oC above pre-industrial levels (IPCC 2007c; Meinshausen 2006).
    3.6.1 What is stabilisation?

‘Stabilisation’ of a greenhouse gas is achieved when its atmospheric concentration is constant. For a group of greenhouse gases, stabilisation is achieved when the combined warming effect (radiative forcing) of the gases is maintained at a constant level.

Stabilisation of long-lived greenhouse gases does not mean the climate will stop changing—temperature and sea-level changes, for example, will continue for hundreds of years after stabilisation is achieved. How does the lifetime of a gas influence stabilisation?

For all greenhouse gases, if emissions continue to increase over time their atmospheric concentration will also increase. However, the way in which the concentration of a gas will change in response to a decrease in emissions is dependent on the lifetime of the gas (IPCC 2007a: 824). Stabilisation of greenhouse gas emissions is therefore not the same as stabilisation of greenhouse gas concentrations in the atmosphere.

Carbon dioxide is naturally removed slowly from the atmosphere through exchange with other parts of the carbon cycle. The current rate of emissions is well above the natural rate of removal. This has caused the accumulation of carbon dioxide in the atmosphere. If carbon dioxide emissions were stabilised at current levels, concentrations would continue to increase over this century and beyond. To achieve stabilisation of carbon dioxide concentrations, emissions must be brought down to the rate of natural removal.

The rate of absorption of carbon by sinks depends on the carbon imbalance between the atmosphere, the oceans and the land, and the amount already contained in these sinks. Once stabilisation in the atmosphere is reached, the rate of uptake will decline (Figure 3.10). Long-term maintenance of a stable carbon dioxide concentration will then involve the complete elimination of carbon dioxide emissions as the net movement of carbon dioxide to the oceans gradually declines (IPCC 2007a: 824; CASPI 2008).

The response of other greenhouse gases to decreases in emissions is more straightforward: the level at which concentrations are stabilised is proportional to the level at which emissions are stabilised. For gases with a lifetime of less than a century (such as methane) or around a century (such as nitrous oxide), keeping emissions constant at current levels would lead to the stabilisation of concentrations at slightly higher levels than today within decades or centuries, respectively (IPCC 2007a: 824). If emissions of these gases were to cease completely, concentration levels would eventually return to pre-industrial levels.

[Figure 3.10 Response of different carbon sinks to the rate of emissions over time]*
Source: Based on CASPI (2008).

For non-carbon dioxide greenhouse gases with lifetimes of thousands of years (such as sulphur hexafluoride), stabilisation would only occur many thousands of years after emissions stopped increasing. The response to decreases in emissions would thus happen over timescales that are largely irrelevant to current considerations. Therefore, in the policy context they should be treated in the same way as carbon dioxide, with the long-term aim of bringing emissions to zero in order to stabilise their warming effect.How to achieve stabilisation?

There are any number of emissions pathways that could lead to stabilisation of a gas at a given concentration. For carbon dioxide, these pathways generally involve a trade-off between the level at which emissions peak and the maximum rate of reductions required in the future. Figure 3.11 shows some of the possible emissions pathways to achieve the same stabilisation target. These curves are stylised—in the real world, annual emissions would fluctuate. The pathways that have a higher peak in emissions have a much greater rate of reduction at a later point in time, shown by the steepness of the curve.

[Figure 3.11 Different pathways of emissions reductions over time to achieve the same concentration target]*
Source: Based on CASPI (2008).

The timing of emissions reductions influences the efficiency of uptake of carbon dioxide by sinks, the rate of temperature increase and potentially the timing of climate–carbon feedback effects. For any given concentration stabilisation target, reaching the stabilisation target later by more rapid mitigation will give greater environmental benefits (O’Neill & Oppenheimer 2004), although small differences will not have material effects. In contrast, delaying mitigation, within limits, can reduce the upfront costs of mitigation (Wigley et al. 2006).Is a target of 450 ppm CO2-e or below scientifically possible?

The concentration of long-lived greenhouse gases in the atmosphere for 2005 is equivalent to the warming effect of 455 ppm of carbon dioxide. However, when the cooling influence of aerosols is included, the equivalent carbon dioxide concentration is estimated at 375 ppm CO2-e. The concentration of carbon dioxide in 2007 was 383 ppm (Tans 2008).

