Feature Article

Argo floats monitor ocean tempertatures, providing vital data about climate change.
Image: CSIRO

The science of climate change

Global temperatures are rising

Over the past century the global average surface temperature has risen by 0.74 ºC. The observed increase in average temperatures is widespread around the globe, with rising trends recorded on all continents and in the oceans.

The warming has been fastest over land and greatest in the upper northern hemisphere. Global ocean temperature rose 0.10 ºC between 1961 and 2003, to a depth of 700 m. In Australia there has been a 0.9 ºC warming since 1950.

A shift of just a few degrees in global temperature can cause major changes

However, this level of warming is not unusual in the Earth’s geological history. For millions of years the planet has experienced a series of ice ages and warmer inter-glacial periods, driven mainly by changes in the Earth’s orbit.

During the last major ice age, the global average temperature was only 3 to 5 ºC cooler than today, and sea levels were up to 120 m lower than present. Around 125 000 years ago our ancestors lived through an interglacial period when the polar regions were 3 to 5 ºC warmer than the present and sea levels were an estimated 4 to 6 metres higher than the twentieth century.

This illustrates that even a few degrees change in global temperatures can create a vastly different environment.

Figure 1: The three most complete global temperature records available – from the UK Hadley Centre, NASA, and the US National Climate Data Centre – all show a clear upward trend in global average temperatures over the last 150 years (calculated using an 11 year running average).

Global sea levels are rising

From 1870 to 2007 the global average sea level rose by close to 20 cm. Sea levels rose at an average of 1.7 mm per year during the 20th century, accelerating to 3.4 mm per year from 1993–2007.

As water warms, it expands in volume. This thermal expansion of the ocean is a major cause of sea-level rise in the 20th century.

The other main contributors are the melting of icecaps and glaciers around the world, and smaller contributions from the Greenland and Antarctic ice sheets.

Extreme weather events and precipitation patterns are changing

Over the last 50 years globally, there have been fewer cold days and nights, and more hot days, hot nights and heatwaves. Heavy rainfall events and extreme sea-level events have increased over most areas.

Since 1900, precipitation has increased significantly over eastern parts of the Americas, northern Europe, parts of Asia and north-western Australia. Reduced precipitation has occurred in central and southern Africa, the Mediterranean and parts of southern Asia. Since 1950, there has been significant drying in south-western and eastern Australia.

These long-term global climate trends are occurring alongside normal weather variations that happen naturally over seasons or decades. The way short-term and long-term variations interact can reduce or worsen the impacts we experience, making it harder to pinpoint all the causes of local temperature changes or specific weather events.