IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3Box 3-4 | Small Island StatesSmall island states represent a distinct category of locations owing to their small size and highly maritime climates, which means thattheir concerns and information needs in relation to future climate change differ in many ways from those of the larger continentalregions that are addressed in this chapter. Their small land area and often low elevation makes them particularly vulnerable to rising sealevels and impacts such as inundation, shoreline change, and saltwater intrusion into underground aquifers (Mimura, 1999). Theirmaritime environments lead to an additional emphasis on oceanic information to understand the impacts of climate change (see CaseStudy 9.2.9). Particular challenges exist for the assessment of past changes in climate given the sparse regional and temporal coverageof terrestrial-based observation networks and the limited in situ ocean observing network, although observations have improvedsomewhat in recent decades with the advent of satellite-based observations of meteorological and oceanic variables. However, the shortlength of these records hampers the investigation of long-term trends in the region. The resolution of GCMs is insufficient to representsmall islands and few studies have been undertaken to provide projections for small islands using RCMs (Campbell et al., 2011). Inregions such as the Pacific Ocean, large-scale climate features such as the South Pacific Convergence Zone ENSO (Section 3.4.2) have asubstantial influence on the pattern and timing of precipitation, yet these features and processes are often poorly represented in GCMs(Collins et al., 2010). The purpose of this box is to present available information on observed trends and climate change projections thatare not covered in the other sections of this chapter as well as discuss key aspects of the climate system that are particularly relevant forsmall islands. The very likely contribution of mean sea level rise to increased extreme sea levels (see Section 3.5.3), coupled with thelikely increase in tropical cyclone maximum wind speed (see Section 3.4.4), is a specific issue for tropical small island states.Although the underlying data sources are limited, some data for the Indian Ocean, South Pacific (Fiji), and Caribbean were available inthe studies of Alexander et al. (2006) and Caesar et al. (2011). Problems of data availability and homogeneity for the Caribbean arediscussed by T.S. Stephenson et al. (2008). Based on standard extremes indices, positive trends in warm days and warm nights andnegative trends in cold days and cold nights 2 have occurred across the Indian Ocean and South Pacific region for the period 1971 to2005 (Caesar et al., 2011) and the Caribbean for the period 1951 to 2003 (based on data from Alexander et al., 2006). Based on thesame data sources, trends in average total wet-day precipitation were positive and statistically significant over the Indian Ocean region,negative over the South Pacific region, and weakly negative over the Caribbean. Trends in heavy and very heavy precipitation werepositive over the Indian Ocean, negative over the South Pacific region, and close to zero over the Caribbean. We have low confidence intemperature trends over the Indian Ocean and South Pacific region due to the shorter record over which trends were assessed,whereas for the Caribbean, we have medium confidence in the temperature trends due to the longer records available for assessment.Because of the spatial heterogeneity exhibited in precipitation trends in general, there is insufficient evidence to assess observedrainfall trends. For the Caribbean, temperatures are projected to increase across the region by 1 to 4°C over 2071-2100 relative to1961-1990 under the A2 and B2 scenarios and rainfall is mainly projected to decrease by 25 to 50% except in the north (Campbell et al.,2011). Based on this study and the evidence for projected temperature increases reported for other regions (see Table 3-3) we havemedium confidence in the projected temperature increases for the Caribbean. However, due to the range of processes that contribute torainfall change, some of which are poorly resolved by GCMs, there is insufficient evidence to assess projected rainfall changes on thesesmall islands.Given the low elevation of many small islands, sea level extremes are of particular relevance. Sea level extremes are strongly influencedby tidal extremes (Chowdhury et al., 2007; Merrifield et al., 2007). When the tide behavior is mostly semi-diurnal (two high and low tidesper day), there will be a clustering of high spring tides around the time of the equinoxes whereas when the tide behavior is diurnal (onehigh and low tide per day), the clustering of high spring tides will occur around the time of the solstices. In addition, ENSO has a stronginfluence such that sea levels and their extremes are positively (negatively) correlated with the SOI in the tropical Pacific west (east) of180° (Church et al., 2006b; Menendez et al., 2010). Tides and ENSO have contributed to the more frequent occurrence of sea levelextremes and associated flooding experienced at some Pacific Islands such as Tuvalu in recent years, and make the task of determiningthe relative roles of these natural effects and mean sea level rise difficult (Lowe et al., 2010). Furthermore, the steep shelf margins thatsurround many islands and atolls in the Pacific support larger wave-induced contributions to sea level anomalies. Unfortunately, waveobservations (including wave direction) that would facilitate more comprehensive studies of tide, surge, and wave extremes in the regionare sparse, including those that are co-located with tide gauges (Lowe et al., 2010).____________2 Termed “cool days” and “cool nights” in that study.Continued next page184