The major constituents of the atmosphere (oxygen and nitrogen) are essentially transparent to both the incoming solar radiation and the infrared radiation emitted upward from the earth’s surface. A number of minor constituents, especially water vapour and carbon dioxide, are also largely transparent to the incoming solar radiation, but strongly absorb the infrared radiation emitted from the ground. The radiation absorbed by these gases is re-emitted in all directions, some back toward the surface leading to a net warming of the surface. These so-called greenhouse gases trap heat in the near surface layers of the atmosphere and thus cause the earth’s surface to be considerably warmer than if there were no greenhouse effect. With no greenhouse gases, the earth would have an average temperature of –18 °C.

The global average surface temperature has increased by 0.6 +/- 0.2°C since the late 19th century. It is very likely that the 1990s was the warmest decade and 1998 the warmest year in the instrumental record since 1861. Globally, 2005 was the second warmest year on record, and the hottest year on record for Australia.

Reconstructions of temperature over the last 1,000 years indicate that the temperature changes over the last hundred years are unlikely to be entirely natural in origin. The Intergovernmental Panel on Climate Change is recognised as the global authority on climate change. The IPCC concluded that most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations. The IPCC projects that global mean temperature will increase by 1.4 to 5.8 ˚C over the period 1990 to 2100. This projection has led many scientists to predict that if society pursues a ‘business as usual’ approach to greenhouse emissions, the collapse of society due to climate change is inevitable.

However, the ‘consensus’ science of the IPCC has been disputed. The majority of arguments put forward by greenhouse ‘skeptics’ can be categorised into three areas: recent warming of the earth’s surface is not unusual in the context of climate history; general circulation models are not adequate representations of climate, and are unreliable to project future climate states; and projected global warming scenarios are exaggerated. In particular, the ability of global climate models to project future climate is disputed. For instance, the IPCC itself acknowledges that scientists do not know if clouds enhance or diminish the greenhouse effect.

‘Contrarian’ climate change scientists dispute the ‘collapse of civilisation’ predictions, with many believing that global warming will be at the lower end of the IPCC temperature range projections.

The challenge for governments is to assess this conflicting science to develop public policy.