Climate change is a change in the average pattern of weather over a long period of time Climate is a statistical description of weather conditions and their variations, including both averages and extremes. Climate change refers to a change in these conditions that persists for an extended period, typically decades or longer.
Weather variables such as temperature and rainfall fluctuate naturally (see Box 1). These changes in weather from day to day, between
seasons, and from one year to the next, do not represent climate changes. The period for estimating climate is usually 30 years or more,
long enough to sample a full range of weather.
Climate can be defined for a particular place or region, usually on the basis of local rainfall patterns or seasonal temperature
variations. Climate can also be defined for the entire Earth. For global climate, a key variable is the average surface temperature.
Sustained and truly global changes in average temperature require some global heating or cooling influence such as variations in heat output by the Sun, changes to the Earth’s orbit around the Sun, changes in cloudiness, changes to the extent of ice on Earth’s surface, or changes in greenhouse gas concentrations in the atmosphere.
Identifying climate change that is truly global in extent requires simultaneous observations from a network of locations around the world (see Question 3). Such a network of instrumental observations has only been available since the second half of the 19th century. Climate changes that occurred before this time can be identified by reconstructing records from climate-sensitive indicators like ocean sediments, ice-cores,
tree rings and coral reefs.
Greenhouse gases play an important role in determining climate and causing climate change Greenhouse gases include water vapour, carbon dioxide (CO2), methane, nitrous oxide and some industrial gases such as chlorofluorocarbons (CFCs). These gases act like an insulating blanket, keeping the Earth’s surface warmer than it would be if they were not present in the atmosphere.
Except for water vapour, the atmospheric concentrations of all of these gases are being directly influenced by human activities (see Question
4). Once released into the atmosphere, many of these gases remain there for a long time: in particular, a significant fraction of CO2 emissions remains in the climate system for hundreds to thousands of years.
Water vapour is an important greenhouse gas but it is not like the greenhouse gases affected directly by human activities. Its concentration in the atmosphere is controlled by the climate itself, rather than by human activities.
Water vapour therefore reacts to, and amplifies, climate change caused by other factors (see Box 2 and Figure 1.1). The effects of changing greenhouse gas levels on climate can be distinguished from the effects of other factors such as changes to the Sun’s radiation. These different causes lead to different patterns or “fingerprints” in the resulting climate changes, which assist in identifying the cause of observed changes.
For example, increases in solar radiation would be expected to warm both the upper and lower parts of the atmosphere and result in days warming more than nights. On the other hand, increases in greenhouse gases would be expected to result in a cooling, not a warming, in the stratosphere (the layer of the atmosphere above 15 km elevation), and cause nights to warm more than days. The observed patterns of change
more nearly match those expected from increasing greenhouse gases.
There are close connections between global temperature, atmospheric water vapour, the extent of polar ice caps and levels of greenhouse gases (GHGs) in the atmosphere. When one of these is disturbed, the others react through processes that amplify the original disturbance until
a new, different climate equilibrium is reached.
In the glacial cycles over the past million years, the disturbance came from fluctuations in the Earth’s orbit around the Sun (grey box in upper diagram). This caused temperatures to change (green box), in turn inducing rapid changes in water vapour (left blue box), and much slower
changes in ice caps (right blue box) and greenhouse gas levels (orange box), which together amplified the temperature change.
In modern climate change, the disturbance comes from human-induced changes in atmospheric CO2 and other greenhouse gas levels (grey box in lower diagram). In both cases, the disturbance is amplified by similar reinforcing processes.
The Australian Academy of Science, which represents Australia’s foremost scientists, provides scientific advice to policy makers and promotes excellence in Australian science, has devoted considerable resources to untangling the science of climate change and presenting it in a simple and easily understood format.