Parts of Australia experience several types of floods, including:
- large, regional floods which occur as water from rainfall over large catchments gradually flows downstream for which there are generally days to weeks of warning. Examples include the floods that occurred in Rockhampton, St George and Brisbane in early 2011
- small river floods which are caused by regional rain where the flood peaks occur within hours or a day of the rain falling. Examples include Lismore and Emerald in early 2011
- local flash flooding, caused by locally intense storms, which can occur with warnings of less than an hour. Examples include Toowoomba, and the Lockyer Valley in early 2011.
The extreme rainfall experienced in parts of Australia in 2011 was associated with one of the strongest La Niña events on record. Flooding in NSW and Qld in 2012 is again associated with strong La Niña conditions.
La Niña events are features of climate variability and reflect year-to-year variations in sea surface temperatures and weather patterns across the Pacific Ocean, commonly referred to as the El Niño – Southern Oscillation or ENSO. The flooding events of 1974 also occurred in a strong La Niña period.
For more information regarding La Niña events, see the Bureau of Meteorology's information on La Niña.
The specific contribution of climate change to such individual events is difficult to assess.
Climate change involves long-term changes to the underlying ocean and atmosphere patterns that generate such events as part of year-to-year climate variability.
Warmer oceans and higher sea surface temperatures that result from climate change tend to increase the amount of moisture that gets transported from the ocean to the atmosphere. A warmer atmosphere can hold more moisture and will most likely increase the intensity of extreme rainfall events.
It is expected that long-term climate change will result in greater climate variability with more intense extreme events than in the past.
CSIRO research shows that Australia is likely to become warmer over the coming decades, with a reduction in average annual rainfall in the south and east, and uncertain changes in average annual rainfall in the north.
Climate variability from year to year and within years will be superimposed on these trends in average conditions. For example, a warming trend will include some cool years and many hot years, and a drying trend will include some very wet years and many dry years. In this highly variable climate, future extreme rainfall events are likely to be more intense resulting in more severe flooding.