The temperature of earth as a whole is determined by the balance between incoming and outgoing energy. Climate change arises largely from changes to the earth’s heat balance. Many factors can influence this—both natural processes and ANTHROPOGENIC (which means human-made) processes. It is the impacts from anthropogenic processes, through the enhanced greenhouse effect, which are likely to be causing contemporary climate change, which in turn could bring about considerable environmental, social and economic disruption if adequate mitigation and adaptation measures are not implemented.
Factors influencing earth's climate
As the diagram shows, the main factors influencing climate change are:
Many of these factors are inter-related, and atmospheric, ocean and land interactions can involve complex FEEDBACK MECHANISMS that can either enhance or dampen changes to the climate system.
The natural influences on the climate system have caused variations in the earth's climate over hundreds of thousands of years, as well as on shorter timescales of decades. Many of these processes are unaffected by human activity, including the extraterrestrial factors and factors associated with the earth's tectonic activity. Exchange of heat and gases between the earth's atmosphere and its oceans and land vegetation are also natural influences on the climate system, but these processes are now being affected by human activities to various degrees. For example, the presence and composition of the earth's atmosphere produces a natural greenhouse effect. The atmosphere contains gases such as water vapour and carbon dioxide that are relatively transparent to the sun's shortwave radiation, but absorb some of the longwave radiation that is re-radiated from the earth's surface. This causes the average temperature on earth to be about 33°C warmer than it would be if there were no atmospheric 'blanket'. As discussed below, changes to atmospheric composition through human activity is the main factor thought to be causing the climate to shift from its natural state.
There is increasing evidence of human influences on the climate system. Human activities are directly influencing the composition of the atmosphere, by increasing the concentration of the naturally occurring greenhouse gases and by adding new ones. We are also changing the reflectivity over parts of the earth's surface.
Increasing levels of greenhouse gases in the atmosphere is by far the most influential amongst the factors forcing contemporary climate change. Greenhouse gas emissions from human activity derive mainly from combustion of fossil fuels, with additional significant contributions from industrial processes, agriculture, and land use change. Changes in levels of black carbon particulates, snow albedo, and atmospheric pollutants have small additional impacts on global warming. Other factors which offset these effects are cloud albedo, aerosols, land albedo change, and, periodically, dust from volcanic eruptions. The estimated relative contribution of these factors to changes in radiative forcing since 1850 are illustrated in the figure below. A positive change represents an enhancement of the greenhouse effect, while a negative change reduces the greenhouse effect.
Globally and annually averaged temporal evolution of the instantaneous all-sky radiative forcing due to various agents, as simulated in the MIROC+SPRINTARS model. This is an illustrative example of the forcings as implemented and computed in one of the climate models participating in the IPCC Fourth Assessment Report. Note that there could be differences in the radiative forcings among models. Most models simulate roughly similar evolution of the long-lived greenhouse gases’ radiative forcing.
Source: Intergovernmental Panel on Climate Change, Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 2 Changes in atmospheric constituents and radiative forcing, Figure 2.23, p. 208.
The concentrations of long-lived greenhouse gases (LLGHGs) have increased rapidly over the past 20 years. These contribute the most to RADIATIVE FORCING, exceeding the level of contribution from all other ANTHROPOGENIC agents throughout the latter half of the 20th century.
The input from solar effects is around 20 per cent of the combined effect of all anthropogenic agents, and about an order of magnitude less than the total greenhouse gas contribution.
Volcanic eruptions have a large but transitory negative effect. From 1950 to 2005, the combined natural forcing has been either negative or slightly positive. The Fourth Assessment Report of IPCC concluded that radiative forcing related to natural systems has played little or no contributing role in driving climate change over the last 50 years.
The presence of dust floating in space between the sun and the earth could influence the amount of heat arriving on our planet. It has been suggested, therefore, that changes in the quantity or type of space dust, as the solar system orbits the galactic centre, may influence global climate. Recent evidence suggests this is not currently occurring and has not played a part in the last few thousand years. The evidence comes from studying some of the 40,000 tons of extraterrestrial matter that is believed to reach the earth every year. Studies of helium isotopes in the arriving dust indicate that the rate of space dust deposition on earth is constant, which means that the quantity of dust in the vicinity of earth must also be constant; this lack of variation therefore suggests that changes in the amount of space dust cannot be responsible for significant change to the earth's climate at the present time.
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 1 Historical overview of climate change science.
Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 2 Changes in atmospheric constituents and in radiative forcing.
Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 9 Understanding and attributing climate change.
W. J. Bouma, G. I. Pearman and M. R. Manning (eds), Greenhouse—coping with climate change, CSIRO Publishing, Collingwood, 1996.
G. Winckler and H. Fischer, '30,000 years of cosmic dust in Antarctic ice', Science, vol. 313, 28 July 2006, p. 491.