The Oxford Dictionary of Ecology defines climate as ‘the average weather conditions experienced at a particular place over a long period, known as the ‘climatological normal’.
Weather forecasters frequently compare current weather conditions to the monthly average, and travellers often check annual rainfall and temperature figures before selecting a holiday destination. This type of information is based on the ‘climatological normal’ – the mean temperature, humidity, or precipitation value calculated from thirty years of data.
The climatic normal is updated every ten years and can be measured at a local, national, regional, or worldwide level. There is a natural tendency for weather conditions to vary from the norm however, and meteorologists would agree that instability is a leading characteristic of climate.
Causes of fluctuations
Climate variability and extremes can be caused by internal and external factors (Bryant, 1997, p. 13). Internal factors include interactions between the atmosphere and hydrosphere, while external factors might include a volcanic eruption or change in the earth’s orbit.
Although climate change occurs naturally it can also be accelerated by human activity. Up to the late nineteenth century most weather events were the result of natural climate fluctuation – human impact was not a major contributing factor. From the twentieth century, however, there has been a significant difference in climate which goes beyond expected fluctuations.
It is now widely accepted that there has been a definite change in climate, and that it is due to anthropogenic influences (Burroughs, 1997, p. 227). Humans have been responsible for an increase in carbon dioxide emissions and this has had a dramatic impact on global atmospheric and sea surface temperatures, and sea levels.