||The increase in global temperature resulting from human activities that enhance the so-called natural \'greenhouse effect\'. This phenomenon is often seen as part of \'climate change\', a term that is less impelling but recognizes the likely variability of climate changes in different parts of the world.
Greenhouse gases are mostly natural compounds (water vapour, carbon dioxide, methane and nitrous oxide) that allow the earth\'s atmosphere to trap heat that has been released as longwave energy from the earth\'s surface. This leads to increases in atmospheric temperature. At an average temperature of fifteen degrees Celsius, the earth is able to support many lifeforms. However, human activity has increased the levels of carbon dioxide (mainly through coal burning, internal combustion engines in transport media and clearing of forests), methane (through burning of natural gas, seepage from landfill sites, rice paddies and from the increased numbers of cattle) and nitrous oxide (mainly through agricultural activities). Artificial gases, such as chlorofluorocarbons (CFCs), have also significantly added to the earth\'s natural greenhouse effect (see pollution). CFCs are also responsible for a different process, ozone depletion in the earth\'s atmosphere (see Harries, 1994; Turco, 1997): they are extremely inert, which means they cannot be removed and are not absorbed. The Montreal Protocol (1987) was a significant attempt, followed by amendments in Copenhagen (1992), to phase out CFCs (see Clayton, 1995).
In 1991, eight countries, including three developing countries with large populations, contributed approximately 62 per cent of the world\'s greenhouse gas emissions (World Resources Institute, 1994). The Intergovernmental Panel on Climate Change (IPCC) provided detailed analyses of various greenhouse scenarios in 1990, 1992, 1994 and 1995 which contributed to the United Nations Framework Convention on Climate Change (1992) at the Rio conference on sustainable development and to the Climate Change Convention in Kyoto, Japan, in 1997. Unfortunately, at this latter convention it was apparent that individual countries and blocs (e.g. the European Union) were more interested in maintaining economic growth than in taking the ecologically necessary steps to reduce emissions. This was evident in the choice of base year against which reduction in emissions were to be set (1990), which still included pollution from the now inoperational heavy industry in the former East Germany. Under the agreement, the European Union aims to reduce emissions to 8 per cent below their 1990 level by the year 2012; the USA\'s target is to reduce its emissions by 7 per cent whereas Japan is aiming for a 6 per cent reduction. The role of Australia (a so-called developed country, with high coal exports) in obtaining the right to increase its emissions by 8 per cent to the year 2012, highlighted the lack of environmental commitment by some countries.
Climate change is critical because it highlights human impacts at a global scale. While there have been major cooling and heating periods throughout the earth\'s history, human input into present changes cannot be ignored. Increased concentrations of greenhouse gases with long lifetimes (Pickering and Owen, 1994), rising sea levels, changes in the location, intensity and frequency of cyclones, drought and flood, and depletion in the earth\'s ozone layer (particularly over the Antarctic pole) are all important impacts of human activity. These changes are yet to be understood because of the time lag between pollution emission and cumulative impacts. The issues of global warming and ozone depletion have also caused political tensions around consumption lifestyles, energy-use patterns, production methods and deforestation. Global warming has divided the so-called Third World countries, more than by wealth alone. Global warming heats the top layer of sea water, which expands in volume (to which is added melting glaciers) and causes sea level rise. Small island states, fearing inundation, want restrictions on fossil-fuel use. Countries that produce and/or consume oil and coal are very reluctant to meet this demand.
The potential for carbon dioxide emissions to affect the earth\'s radiation balance was suggested as early as 1861 (Pickering and Owen, 1994). Current monitoring of ice sheets, tree rings and other phenomena indicates that human impact has recently altered the earth\'s radiation system. Computer modelling, otherwise known as General Circulation Models (GCMs), is used to predict possible future changes. Initial predictions of the extent of climate change have been lowered recently by 20 to 30 per cent, due mainly to incorporation of sulphate aerosols emitted by volcanoes which have contributed to stratospheric cooling (Pickering and Owen, 1994) and there is still uncertainty about the significance of clouds and the cumulative impacts of changes.
What is certain is that emissions of carbon dioxide and some other greenhouse gases are increasing, that they have a long lifetime, and that the earth is getting warmer. The lowering of GCM predictions does not dismiss the issue. While the processes are still occurring, it merely means that if the predicted impacts eventuate, they will be delayed by only a few years (see global futures).Â (PM)
References Clayton, K. 1995: The threat of global warming. In T. O\'Riordan, ed., Environmental science for environmental management. Harlow, Essex: Longman, 110-30.Â Harries, J. 1994: Earthwatch â€” the climate from Space. Chichester: John Wiley and Sons.Â Pickering, K. and Owen, L. 1994: An introduction to global environmental issues. London and New York: Routledge.Â Turco, R. 1997: Earth under siege: from air pollution to global change. Oxford and New York: Oxford University Press.Â World Resources Institute 1994: World resources 1994-95. New York and Oxford: Oxford University Press.