Causes of climate change

What is climate change?

The Earth’s climate varies over times scales from months through to centuries and beyond. Factors that affect the climate over time-scales from hundreds to millions of years include:

  • energy output from the sun,
  • variation in the earth’s orbit and the orientation of its axis,
  • the greenhouse effect of water vapour and other trace gases,
  • volcanic and meteorite activity and plate tectonics (movement of the continents).

For example, glacial cycles are driven by wobbles in the Earth’s orbit. Greenhouse gases and polar ice sheets respond to this wobble and enhance the warming/cooling cycle of the earth by 2-3°C. Other natural phenomena cause variability at decadal and inter-annual scales: changes in sea surface temperature, ocean currents and the associated changes in the atmospheric circulations (eg. the El Niño-Southern Oscillation). The resultant changes to the climate from these phenomena are considered to be natural variation.

The term ‘Climate Change’ commonly refers to influences on climate resulting from human practices. Increases in the concentration of so-called greenhouse gases in the atmosphere resulting largely from burning of fossil fuels and deforestation, have led to an observed and projected warming of the earth, known as the enhanced greenhouse effect. It is not easy to distinguish this anthropogenic climate change from the natural variations in the drivers described above.

The Murrumbidgee River at Darling Point, NSW

The Murrumbidgee River at Darlington Point, NSW

The study of the climate and its variations, extremes and shifts is not a new science; Svante Arrhenius suggested in 1896 that burning of fossil fuels might cause an increase in carbon dioxide in the atmosphere and in turn warm the Earth (Pittock 2003). However, the recent succession of unusually warm years and extreme climatic events has heightened awareness and climate change is now a mainstream media topic, in recent times moving to headline status. The intensity of interest is placing considerable pressure on the capacity of the scientific community to respond to the concerns being raised.

Until very recently much of the media attention centred on the sceptics who put up counter arguments to the existence of climate change, pointing to the 5°C difference in global average temperatures between the glacial and inter-glacial periods (>10,000 year cycles) as evidence that recent temperature trends are within normal variation (Pittock 2003). These sceptics played a very important role in forcing the scientists to produce evidence of rigorous analysis of their scenarios and projections. If current trends continue, scientists predict temperature rises of up to 5°C over the next century, causing major perturbation of natural and human systems.

What causes climate change?

The Earth would be much colder if not for the ‘greenhouse’ gases that provide a blanket that warms the atmosphere. Some of the gases in the atmosphere transmit the short-wave radiation from the Sun to the Earth, warming its surface.

Some of this warmth is emitted in the form of long-wave (infrared) radiation from the Earth to the atmosphere and some of the gases in the atmosphere absorb and re-emit radiation of this wavelength, effectively enhancing the warming of the lower atmosphere (Figure 1). These gases are called greenhouse gases because their effect is similar to the function of a glass greenhouse that heats up as infrared radiation is trapped by the glass. The main greenhouse gases are water vapour, carbon dioxide, methane and nitrous oxide, all of which occur naturally in the atmosphere.

The greenhouse effect

Graphical representation of the greenhouse effect.  Source: CO2 Cooperative Research Centre

Water vapour is the major contributor to the greenhouse effect. Water vapour concentrations fluctuate regionally due to natural impacts, and human activity generally does not directly affect water vapour concentrations except at very local scales. However, climate models are now predicting the concentration of water vapour in the upper troposphere may increase in response to increasing concentrations of other greenhouse gases (Steffen 2006). This increase in water vapour could play a key role in amplifying the rate at which the climate warms (Soden et al. 2005).

The gases that contribute directly to the enhanced greenhouse effect as a result of anthropogenic activities are carbon dioxide, methane and nitrous oxide emitted from combustion of fossil fuels, deforestation and agriculture, and sulphur hexafluoride, perfluorocarbons and hydrofluorocarbons arising from industrial processes. It is these six gases that are controlled under the UN Framework Convention on Climate Change. Some other gases, including carbon monoxide, nitrogen oxides and volatile organic compounds, contribute indirectly to global warming through chemical reactions in the atmosphere. Other emissions, such as sulfate aerosols have a cooling or dimming effect on the climate as they reflect some of the short-wave radiation before it reaches the earth’s surface.

The contribution of each of the greenhouse gases to global warming is dependent on its Global Warming Potential (GWP), expressed as carbon dioxide equivalent (CO2 e). The GWP takes into account:

  • the amount of radiation that the gas absorbs and the wavelength at which it absorbs,
  • the time that the gas stays in the atmosphere before reacting or being dissolved in rainwater or the ocean,
  • the current concentration of the gas in the atmosphere,
  • and any indirect effects of the gas (eg. methane will produce ozone gas in the lower atmosphere and water vapour in the stratosphere).

The GWP of nitrous oxide is 298 times that of carbon dioxide and methane is 25 times that of carbon dioxide when considered over 100 years (Solomon et al. 2007).

The concentration of CO2 in the atmosphere in 2005 was 379 parts per million (ppm), compared with the pre-industrial value of 280 ppm, and is rising at 1.9 ppm per year (1995-2005 average) (Solomon et al. 2007). The increase in concentration of greenhouse gases in the atmosphere has altered the earth's radiative balance, resulting in more of the sun's heat being absorbed and trapped inside the earth's atmosphere, producing global warming. Without mitigation measures, the concentration of CO2 in the atmosphere is predicted to rise to at least 650 ppm and up to 1200 ppm by 2100 (IPCC 2001a), which is expected to increase average global temperature by 1 to 6°C.

Most scientists agree that global warming caused by anthropogenic greenhouse gas emissions is one of the most serious environmental problems facing the world today, with far-reaching consequences for all sectors of society. To avert catastrophic impact it is generally agreed that atmospheric CO2 concentration should be constrained to 550 ppm, which is believed will limit the temperature increase to 2°C.