9.1  Weather, climate variability and climate change

Everyone is familiar with what is meant by the weather – the temperature, rainfall, wind strength, sunshine, cloud cover, etc. that we experience from hour to hour and from day to day in our local environment. For example, the weather in Addis Ababa on Wednesday 11 March 2015 was light winds (wind speed 10 km/hour), humidity 28%, pleasantly warm (a maximum temperature of 22 °C) and sunny with some clouds.

By contrast, the climate refers to a general description of longer-term features of the weather in a particular location, such as the average temperature or rainfall for each month of the year, calculated over a period of 30 years or more (Pollution Probe, 2004). You can think of the weather as being a daily expression of the fluctuations of climate around the long-term average pattern.

Climates vary widely around the world – from the hot, rainy climates of tropical regions near the equator, to the cold, icy climates near the poles, both of which experience much the same temperature all year round. There are hot, dry deserts and milder ‘temperate’ regions where there is a large difference in temperature between summer and winter. Some regions can have rain in any month, others have a well-defined wet or dry season, and some receive little rain or snow throughout the year.

Although climate is the average weather over a period of a few decades, the climate in a particular country or region isn’t exactly the same every year – it varies around the average climate for that location. You probably remember a year that was hotter or cooler than this year, but the difference was probably not outside the normal range you would expect in your region of Ethiopia. These fluctuations around the average climate are termed climate variability. They are mainly due to fluctuations in natural conditions in the environment such as changes in the patterns of ocean currents or atmospheric pressure (Pollution Probe, 2004).

The global climate has undergone many long-term cyclical variations during millions of years of Earth’shistory, owing to shifts in natural environmental factors (described in Section 9.3). But from about the beginning of the 20th century, climate scientists began to detect changes in the global climate over time that were happening at a much faster rate than could be explained by normal climate variability. As the evidence for widespread, rapid changes in the world’s climate increased, scientists began using the term climate change to distinguish these trends from normal climate variation.

A frequently mentioned feature of climate change is global warming – a sustained increase in the Earth’s average surface temperature, which is predicted to continue rising during the 21st century. However, climate change has other important features, including changes in ocean currents, sea surface temperature, wind strength and direction, and the distribution and extent of rainfall. Many scientists from all over the world agree that climate change is a reality (IPCC, 2013). In the next section, we describe what climate scientists have observed; then in Section 9.3 we examine what is causing the climate to change.