Natural climate changes
The world’s climate has been changing since the beginning of time. It never stays the same. Some climate changes are natural. Others are caused by human behaviour (refer to Chapter 2). Natural climate changes are gradual processes and can often take many hundreds of years to notice.
The most significant and well-known climatic changes have been the Ice Ages. During an Ice Age the weather becomes so cold that most of the water on Earth begins to freeze and form massive ice sheets. The last Ice Age experienced on Earth ended about 10 000 years ago. It created an ice sheet that covered almost one-third of the Earth. The sheet was approximately 240 metres thick.
Climate change in Australia
Australia has not avoided significant climatic changes. Many thousands of years ago Australia looked very different, especially central Australia which today is very dry. Approximately 15-20 million years ago, in prehistoric times, rainforests were thought to cover all of the Australian landscape. Weather and climate during this time would have been wetter than it is today, as more moisture would have been released by plants and held in the land. Scientists believe that as rainfalls decreased lakes and rivers slowly began to dry out, leading to the drier environment that we know today.
Nature is able to tell us a lot about the climates of the past. Tree trunks, when cut in half, have many rings inside which signify different periods. Rings that are very close together mean that the period of time was very cold, whilst rings that are far apart mean the climate was warmer.
The discovery of dinosaur bones has shown scientists and archaeologists that they could possibly have been killed off due to great climatic changes. Some dinosaurs have even been found in their original condition frozen in ice.
Droughts occur when an area receives less rain than normal. This rain shortage must occur over a length of time for a drought to be declared. Droughts can have devastating consequences, especially on crops, plants and animals, which can all die in drought conditions. Creeks and lakes can also dry up, cutting off water supplies to people and other living things.
The Federation Drought
Australia often experiences droughts due to its naturally dry landscape. Australia’s worst recorded drought occurred during 1895-1903. This drought was named the Federation Drought and had devastating consequences for farmers, especially sheep and cattle farmers. Approximately 40 percent of Australia’s cattle population were killed, whilst half of the sheep population died.
Droughts are difficult to predict but they have often been linked to the climatic phenomenon called El Nino (see below).
Floods occur after extremely high levels of rain falls to earth. Heavy rains may strain the Earth’s natural and unnatural water processing systems (ground absorption, street drains). There are three types of floods: river floods; flash floods; and flooding in mountainous regions. (Refer to Topic 5, Chapter 3 for more information about floods).
Australia is also susceptible to floods which are often linked to the La Nina effect (see below). There have been many devastating floods in Australia which have seen rivers burst, cities and towns under water and crops ruined due to the high amounts of water.
El Nino is a natural phenomenon which occurs in the Pacific Ocean. As water currents warm to a temperature higher than normal, oceans winds change their paths. This in turn changes air temperatures in different parts of the atmosphere.
These warmer winds result in heavy rain and floods in North and South America. It also results in less rain falling in Africa, Asia and the Pacific region, where Australia is. Less rain increases the possibility of droughts occurring. Australia’s east and north coasts often feel the affects of El Nino.
It has been predicted that the El Nino effect occurs every 7-10 years and usually lasts for a year or two.
La Nina has the opposite effect of El Nino. Water currents in the Pacific Ocean are cooler during La Nina. This changes the circulation of air and increases the amount of clouds over Australia, Papua New Guinea and Indonesia, which means greater rainfall.
This increase in rainfall has been associated with major floods in Australia.
Nature has a way of disrupting weather conditions
Volcano eruptions, bushfires, cyclones, hurricanes and other natural occurrences can cause solid particles, such as ash, sand, lava and leaf litter to enter the atmosphere. These particles can interfere with the sun’s rays, therefore disrupting the amount of heat and light that reaches the Earth. The sun plays a major role in the weather (refer to Topic 2, Chapter 1) and any interference with it will disrupt the climate of an area.