Changes in extreme events

Regional models are most suitable to evaluate the likelihood of changes in extreme events, and while they have been widely applied in Europe and North America, other areas such as Africa, Latin America and some parts of Asia mainly depend on GCM projections, and detailed regional analyses are still limited in number (Solman et al., 2008; Nunez et al., 2009). However, climate change will certainly influence the frequency and intensity of drought and waterlogging (Timmermann et al., 1999; IPCC, 2001, 2007; Ekstrom et al., 2005; Fowler et al., 2005; Blenkinsop and Fowler, 2007). Regional models also suggest that rainfall intensity is to increase over southern Africa (Tadross et al., 2005; IPCC, 2007). Compensation between intensity and frequency of rain is expected when the total annual rainfall tends to be maintained over time (i.e. more intense rainfall events spread throughout the year). In almost all global land areas, warm seasons will probably be extremely warm by the end of the 21st century, with very high confidence under the A1B scenario (IPCC, 2007), with increases in the wet season rainfall intensity as well, due mainly to increased cloud loadings and surface evaporation (IPCC, 2007).

The degree to which changes in atmospheric processes led by changes in concentrations of GHGs influences the likelihood or frequency of extreme storms is still not known. Hurricanes generally occur over the oceans in regions where sea surface temperatures exceed a certain threshold (Trenberth, 2005). There is a non-linear upward trend in sea surface temperatures over the 20th century. This trend is most pronounced in the past 35 years in the extra-tropical North Atlantic (Trenberth, 2005), and will be likely to continue during the whole of the 21st century. It is associated with global warming and has been attributed to human activity (IPCC, 2001, 2007). In the tropical North Atlantic (the region of most relevance to hurricane formation), multi-decadal variability dominates sea surface temperatures, leading to different temporal patterns of hurricane formation (Trenberth, 2005). However, there is no sound theoretical basis for drawing any conclusions about how anthropogenic climate change affects hurricane numbers or tracks (Vecchi et al., 2008). Some model results suggest a shift in hurricane intensities towards stronger hurricanes (Knutson and Tuleya, 2004; Elsner et al.,

2008), while others indicate a likely drop in hurricane frequency (Knutson et al., 2008).

The ENSO phenomenon is the strongest natural inter-annual climate fluctuation across the globe. ENSO originates in the tropical Pacific Ocean and has large effects on the ecology of the region. It can be understood as an irregular low-frequency oscillation between a warm (El Niño) and a cold (La Niña) state (Timmermann and Menviel,

2009). Recent changes in frequencies and intensities of ENSO, especially regarding a perceptible increase in El Niños, suggest that anthropogenic activities could have influenced these changes. This has been addressed via several GCMs, but the inability of the models to fully simulate ENSO has been debated (Timmermann et al., 1999). Whether the frequency and/or intensity of El Niños will increase with human-led climate change is still not known.

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