AR4 highlighted the importance of understanding changes in extreme climate events (Annex III: Glossary) because of their disproportionate impact on society and ecosystems compared to changes in mean climate (see also IPCC Working Group II). More recently a comprehensive assessment of observed changes in extreme events was undertaken by the IPCC Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX) (Seneviratne et al., 2012; Section 1.3.3).

Data availability, quality and consistency especially affect the statistics of extremes and some variables are particularly sensitive to changing measurement practices over time. For example, historical tropical cyclone records are known to be heterogeneous owing to changing observing technology and reporting protocols (Section 14.6.1) andwhen records from multiple ocean basins are combined to explore global trends, because data quality and reporting protocols vary substantially between regions (Knapp and Kruk, 2010). Similar problems have been discovered when analysing wind extremes, because of the sensitivity of measurements to changing instrumentation and observing practice (e.g., Smits et al., 2005; Wan et al., 2010).

Numerous regional studies indicate that changes observed in the frequency of extremes can be explained or inferred by shifts in the overall probability distribution of the climate variable (Griffiths et al., 2005; Ballester et al., 2010; Simolo et al., 2011). However, it should be noted that these studies refer to counts of threshold exceedance— frequency, duration—which closely follow mean changes. Departures from high percentiles/return periods (intensity, severity, magnitude) are highly sensitive to changes in the shape and scale parameters of the distribution (Schär et al., 2004; Clark et al., 2006; Della-Marta et al., 2007a, 2007b; Fischer and Schär, 2010) and geographical location. Debate continues over whether variance as well as mean changes are affecting global temperature extremes (Hansen et al., 2012; Rhines and Huybers, 2013) as illustrated in Figure 1.8 and FAQ 2.2, Figure 1. In the following sections the conclusions from both AR4 and SREX are reviewed along with studies subsequent to those assessments.