Title: 2. How has climate changed? | Australian Academy of Science
Headings: 2. How has climate changed?
2. How has climate changed?
Past climate has varied enormously on a variety of time-scales
Past records demonstrate that global climate is sensitive to small but persistent influences
Box 2.1: How do we detect climate change?
Box 2.2: Has climate warming recently stopped?
Global average temperatures have increased over the past century
Changes are evident in many parts of the climate system
There are regional differences to climate change including within Australia

Content: Photo: ©iStockphoto.com/John Carnemolla
Past climate has varied enormously on a variety of time-scales
Scientists have been using specialised equipment to measure and record weather and climate since 1850. NASA’s Global Precipitation Measurement (GPM) Core Observatory satellite is designed to provide rain and snow observations worldwide. Visualisation: NASA
Earth’s climate has changed dramatically many times since the planet was formed 4.5 billion years ago. These changes have been triggered by the changing configuration of continents and oceans, changes in the Sun’s intensity, variations in the orbit of Earth, and volcanic eruptions. Natural variations in the concentrations of greenhouse gases in the atmosphere, the evolution of life and meteorite impacts have also caused climate change in the past. Several million years ago, for example, global average temperature was a few degrees higher than today and warm, tropical waters reached much farther from the equator, resulting in very different patterns of ocean and atmospheric circulation from today. Over the past million years, Earth’s globally averaged surface temperature has risen and fallen by about 5˚C in ice-age cycles, roughly every 100,000 years or so (Figure 2.1a). In the coldest period of the last ice age, about 20,000 years ago, sea level was at least 120 metres lower than today because more water was locked up on land in polar ice sheets.