We grow plants in greenhouses (glass-houses) because they provide a warmer environment than in the open air. The common explanation for the increase in temperature is that the glass is transparent to short-wave (solar) radiation and opaque to longwave (infrared) radiation. More radiant energy is trapped therefore temperature is increased. While this explanation turns out to be rather incomplete for glass-houses (it's actually suppression of convection which accounts for most of the increase in temperature), it is a very accurate explanation of why the surface of the Earth is kept warm by the presence of the so-called greenhouse gases in the atmosphere (see Figure 1).
Figure 1. A simplified diagram illustrating the global long-term radiative balance of the atmosphere. Net input of solar radiation (240 Wm−2) must be balanced by net output of infrared radiation. About a third (103 Wm−2) of incoming solar radiation is reflected and the remainder is mostly absorbed by the surface. Outgoing infrared radiation is absorbed by greenhouse gases and by clouds keeping the surface about 33°C warmer than it would otherwise be (from IPCC, 1994. With permission).
While most planets exhibit a greenhouse effect, the magnitude of the warming depends on the composition of the planetary atmosphere, because it is only those components which absorb and re-emit infrared radiation which contribute to the effect. The atmosphere of Venus is composed almost entirely of carbon dioxide (CO2), giving rise to a greenhouse effect of more than 500K and a surface temperature of around 750K. The atmosphere of Mars, also composed chiefly of CO2 but much thinner than that of Venus, provides a greenhouse warming of around 10K. The Earth's atmosphere, with a greenhouse effect of around 30K, contrasts with the other planets in that water vapor is the most important greenhouse gas, followed by carbon dioxide, and that together these gases represent only a small fraction of the total mass of the atmosphere; the major components nitrogen and oxygen do not absorb infrared radiation and play no direct part in the Earth's greenhouse effect.
Changes in the atmospheric concentration of greenhouse gases will alter the greenhouse effect, leading to a change in climate as the atmosphere adjusts to a new equilibrium. Because marine and terrestrial vegetation plays such a large part in the exchange of greenhouse gases between the atmosphere and the surface, it is likely that past changes in climate occurring over time scales of thousands to millions of years, even when caused by external factors, have been modulated by changes in the marine and terrestrial biospheres. More recently, the increasing use of fossil fuels since the Industrial Revolution has demonstrably increased atmospheric concentrations of CO2. Concentrations of methane (CH4) and nitrous oxide (N2O) have also increased due, at least in part, to human activities, and some widely-used synthetic chemicals such as the chlorofluorocarbons (CFCs) are new and powerful greenhouse gases.
In the absence of other changes, a doubling of CO2 concentration (or equivalent increases in other greenhouse gases) would increase the Earth's surface temperature by around 1K. In practice, other complex changes would be expected to occur in the climate system. Some of the associated changes will tend to reduce the warming and some will tend to increase it, and the greatest uncertainty in calculating climate change under the enhanced greenhouse effect is due to uncertainty in these climate feedbacks, particularly those involving the amount and distribution of water vapor. Numerical models of climate generally predict that the net effect of climate feedbacks would be to increase the warming significantly above that by the simple calculation based on radiation (see Figure 2).
Figure 2. Future temperature rise will depend on future emissions of greenhouse gases. The diagram shows projected changes in mean surface temperature under the IS92 scenario of future emissions (see IPCC, 1992). Climate sensitivity is an index of the equilibrium change in global mean temperature that would occur in response to a doubling of CO2 concentration. Low climate sensitivity = 1.5°C, best estimate = 2.5°C and High = 4.5°C (from IPCC, 1992. With permission). The calculations do not include the effect of atmospheric dust and particulates (aerosols).
Atmospheric particulates (aerosols) derived from industrial emissions and from biomass burning exert a cooling influence on climate because they increase the amount of sunlight reflected to space. Though linked to the enhanced greenhouse effect through emissions (greenhouse gases often come from the same sources as the aerosols or their precursors, e.g., power station chimneys) atmospheric particulates are not themselves greenhouse agents.
The warming effect of greenhouse gases in the atmosphere was first recognized in 1827 by Jean-Baptiste Fourier. Around 30 years later John Tyndall measured the infrared absorption characteristics of water vapor and carbon dioxide, and suggested that reduced levels of atmospheric carbon dioxide may have been a cause of ice ages. In 1896, Svante Arrhenius estimated that doubling atmospheric concentrations of carbon dioxide would increase global average temperature by 5-6°C (not very different from current estimates), but it was about a half century before G. S. Callendar made a similar calculation for the effect of the burning of fossil fuels.
Such calculations received little attention outside of scientific circles and it was not until the mid-1970s that "the Greenhouse Effect" or "Global Warming" began to move into the public—and therefore political—spotlight, prompted by a variety of factors including: clear evidence of increasing atmospheric concentrations of carbon dioxide and of its link to human activities; results from computer models which suggested significant regional changes of climate change associated with an enhanced greenhouse effect; and, not least, media coverage which tended to associate the greenhouse effect with images of disaster and chaos, leaving the public (and governments) disturbed but not necessarily better informed.
To provide an authoritative assessment of current scientific understanding of the enhanced greenhouse effect the World Meteorological Organisation (WMO) and the United Nations Environmental Programme (UNEP) jointly established an Intergovernmental Panel on Climate Change (IPCC) in 1988. The IPCC reports in 1990 provided impetus to the negotiation of the UN Framework Convention on Climate Change, which was eventually signed in Rio de Janeiro, Brazil by over 150 countries in June 1992.
Basic physics of the greenhouse effect
Houghton, J. T. (1986) The Physics of Atmospheres, 2nd edn., Cambridge University Press, Cambridge.
The enhanced greenhouse effect and climate change
Climate Change 1990: The IPCC Scientific Assessment, J. T. Houghton, G. J. Jenkins and J. J. Ephraums, Eds., Cambridge University Press, Cambridge.
Climate Change 1995: The Science of Climate Change Contribution of WGI to the Second Assessment Report of the Intergovernmental Panel on Climate Change, J. T. Houghton, L. G. Meira Filho, B. A. Callander, N. Harris, A. Kattenberg and K. Maskell, Eds. Cambridge University Press, Cambridge.
Impacts of climate change
Climate Change 1990: The IPCC Impacts Assessment, W. J. McG. Tegart, G. W. Sheldon and D. C. Griffiths, Eds., Australian Government Publishing Service, Canberra.
Climate Change 1995: Impacts, Adaptations and Mitigation of Climate Change: Scientific-Technical Analyses,Contribution of WGII to the Second Assessment Report of the Intergovernmental Panel on Climate Change, R. T. Watson, M. C. Zinyowera, and R. H. Moss, Eds. Cambridge University Press, Cambridge.
Houghton, J. T. (1994) Global Warming: the Complete Briefing, Lion Publishing, Oxford
Greenhouse gases: emissions and strategies and options for reducing emissions
Climate Change 1990: The IPCC Response Strategies, Island Press, Washington, DC.
Climate Change 1995: Economic and Social Dimensions of Climate Change, Contribution of WGIII to the Second Assessment Report of the Intergovernmental Panel on Climate Change. J. Bruce, Hoesung Lee, and E. Haites, Eds., Cambridge University Press, Cambridge.
The UN Framework Convention on Climate Change
The Earth Summit 1993: The United Nations Conference on Environment and Development (UNCED) S. P. Johnson, Ed., International Environmental Law and Policy Series, Graham and Trotman/Martinus Nijhoff, London.