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A greenhouse effect has always existed, keeping the Earth warmer than it would be without an atmosphere. What is popularly referred to today as the 'greenhouse effect' is really the anthropogenically enhanced greenhouse effect by which an extra warming of the surface and the lower atmosphere is produced, leading to disturbances in the geosphere/biosphere system and, notably, an increase in the mean global surface temperature and in the mean sea level.

The natural presence of 'radiatively-active' gases in the atmosphere is essential for life: they trap heat in the lower atmosphere, thus creating ­ like a greenhouse ­ an environment which is far warmer (by about 33°C) inside than out. By increasing the concentration of greenhouse gases, however, additional infra-red radiation, which otherwise would have been lost to space, is absorbed in the lower atmosphere and the Earth's radiation balance is upset. This energy is re-emitted in all directions, a large portion being sent back to the Earth's surface or elsewhere in the troposphere. This yields a radiative imbalance which can be restored only through a warming of the troposphere. On the other hand, the enhanced greenhouse effect will also cool the upper layers of the atmosphere (ie, the stratosphere, and above between 25 and 70 km).

Greenhouse gases in the atmosphere have increased since pre-industrial times by an amount that is radiatively equivalent to about a 50 per cent increase in carbon dioxide (CO2), although CO2 itself has risen by about 25 per cent; other gases have made up the rest. The global emissions of most greenhouse gases are expected to rise in the next decade: CO2 at 0.5 per cent/year, methane (CH4) at 0.9 per cent/year, and nitrous oxide (N2O) at about 0.3 per cent/year. In contrast, CFC emissions are expected to decrease to near zero around the year 2000 following internationally agreed phase-out measures. As a result of these trace gas trends, an effective doubling of greenhouse gas concentrations (as commonly measured as 'CO2-equivalents') is expected around 2030.

There has been much scientific activity aimed at identifying clues of the enhanced greenhouse effect by searching for trends in climatic and hydrological records (mainly temperature, precipitation, glaciers, runoff, freezing/melting date and sea level). Conclusions have so far been mainly tentative because definite signals from an enhanced greenhouse effect are weak and/or smaller than natural fluctuations, most of the homogeneous and comparable records, if any, are too short, and regional differences have added to the confusion (see Chapter 4). Our partial understanding of multimedia mechanisms involved has also been a limitation to making a safe forecast of future trends, although this has fostered large international research efforts such as the World Climate Research Programme (WCRP, of the World Meteorological Organisation (WMO), the International Council of Scientific Unions (ICSU) and the Intergovernmental Oceanographic Commission (IOC) of UNESCO) and the International Geosphere-Biosphere Programme (IGBP of ICSU), or international expert assessments such as under the Intergovernmental Panel on Climate Change (IPCC of WMO and UNEP).

At first glance, uncertainties about the distribution and timing of consequences of climate change may make some people feel that the cost of rapid implementation of actions intended to curb the man-made emissions responsible might be prohibitive. However, regardless of what happens to climate, application of relevant measures would bring its own benefits, such as longer availability of fossil fuels and other natural resources, as well as more rational and efficient energy use. Additionally, consequences such as stronger and more frequent storms, higher seas and less water in rivers are so far-reaching that taking this 'precautionary' approach should be rewarding. This approach is also known as the 'no regrets' policy, because of the argument that preventing climate change would be far better than attempting to cure it, despite the uncertainties of how fast and how soon it might be happening.

Although Europe's land mass covers less than 10 per cent of the world's total area and its population is some 15 per cent of the world population, European countries contribute a significant amount to the global emissions of greenhouse gases and other substances which bring about large-scale changes in the atmosphere (see Chapters 4 and 14). Conversely, climate change may greatly affect Europe's 680 million people and its natural environment.

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