Due to the short lifetime of aerosols in the atmosphere, it is not appropriate to include their influence in a long-term target. Aerosols are expected to lessen through a reduction in the burning of fossil fuels as a result of climate change policies as well as through separate efforts to reduce air pollution.

The 2005 long-lived greenhouse gas concentration of 455 ppm CO2-e includes the warming influence of gases such as methane, which can be reduced in a relatively short period of time. If the target were set for some point in the future (such as 2050 or 2100), it would be scientifically feasible to bring CO2-e emissions down to a target level of 450 ppm CO2-e if immediate and deep cuts were made in emissions of most greenhouse gases.

However, to achieve a target of 450 ppm CO2-e would mean that global emissions would have to peak and fall almost immediately and a very high rate of reduction would be required. When viewed in the context of current emissions trends these fairly dramatic changes in emissions are not considered feasible. Hence, the 450 ppm CO2-e stabilisation scenario being considered by the Review includes the assumption of an ‘overshoot’ in greenhouse gas concentrations.Overshooting

There is increasing recognition in both science and policy communities that stabilising at low levels of CO2-e (around or below 450 ppm) requires ‘overshooting’ the concentration target (den Elzen et al. 2007; Meinshausen 2006; IPCC 2007a: 827).

The climate change impacts of the higher levels of greenhouse gas concentrations reached in an overshoot profile are dependent on the length of time the concentrations stay above the desired target, and how far carbon dioxide overshoots.

[Figure 3.12 Temperature outcomes of varying levels of overshooting]*
Source: Concentration and temperature pathways developed using SIMCAP (Meinshausen et al. 2006).
Figure 3.12 shows the different temperature outcomes for a range of cases of overshooting. All three cases show stabilisation at the same level in a similar time frame, but with varying amounts of overshooting. The temperature output demonstrates that while the ’small overshooting’ case remains under the target temperature, the other cases do not. Hence, due to inertia in the climate system, a large and lengthy overshooting will influence the transient temperature response, while a small, short one will not (den Elzen & van Vuuren 2007).

Increasing attention is being paid in the environmental and scientific communities to low stabilisation scenarios. In particular, a number of organisations in Australia have suggested that the Review should focus as well on a 400 ppm objective. They argue that the risks of immense damage to the Australian environment, including the Great Barrier Reef and Kakadu National Park, are unacceptably high at 450 ppm. Some scientists have also expressed the view that stabilisation at 450 ppm is too high (Hansen et al. 2008). For any such scenarios to be feasible, there will need to be a considerable period of overshooting.What is a peaking profile?

An overshooting profile requires a period in which emissions are below the natural level of sequestration before they are stabilised. Another mitigation option is to follow a ‘peaking profile’.

Under a peaking profile, the goal is to cap concentrations at a particular level (the peak) and then to start reducing them indefinitely, without aiming for any explicit stabilisation level. Stabilisation is therefore not conceived as a policy objective for the foreseeable future.

The key benefit of a peaking profile is that it allows concentrations to increase to or above the level associated with a given long-term temperature outcome, but reduces the likelihood of reaching or exceeding that temperature outcome. The higher level of peak concentrations means that current trends in emissions growth do not need to be reversed as quickly to achieve any given temperature goal. This decreases the costs of meeting a given temperature target (den Elzen & van Vuuren 2007).

A disadvantage of a peaking profile is that if the climate is found to be more sensitive to increases in greenhouse gases than anticipated, the more of the mitigation task left until later by delaying emissions reductions, the less flexibility there is to adjust to a lower concentration target later and an increased risk that a threshold may be crossed. Is overshooting feasible?

Designing a mitigation pathway—whether an overshooting or a peaking profile—that requires a decrease in the concentration of greenhouse gases assumes that emissions can be brought below the natural level of sequestration. Figure 3.13 shows overshooting profiles. A lower concentration target following an initial overshoot will require negative emissions net of natural sequestration for a longer period.

[Figure 3.13 Emissions pathways required to achieve a low concentration target following an overshooting]*
Source: Based on CASPI (2008).

