An Introduction to Climate Change
The Greenhouse Effect
The Sun, which is the Earth's only external form of heat, emits solar radiation mainly in the form of visible and ultraviolet (UV) radiation. As this radiation travels toward the Earth, 25% of it is absorbed by the atmosphere and 25% is reflected by the clouds back into space. The remaining radiation travels unimpeded to the Earth and heats its surface. The Earth is much cooler than the Sun, this means that the energy re-emitted from the Earth's surface is lower in intensity than that emitted from the Sun, i.e. in the form of invisible infra-red (IR) radiation.
The Greenhouse Effect
Greenhouse gases like water vapour, carbon dioxide, methane and nitrous oxide trap the infrared radiation emitted by the Earth's surface. The atmosphere acts like the glass in a greenhouse, allowing short-wave UV radiation to travel through relatively unimpeded, but trapping some of the long-wave IR radiation which is trying to escape. This process makes the temperature rise in the atmosphere just as it does in the greenhouse. This is the Earth's natural greenhouse effect and keeps the Earth 33°C warmer than it would be without an atmosphere, at an average 15°C.
The Earth is like a Greenhouse
The Enhanced Greenhouse Effect
Increasing the concentrations of the greenhouse gases traps more terrestrial radiation in the lower atmosphere (troposphere), enhancing the natural greenhouse effect. The "enhanced" greenhouse effect is the direct result of human activities. Processes such as the burning of fossil fuels, industrial operations and forest clearing increase carbon dioxide and methane in the atmosphere. Chlorofluorocarbons, or CFCs, are also potent greenhouse gases, and as an added danger, they also destroy the stratospheric ozone layer. Scientists call this trapping of infrared radiation in the lower atmosphere radiative forcing.
Has the increase in the enhanced greenhouse effect, due to the higher atmospheric concentrations of greenhouse gases, caused a global rise in temperature? According to surface measurements the Earth has warmed by between 0.4 and 0.8°C during the last century. This level of global warming is not inconsistent with computer model predictions of the impact of the change in atmospheric composition over the last 100 years.
Global Warming in the 20th Century
This finding does, however, have to be interpreted with care. For example, the observed warming has not been constant, although the steady rise in greenhouse gas concentrations would suggest that any greenhouse-induced warming should have been steady over the past 100 years. There have been two periods of warming during 1920 to 1940 and again since 1980. From the 1940s to the 1970s, the global temperature remained relatively steady. Furthermore, aerosols which form in the lower atmosphere as a result of the same processes that release greenhouse gases can cool the Earth, offsetting some of the warming due to the enhanced greenhouse effect. Finally, temperature observations made by satellites have revealed little or no warming trend at all.
Global Warming Uncertainties
While the surface warming is not inconsistent with the climate model predictions, there are other possible explanations for the warming trend: natural mechanisms of climate change such as a change in the degree of volcanic and solar activity, or a changing distribution of ocean temperatures. In addition, the Earth's climate can fluctuate purely at random by as much as 0.3°C over 100 years. These mechanisms may obscure the greenhouse-induced trend which may be less than, or greater than, the observed warming of around 0.6°C.
The temperature rise must surpass the natural variability in the climate before natural changes in climate can be discounted. Increasingly, however, the enhancement of the greenhouse effect seems to offer the most plausible explanation for the warming trend that has occurred since the mid-19th century. In light of the most recent evidence, and taking into account the remaining uncertainties, the Intergovernmental Panel on Climate Change (IPCC) have stated that most of the observed warming in the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.
Future Climate Change
According to current projections global temperature may stand as much 5.8°C above the 1990 temperature by the year 2100. To place this change in climate in context, the temperature rise that brought the planet out of the most recent ice age was only of the order of 4 to 5°C. Carbon dioxide concentrations in the atmosphere are already higher than at any time during the last 160,000 years. If concentrations continue to rise, the Earth's temperature may become warmer than at any time during the last 40 million years.
If such a rise in global temperature occurs, sea levels will rise in response to both thermal expansion of the (warmer) oceans and melting of land-based ice sheets. The rapidity of the temperature rise will mean that many ecosystems will find it hard to adapt and many animal and plant species will become extinct. Humans, too, will face difficulties, as impacts to agriculture, forestry and water supplies can be expected.
Reducing the Threat of Global Warming
What can be done to reduce or remove the threat of global warming? Models estimate that huge cuts in greenhouse gas emissions would be required to stabilise greenhouse gas concentrations in the atmosphere at their present levels.
The Framework Convention of Climate Change (FCCC) represents a first step to achieve this goal. Signed in June 1992 at the Rio Earth Summit, by 162 Governments, the Convention calls for nations to adopt a precautionary approach towards the threat of global warming, to avoid dangerous interference with the global climate. At the Kyoto Conference of Parties (1997) nations agreed to reduce greenhouse gas emissions by 5.2% by 2008-2012, relative to 1990 levels (the Kyoto Protocol).
This fact sheet has examined the evidence for man-made global warming, as a result of an enhanced greenhouse effect. Although other mechanisms could be linked to the observed climate change, it seems most likely that emissions of greenhouse gases since the Industrial Revolution are to blame. Despite uncertainties in current understanding of the behaviour of the global climate, we should adopt a precautionary approach to averting detrimental impacts of climatic change. The Framework Convention on Climate Change and the Kyoto Protocol represent the first steps.