The Science

“The world’s scientists have spoken clearly and with one voice. I expect the world’s policy makers to do the same.” UN Secretary-General Ban Ki-moon, commenting at the launch of the Intergovernmental Panel on Climate Change, Fourth Assessment Synthesis Report.

What is the greenhouse effect?

All materials radiate heat. The amount they radiate depends on their temperature: the warmer they are the more they radiate. Planets like the Earth thus warm until their temperature is such that the amount of heat they radiate equals that which they receive from the sun.¹ But planets have atmospheres. In the case of the Earth, the atmosphere is thin (80% of the air in approximately the first 12 km of altitude compared with the diameter of the Earth being about 12 thousand km). The gases that make up the Earth’s atmosphere are transparent to sunlight that heats the Earth’s surface. But some gases in the atmosphere absorb part of the heat that the Earth radiates (see Figure 1). So the temperature needs to be higher than it would otherwise be to ensure that sufficient heat is lost to balance the energy coming in. This is called the greenhouse effect, because, like the glass in a greenhouse, the atmosphere enhances the temperature of the Earth’s surface.² Gases in the Earth’s atmosphere that have these properties are called greenhouse gases and include water vapour, carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O).

This greenhouse effect has a major impact on the temperature of the Earth and the neighbouring planets, Venus and Mars (see Table 1). The greenhouse effect is a natural feature of the Earth’s climate. Without these gases, the surface of the Earth would be about 33^oC cooler than at present.

Figure 1: The greenhouse effect. Source: UNFCCC

Table 1: The observed temperatures of the Earth and neighbouring planets is explained by both their distance from the sun, but significantly modified by the amount of greenhouse gases in their atmosphere. From IPCC (1990).

Around 150 years ago, physicists who understood the heat-trapping properties of carbon dioxide suggested that it would influence the climate of the Earth and, indeed, that changes in its concentration would bring about climate change. Today we understand that these natural greenhouse gases together with some additional greenhouse gases, chlorofluorocarbons (manufactured by human activities), are increasing in the atmosphere. Increasing levels of these gases have already caused warming of the Earth and are anticipated to lead to climate change through this century. This is called the enhanced greenhouse effect.

While precise and regular measurements of carbon dioxide did not commence until 1958 and in the 1960s and 1970s for the other gases, we now have clear evidence of changes in their concentration since that time. In addition, through the retrieval, measurement and dating of air trapped in polar ice, we have been able to trace changes in composition over the last half million years (see, for example, Figure 2).

In any sample of a greenhouse gas, there will be different ratios of molecules that share slightly heavier or lighter atoms. These are called the isotopes of the gas.³ From isotopic studies and understanding of the global budgets4 of the gases and their time and spatial distributions, we know that the rise in concentration of these gases has resulted from modern industrialisation over the last century or so. Table 2 shows their concentration prior to industrialisation, current levels and their calculated impact on global surface temperatures thus far.
These changes to the composition of the Earth’s atmosphere have resulted in a small but significant warming of the Earth with concomitant changes to the details of the planet’s climate system and changes to biological systems that are influenced by or depend on climate.

They have also led to a very significant growth in research effort aimed at better understanding global vulnerability to future climate change. At the same time, an International Framework Convention on Climate Change has been established to develop a global response to the issue (see Box 1).

Figure 2: Changes in the atmospheric concentration of the greenhouse gases carbon dioxide and methane since the year AD 1. From published and new ice-core data of Etheridge and MacFarling, CSIRO. Different colours represent different sources of data. Black solid lines in recent times are direct observations at Cape Grim.

Table 2: Past, current and estimated future levels of greenhouse gases in the global atmosphere with estimates of their impact on energy flow into the lower atmosphere and thus temperature (based on IPCC 2001a, 2007a): ppmv (parts per million by volume).

