An extreme weather event is an event that is rare at a particular place and time of year. Definitions of rare vary, but an extreme weather event would normally be as rare as or rarer than the 10th or 90th percentile of a probability density function estimated from observations.
Globally, the frequency and severity of extreme temperature events has increased since the middle of the 20th century. Patterns of change in extreme rainfall events vary with region. This is consistent with scientific understanding of the physics of a warming climate. Download the IPCC Working Group 1 FAQs for more detail.
In Australia in recent decades, anomalously warm months have occurred more often than anomalously cold months. Many heat-related records were broken in the summer of 2012-13 and in the year of 2013, including Australia’s hottest day, week, month and year averaged across Australia. Extreme summer temperatures during 2012- 13 were unlikely to have been caused by natural variability alone, and such temperatures are now five times more likely due to the enhanced greenhouse effect. Since 2001, the number of extreme heat records in Australia has outnumbered extreme cool records by almost 3 to 1 for daytime maximum temperatures and almost 5 to 1 for night-time minimum temperatures. Heat waves have increased in duration, frequency, and intensity in many parts of the country. For a full discussion, see the Technical Report (Section 4.2.1).
Heavy daily rainfall has accounted for an increased proportion of total annual rainfall over an increasing fraction of the Australian continent since the 1970s (Technical Report Section 4.2.3). Record rainfall totals occurred in many areas during 2010 and 2011, attributable in part to the presence of strong La Niña conditions and higher than average sea surface temperatures to the north of Australia (Technical Report Section 4.2.2).
There is some observational evidence for a decrease in the occurrence of tropical cyclones. However, the short period of consistent observational records and high year to year variability make it difficult to discern clear trends in tropical cyclone frequency or intensity. For a full discussion, see the Technical Report (Section 4.2.7).
Extreme fire weather days have increased at 24 out of 38 Australian sites from 1973-2010, due to warmer and drier conditions. For a full discussion, see the Technical Report (Section 4.2.12).