In this way the Earth's atmosphere acts like the glass in a greenhouse. The greenhouse lets in the UV and visible light, but once the interior warms up and IR energy is re-emitted, the glass absorbs the radiation and does not allow it to escape back to space. This warms the greenhouse. The fact that all incoming wavelengths are absorbed by the Earth and re-emitted as IR is the key to the greenhouse effect, both on the Earth, and in the greenhouse. The "greenhouse effect" is important to the Earth because it keeps the planet at a nice, warm, habitable temperature. However, many scientists, policy makers and citizens are worried that humans are altering the composition of the atmosphere in such a way as to enhance the greenhouse effect and cause undesirable global warming.
A balance between energy coming in from the sun in the form of visible radiation (sunlight) and energy constantly being emitted from the surface of the earth to space determines the temperature of the earth. The energy coming in from the sun can pass through the atmosphere almost unchanged and warm the earth, but the infrared radiation emanating from the earth's surface is partly absorbed by some gases in the atmosphere and some of it is re-emitted downwards. This further warms the surface of the earth and the lower atmosphere. The gases that do this naturally are mainly water vapour and carbon dioxide. An analogy is made with the effect of a greenhouse, which allows sunshine to penetrate the glass that in turn keeps the heat in, hence the greenhouse effect.
Without this natural greenhouse effect, the earth would be over 30°C cooler and would be too cold to be habitable. But as greenhouse gas concentrations rise well above their natural levels, the additional warming that will take place could threaten the future sustainability of the planet.
The extent to which changes in temperature over the last 100 years are due to human activities has been studied by looking at patterns of change across the surface of the earth, and vertically through the depth of the atmosphere and the ocean. Climate models predict a characteristic 'fingerprint' pattern of change in response to increasing greenhouse gas concentrations. Statistical analysis shows that this fingerprint can be detected in observed temperature changes, indicating that most of the change which has occurred can be attributed to human activities. The individual contributions of natural effects, for example, variations in the sun's output and volcanoes, have also been studied. They were found to be unable to account for all of the observed warming. Only when greenhouse gases emitted by human activities are invoked can the observed warming be explained. Borehole measurements worldwide imply a global surface warming of around 1°C during the last 500 years, with about half of this warming occurring in the 20th Century.
The devastating floods, droughts and storms we have seen in the UK and across the world in recent years show all too clearly how vulnerable we are to climate extremes and how high the economic, human and environmental cost can be, particularly in developing countries.
The greenhouse effect causes trouble by raising the temperature of the planet. The actual rise is not very much, but the Earth's is very fragile, and small changes can have large effects
Inter-Governmental Panel on Climate Change has predicted that this rise of one degree will happen by the year 2025
The other serious worry is that rising sea levels from the melting of the polar ice caps could severely flood many countries. A rise in sea levels of one metre, which many experts are predicting by the year 2100 (and some as soon as 2030),