1 This is called the Stefan-Boltzmann Law, I = ??T4, where ? is the emissivity (“blackness” of the body which in the case of the Earth this is close to 1.0), ? is a constant (5.67 x 10-8 W m-2 K-4) and T is the temperature in Kelvin degrees. The Earth, when at equilibrium with the incoming sunlight, calculated by this formula, would about -18°C, 33°C below its current temperature.
2 Actually, this comparison is not a very good one as a greenhouse achieves much of its effect by limiting vertical mixing of heated air, unlike the free atmosphere. But the usage of the term is now well entrenched.
3 For example, most carbon dioxide (one atom of carbon plus 2 atoms of oxygen) has carbon that has an atomic weight of 12. But about 1% has a carbon mass of 13, and a very small trace, the radioactive carbon, a mass of
14. The ratios of these different masses in the carbon dioxide often indicate where the gas came from. For example, as carbon 14 is radioactive and decays on average in around 6000 years, fossil carbon such as that in coal, oil and natural gas, contains no carbon 14.
4 The global budget of a particular gas includes how much of the gas resides in the atmosphere, is dissolved in the oceans, or contained in plant and animal material. It also includes how much of the gas moves between these reservoirs, and what are the causes of these movements. It is, in the case of carbon dioxide, the basis of our understanding of how much of any carbon dioxide added to the atmosphere by human activities will stay there, and how much and at what rate it will move into the oceans and/or plants and animals (biosphere).

Climate system - carbon cycle

Over millennia, the amount of carbon dioxide in the atmosphere is determined by the temperature and chemical properties of the ocean. But for shorter periods of decades to centuries, equilibration between the atmosphere and the oceans can be incomplete as re-adjustment to changes in one reservoir or the other is slow. For example, each year photosynthesis by the world’s living plants absorbs from the atmosphere around 50,000 million tonnes (50 Gt) of carbon⁵ , replacing most of it by respiration during that same year. This causes seasonal variations in the amount of carbon dioxide in the atmosphere observed in records of atmospheric carbon dioxide, particularly in the Northern Hemisphere. From year to year, climate fluctuations, such as those caused by the familiar El Nino phenomenon, cause slight imbalances that lead to variations in the amount of carbon dioxide in the atmosphere. During the last ice age, as a result of changed biological conditions (half the Earth was covered with ice, sea levels were 80 metres lower than at present and ecosystems significantly different), cooler ocean temperatures and changed ocean circulation, carbon dioxide concentrations were about 40% lower than they were pre-industrially.

But over the last century or so humans have been adding carbon dioxide to the atmosphere, today at a rate of around 8 Gt of carbon per year, primarily as a result of the combustion of fossil fuels, coal, oil and natural gas. The oceans are currently only capable of removing about 2 Gt of carbon per year from the atmosphere into the deep ocean. Thus, as a result, concentrations in the atmosphere are rising steadily. Since industrialisation concentrations have increased by about 32% and each year are increasing at a little less than 1%.

Small imbalances in the seasonal, year-to-year and decade-long exchange of carbon dioxide from the combustion of fossil fuels, activities of the global biosphere, and dissolution and transport into the deep oceans leads to both temporal and spatial variations of atmospheric carbon dioxide concentration. These variations are small, but observations at several high-precision observatories around the world are used to identify the behaviour of the carbon cycle. One such observatory is at Cape Grim, North West Tasmania (see Figure 3).

Combined with measurements of the isotopic composition of the carbon and atmospheric oxygen these observations enable us to complete a picture of the past (Figure 4) and current (Table 3) global carbon budget. One surprising finding has been that, despite deforestation around the world, which must release additional carbon dioxide into the atmosphere, the global biosphere has actually grown over the past 20 years or so, partially offsetting the release of carbon from the burning of fuels and thus slowing the rate of concentrations increase from what it might otherwise have been.

Figure 3: Annual mean carbon dioxide concentration measured at Cape Grim, Tasmania , (http://www.bom.gov.au/inside/cgbaps/ (Paul Fraser, Personal Communication).

Figure 4: Global carbon emissions (left) and uptakes (right) since 1800 (in thousand million tonnes, Gt, of carbon).

Table 3: Global budget of carbon dioxide showing the rate of accumulation in the atmosphere and the main annual exchanges between the atmosphere, oceans and biosphere in thousand million tonnes of carbon per year (Gt yr-1). (After IPCC 2001).

