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).