The costs of reducing emissions below natural sequestration levels would be lower if controls on gross emissions were supported by cost-effective means of removing carbon dioxide from the atmosphere. Bringing emissions below the natural rate of sequestration would require rigorous reduction of emissions from all sources, but might also require extraction of carbon dioxide from the air. Possible methods include:
  • increasing absorption and storage in terrestrial ecosystems by reforestation and conservation and carbon-sensitive soil management
  • the harvest and burial of terrestrial biomass in locations such as deep ocean sediments where carbon cycling is slow (Metzger et al. 2002)
  • capture and storage of carbon dioxide from the air or from biomass used for fuel
  • the production of biochar from agricultural and forestry residues and waste (Hansen et al. 2008).

The simplest way to remove carbon dioxide from the air is to use the natural process of photosynthesis in plants and algae. Over the last few centuries, clearing of vegetation by humans is estimated to have led to an increase in carbon dioxide concentration in the atmosphere of 60 30 ppm, with around 20 ppm still remaining in the atmosphere (Hansen et al. 2008). This suggests that there is considerable capacity to increase the level of absorption of carbon dioxide through afforestation activities. The natural sequestration capacities of algae were crucial to the decarbonisation of the atmosphere that created the conditions for human life on earth, and offer promising avenues for research and development.

Technologies for capture and storage of carbon from the combustion of fossils fuels currently exist (Chapter 16), and the same process could be applied to the burning of biomass.

Today, there are no large-scale commercial technologies that capture carbon from the air. Yet some argue that it will be possible to develop air capture technologies at costs and on timescales relevant to climate policy (Keith et al. 2006). Captured carbon dioxide could be stored underground or used to produce biofuels.

Under a carbon price applying broadly across all opportunities for carbon dioxide reduction and removal, and with strong research and development support, there will be more rapid commercial development of both existing and new technologies to achieve negative emissions at a large scale (see chapters 13 and 16).
    3.6.2 Other methods of mitigating climate change

So far this section has focused on efforts to mitigate climate change by reducing the concentrations in the atmosphere of greenhouse gases. But other factors influence global temperatures, which could be influenced by humans. Geo-engineering is a term used to describe ‘technological efforts to stabilise the climate system by direct intervention with the energy balance of the earth’ (IPCC 2007c: 815).

A range of geo-engineering proposals have been put forward, including:
  • the release of aerosols into the stratosphere to scatter incoming sunlight (Crutzen 2006)
  • cloud seeding through the artificial generation of micro-meter sized seawater droplets (Bower et al. 2006)
  • fertilisation of the ocean with iron and nitrogen to increase carbon sequestration (Buesseler & Boyd 2003)
  • changes in land use to increase the albedo (reflectivity) of the earth’s surface (Hamwey 2005).

Geo-engineering proposals appear to have several advantages. First, they may be very cheap in comparison to reductions in greenhouse gas emissions. They can be implemented by one or a small number of countries and thus do not require the sort of widespread global action which stabilisation of greenhouse gases will require (Barrett 2008). They may be quick acting, with a lag from implementation to impact of months rather than decades. Geo-engineering techniques could potentially be deployed to avoid reaching a tipping point related to temperature increase.

However, geo-engineering proposals also have several disadvantages.

Those that focus on reducing solar radiation will do nothing to prevent the acidification of the ocean as a result of increased atmospheric concentrations of carbon dioxide, and therefore only provide a part solution to the wider environmental problem.

Geo-engineering techniques are generally untried. Some studies have been undertaken including through small-scale experiments on ocean fertilisation (Buesseler & Boyd 2003), investigation of similar natural phenomenon such as the release of aerosols from Mount Pinatubo in 1991, and computer simulations (Wigley 2006; Govindasamy & Caldeira 2000), but there will always be the risk of unanticipated consequences which could be significant and need to be further understood.

The fact that these solutions can be implemented unilaterally may also give rise to risks of conflict.

So far, the disadvantages of geo-engineering approaches have tended to outweigh the advantages in most minds that have turned to the issue. However, in recent years such proposals have received more support from a number of prominent scientists and economists, with calls for more research into the feasibility, costs, side effects and framework for implementation (IPCC 2007c: 79; Crutzen 2006; Cicerone 2006; Barrett 2008).


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Barrett, S. 2008, ‘The incredible economics of geoengineering’, Environmental and Resource Economics, 39(1): 45–54.

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