Earth's climate has always been affected by a range of natural factors and is known to have changed significantly during recorded history, before gaseous emissions from human activity could have posed any significant threat to atmospheric or climatic conditions. For example, in the Middle Ages Europe was far warmer than it is today. Then, during the 17th and 18th centuries the ‘LITTLE ICE AGE’ brought colder conditions across Europe. Although it appears that these conditions may have represented regional rather than global climate change, it is certain that the earth's climate has differed considerably in the past, as has the concentration of CO2 in the atmosphere, and sea level has also fluctuated.
The graph below shows average global temperature changes plotted against a scale for time increasing in powers of ten, from millions of years ago to the present.
Earth's temperature trends
A schematic representation of recent climate trends and future projections in historical perspective. The 20th century is shown on a linear scale. Earlier periods are shown in terms of increasing powers of ten years ago but are linear within each period.
Source: Bureau of Meteorology, The greenhouse effect and climate change, Bureau of Meteorology, 2003, p. 33.
However, just because climate can change naturally does not mean that the current change is natural. Data synthesised by the IPCC suggests that the warming observed over the last two hundred years (see Temperatures are rising) is unusual in at least the last 1300 years. Though there have been various challenges to the validity of the temperature data and its interpretation, the IPCC has found that the recent warming trend is unequivocal, and furthermore that most of the observed warming is very likely due to human influences on the climate system. There is strong evidence that it is caused by the increased concentrations of carbon dioxide, methane and other gases caused by human activity. In addition, the rate of change is much faster than appears to have been the case in the past.
The challenge remains to understand how the complex interplay of natural and anthropogenic driving forces will impact on the earth's climate into and beyond the 21st century. The correlation of global average temperature with solar activity is of great interest. The sun's output waxes and wanes slightly in a periodic regular cycle (the sunspot cycle), which is well known. The sun can also vary in its luminosity independently of the sunspot cycle. Other important natural factors influencing global climate are changes in earth orbit, volcanic eruptions, continental drift and mountain building, heat exchange between the oceans and the atmosphere, atmospheric composition and the earth's reflectivity.
There appears to be no direct relationship between the PALAEOCLIMATE of the northern hemisphere and of the Australian region. In our region the present climate regime which began about 3000–2000 years ago has been highly variable in precipitation and temperature, but there appear to be no direct correlations with the northern warm and cool episodes such as the MEDIEVAL WARM PERIOD or the LITTLE ICE AGE. However, the tropical glaciers of New Guinea have advanced and retreated several times over the last 3000 years. New Zealand glaciers show multiple periods of advance, the most recent being about 100 years ago which coincided with the coolest mean annual temperatures ever measured. The relatively rapid retreat of glaciers in Papua New Guinea and New Zealand that began about 60 years ago is not likely to be primarily related to natural variability. Rather, it is thought to be a response to warming due to the enhanced greenhouse effect brought about by alteration to the earth's atmosphere from human activities.
Australian Bureau of Meteorology, The greenhouse effect and climate change, Bureau of Meteorology, 2003.
Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 6 Palaeoclimate.
Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 1 Historical overview of climate change science.