Just as weather changes from day to day, so climatic conditions vary from year to year. One year may be cool and wet, the next warm and dry. Such year-to-year variability in climate conditions may conceal gradual trends from one type of climate regime to another. To study these trends information on the weather must be collected over many years. Usually, climatic averages of the common weather variables, such as temperature, rainfall, sunshine and atmospheric pressure, are calculated over a 30 year period. The effect of year-to-year irregularities can then be removed by various mathematical techniques, to reveal possible trends in climate.
Trends in climate can be apparent at many geographical scales, from local and regional climates to the climate of the Earth as a whole, the global climate. Such climate changes however, usually happen relatively slowly, taking generations or longer to occur, and are not often noticed. In fact, some climate changes occur over tens of thousands and even millions of years, but obviously these changes do not affect our day-to-day living.
There are many causes of climate change. Many are natural and involve processes which influence the flows of energy into, out of and within the climate system, such as changes in ocean circulation and changes in the amount of energy received by the Sun. Recently however, concern has grown that mankind's pollution of the atmosphere may be causing global-scale changes in climate, with accompanying shifts in regional climate regimes all over the world. By increasing the amount of greenhouse gases in the atmosphere through the burning of fossil fuels and deforestation, we have enhanced the Earth's natural greenhouse effect. During the last 100 years, the Earth's average surface temperature (about 15°C) has increased by over 0.5°C, much of the rise occurring since the 1980s. Whilst this global warming does not seem very much, further increases in greenhouse gas emissions may increase the warming by a further 2 to 3°C over the next 100 years. This change should be compared to the 5°C warming that occurred over a period of about 5,000 years at the end of the last Ice Age 14,000 years ago.