The IPCC Fourth Assessment Report (IPCC 2007a) concluded that over the coming decades, the capacity of both the oceans and the biosphere to absorb carbon dioxide from the atmosphere is likely to decline. It was somewhat of a surprise to some scientists that this year, observational evidence was found to show that oceanic uptake has been declining (as a proportion of emitted fossil carbon dioxide) through the past 2 decades (see Canadell et al. 2007: Le Quere et al. 2007).

The combustion of carbon-based fuels that release carbon dioxide into the atmosphere is the way humans around the world generate energy to underpin their economic and social wellbeing. It includes the energy required for the heating and cooling of the houses, businesses and industry; the energy required for transportation for ourselves and products that we trade; and the energy required to power our production and manufacturing industries. The relative level of each of these activities is different for each economy around the world. Figure 5 shows the breakdown of energy production, in terms of greenhouse gases emitted, for the Australian economy.

Each greenhouse gas has a different greenhouse effect, first because its molecules absorb more or less heat and second because they stay in the atmosphere for longer or shorter periods (eg. carbon dioxide for about 80 years and methane for about 10 years). Thus, when adding up the effects of all gases, the minor gases are added according to their impacts as an equivalent amount of carbon dioxide.

Figure 5 shows that while stationary energy (that produced by the major power stations around the country) is the dominant source of emissions, other sources are important. In total Australians emit 550 million tonnes of carbon dioxide equivalent each year to produce their energy needs. This is the average for all Australians, and converts to each Australian annually emitting the equivalent of about 28 tonnes of carbon dioxide (or about 7 tonnes of carbon); one of the highest, if not the highest, rates in the world.

Figure 5: Mega tonnes of carbon dioxide equivalents6 emitted by Australia based on the Australian Greenhouse gas inventory. (see http://www.greenhouse.gov.au/inventory/2003/pubs/inventory2003.pdf).

The climate system - Modelling its behaviour

Understanding how these changes in greenhouse-gas concentrations have influenced our climate, and anticipating how further changes may lead to future change to climate, requires that we have a comprehensive understanding of the total global climate system. This system is complex (see Figure 6). It entails how sunlight penetrates the Earth’s atmosphere, including being partially reflected by clouds, or dust and heats the Earth’s surface; how this heating varies from day to day, season to season, and Equator to Pole influencing, the regional heating rates of the Earth; and how the climate responds. But further, it involves understanding how this heat is then redistributed by the circulation of the atmosphere and the oceans as they respond to the uneven heating, and by the evaporation of water which transfers energy into the atmosphere and drives some of the circulation processes (eg. tropical storms); and how that water is condensed to form clouds that influence the reflection of sunlight or is precipitated elsewhere as part of the climate system.

Scientists have approached this challenge by, in the first place, making observations and trying to understand each of the component processes that make up the total system. They then write equations that represent each component, and combine the equations for all components of the system into a single computer model of the climate. Such models are enormously complex, as one might expect from the complexity of the system they attempt to represent. They may consist of 1,000,000 lines of computer code to instruct the computer as to how to represent the climate. These models do not necessarily incorporate existing meteorological information. Rather they attempt, from first principles, to represent the physics and dynamics (motions) of the climate system.

Figure 6: Diagrammatic representation of the components of the global climate system, from the
Australian Bureau of Meteorology, http://www.bom.gov.au/info/climate/change/gallery/6.shtml.

Confidence in the realism of the models is obtained from the fact that when the equations are solved in a supercomputer, they produce a model climate system that has substantial similarities with that of the real world. These may be similarities in how the models reproduce observed diurnal variations in temperature, pressure, winds, humidity; seasonal variations of temperature, rainfall; or the response of the global system to major volcanic eruptions, or the tilting of the earth on its axis that caused the ice ages of the past million years. These and other factors have led scientists in the last few years to have confidence that these models can be used both to assess the changes in climate thus far, and what we might expect over this century due to ongoing increases of greenhouse gases.

In the IPCC Fourth Assessment Report some 23 models are used from laboratories all over the world (including Australia). This huge increase in the number of modelling groups simulating the Earth’s climate, and the increased number of experiments possible as a result of growing availability of computing power, means that this Assessment provides far more confident projections of what might occur to the Earth’s climate in the future.