Climate change is best described as the effects on our climate which can be attributed to the enhanced greenhouse effect. The greenhouse effect describes the way in which greenhouse gases (GHGs) build up in our atmosphere and help trap heat. The enhanced greenhouse effect is due to an unnatural and increasing build up of GHGs in our atmosphere since industrialisation.

In 2007 the Intergovernmental Panel on Climate Change (IPCC) reported that it is “very likely” (>90 percent) that heat-trapping emissions from human activities have caused “most of the observed increase in globally averaged temperatures since the mid-20th century.”

Anthropogenic sources of greenhouse gases

(From the Australian Bureau Of Meteorology) More than a hundred years after the first scientific explanation of the earth’s natural greenhouse effect and sixty years after the Swedish scientist Svante Arrhenius first calculated the additional warming that might be expected from increased carbon dioxide in the atmosphere (1895), the distinguished US oceanographer and meteorologist Roger Revelle forcefully drew attention to the problems ahead: ‘Mankind, in spite of itself, is conducting a great geophysical experiment unprecedented in human history. We are evaporating into the air the oil and coal and natural gas that has accumulated in the earth for the past 500 million years.....This might have a profound effect on climate.’ The concerns of Revelle and others were instrumental in the initiation, in 1957, of what was arguably the most important single geophysical record ever established: the ongoing monitoring of the atmospheric concentration of carbon dioxide in the free atmosphere on the top of Mauna Loa, Hawaii.

Over the past two decades, the evidence for a continuing build-up of carbon dioxide and other greenhouse gases as a result of human activities has become conclusive. These changes have come about as a combined effect of increases in emissions, such as fossil fuel burning, and decreases in sinks, such as reduced forest cover.

(From the Australian Bureau Of Meteorology) Any changes in the relative mix and atmospheric concentration of greenhouse gases, whether natural or human-induced, will lead to changes in the radiative balance of the atmosphere, and hence the level of greenhouse warming. Calculations with global climate models have drawn clear links between increased concentrations of greenhouse gases and large-scale surface warming and other changes of climate. It seems likely that, through the 21st century, enhanced radiative forcing by increases in these gases will have a significant influence on global climate, including a detectable warming ‘signal’ above and beyond the ‘noise’ of natural variability.

(From the Australian Government Department of Climate Change) Throughout history, the Earth has experienced cold and warm periods, including ice ages and interglacial periods. Over the past million years, these natural climate changes were due to periodic variations in the Earth's orbit that affect the amount of sunlight reaching the surface. Greenhouse gas concentrations changed in concert with the temperature variations and caused about half of the magnitude of the climate changes.

The past 11,000 years is known as the Holocene Warm Period. Over the past 2000 years, regional fluctuations of 1.0-1.5°C have occurred. For example, northern Europe was cold until the 7th century, after which temperatures warmed to a peak, known as the Medieval Warm Period (900-1300 AD).

Complex natural fluctuations still affect the Earth's surface temperature and climate over long timescales. However, simulations using sophisticated computer-based climate models confirm that global warming during the past 50 years was mainly caused by human activities that have increased atmospheric concentrations of greenhouse gases, while the variations in the Sun's output, aerosols and land use have had smaller impacts.

The current global concentrations of carbon dioxide in the atmosphere (approaching 380 parts per million) are the highest in the last 650,000 years. The rate of increase in carbon dioxide during the industrial era is very likely to have been unprecedented in more than 10,000 years.
Climate models driven by scenarios of greenhouse gas emissions indicate that, over the next century, a global warming of 1.1-6.4 °C will occur.

This rate and magnitude of warming are significant in the context of the past 400,000 years. History has shown us that a warming of 1-2°C can have dramatic consequences. Even the 0.7°C warming in the past 100 years has been associated with increasing heat waves and floods, fewer frosts, more intense droughts, retreat of glaciers and ice sheets, coral bleaching and shifts in ecosystems. A further warming of 1.1-6.4°C could challenge the adaptive capacity of a range of human and natural systems.

(From the United Nations Framework Convention on Climate Change) The average sea level is predicted to rise by 9 to 88 cm by 2100. This would be caused mainly by the thermal expansion of the upper layers of the ocean as they warm, with some contribution from melting glaciers.

Global precipitation is predicted to increase, but at the local level trends are much less certain. By the second half of the 21st century, it is likely that wintertime precipitation in the northern mid- to high latitudes and in Antarctica will rise. For the tropics, models suggest that some land areas will see more precipitation, and others less. Australia, Central American and southern Africa show consistent decreases in winter rainfall.

The frequency and intensity of extreme weather events such as storms and hurricanes are likely to change. With increasing global temperatures the world is likely to experience more hot days and heat waves and fewer frost days and cold spells. Climate models also consistently show extreme precipitation events becoming more frequent over many areas and the risk of drought becoming greater over continental areas in summer. There is also some evidence to show that hurricanes could be more intense (with stronger winds and more rainfall) in some areas.