IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentAustralia if current rates of CO 2 emission are sustained,” assuming thestrong relationship between hail incidence and the CAPE for 1980-2001remains unchanged under enhanced greenhouse conditions.In summary, it is likely that there have been statistically significantincreases in the number of heavy precipitation events (e.g., 95thpercentile) in more regions than there have been statisticallysignificant decreases, but there are strong regional and subregionalvariations in the trends (i.e., both between and within regionsconsidered in this report; Figure 3-1 and Tables 3-2 and 3-3). Inparticular, many regions present statistically non-significant ornegative trends, and, where seasonal changes have beenassessed, there are also variations between seasons (e.g., moreconsistent trends in winter than in summer in Europe). The overallmost consistent trends toward heavier precipitation events arefound in North America (likely increase over the continent). Thereis low confidence in observed trends in phenomena such as hailbecause of historical data inhomogeneities and inadequacies inmonitoring systems. Based on evidence from new studies and thoseused in the AR4, there is medium confidence that anthropogenicinfluence has contributed to intensification of extreme precipitationat the global scale. There is almost no literature on the attributionof changes in hail extremes, thus no assessment can be providedfor these at this point in time. Projected changes from both globaland regional studies indicate that it is likely that the frequencyof heavy precipitation or proportion of total rainfall from heavyfalls will increase in the 21st century over many areas on theglobe, especially in the high latitudes and tropical regions, andnorthern mid-latitudes in winter. Heavy precipitation is projectedto increase in some (but not all) regions with projected decreasesof total precipitation (medium confidence). For a range of emissionscenarios (A2, A1B, and B1), projections indicate that it is likely thata 1-in-20 year annual maximum 24-hour precipitation rate willbecome a 1-in-5 to -15 year event by the end of 21st century in manyregions. Nevertheless, increases or statistically non-significantchanges in return periods are projected in some regions.3.3.3. WindExtreme wind speeds pose a threat to human safety, maritime andaviation activities, and the integrity of infrastructure. As well as extremewind speeds, other attributes of wind can cause extreme impacts. Trendsin average wind speed can influence potential evaporation and in turnwater availability and droughts (e.g., McVicar et al., 2008; see alsoSection 3.5.1 and Box 3-3). Sustained mid-latitude winds can elevatecoastal sea levels (e.g., McInnes et al., 2009b), while longer-termchanges in prevailing wind direction can cause changes in wave climateand coastline stability (Pirazzoli and Tomasin, 2003; see also Sections3.5.4 and 3.5.5). Aeolian processes exert significant influence on theformation and evolution of arid and semi-arid environments, beingstrongly linked to soil and vegetation change (Okin et al., 2006). A rapidshift in wind direction may reposition the leading edge of a forest fire(see Section 4.2.2.2; Mills, 2005) while the fire itself may generate alocal circulation response such as tornado genesis (e.g., Cunninghamand Reeder, 2009). Unlike other weather and climate elements such astemperature and rainfall, extreme winds are often considered in thecontext of the extreme phenomena with which they are associated suchas tropical and extratropical cyclones (see also Sections 3.4.4 and 3.4.5),thunderstorm downbursts, and tornadoes. Although wind is often notused to define the extreme event itself (Peterson et al., 2008b), windspeed thresholds may be used to characterize the severity of thephenomenon (e.g., the Saffir-Simpson scale for tropical cyclones).Changes in wind extremes may arise from changes in the intensity orlocation of their associated phenomena (e.g., a change in local convectiveactivity) or from other changes in the climate system such as themovement of large-scale circulation patterns. Wind extremes may bedefined by a range of quantities such as high percentiles, maxima overa particular time scale (e.g., daily to yearly), or storm-related highestvalues. Wind gusts, which are a measure of the highest winds in a shorttime interval (typically 3 seconds), may be evaluated in models usinggust parameterizations that are applied to the maximum daily nearsurfacewind speed (e.g., Rockel and Woth, 2007).Over paleoclimatic time scales, proxy data have been used to infercirculation changes across the globe from the mid-Holocene (~6000 yearsago) to the beginning of the industrial revolution (Wanner et al., 2008).Over this period, there is evidence for changes in circulation patternsacross the globe. The Inter-Tropical Convergence Zone (ITCZ) movedsouthward, leading to weaker monsoons across Asia (Haug et al., 2001).The Walker circulation strengthened and Southern Ocean westerliesmoved northward and strengthened, affecting southern Australia, NewZealand, and southern South America (Shulmeister et al., 2006; Wanneret al., 2008), and an increase in ENSO variability and frequency occurred(Rein et al., 2005; Wanner et al., 2008). There is also weaker evidence fora change toward a lower Northern Atlantic Oscillation (NAO), implyingweaker westerly winds over the north Atlantic (Wanner et al., 2008).While the changes in the Northern Hemisphere were attributed tochanges in orbital forcing, those in the Southern Hemisphere were morecomplex, possibly reflecting the additional role on circulation of heattransport in the ocean. Solar variability and volcanic eruptions may alsohave contributed to decadal to multi-centennial fluctuations over thistime period (Wanner et al., 2008).The AR4 did not specifically address changes in extreme wind althoughit did report on wind changes in the context of other phenomena such astropical and extratropical cyclones and oceanic waves and concluded thatmid-latitude westerlies had increased in strength in both hemispheres(Trenberth et al., 2007). Direct investigation of changes in windclimatology has been hampered by the sparseness of long-term, highqualitywind measurements from terrestrial anemometers arising from theinfluence of changes in instrumentation, station location, and surroundingland use (e.g., Cherry, 1988; Pryor et al., 2007; Jakob, 2010; see alsoSection 3.2.1). Nevertheless, a number of recent studies report trends inmean and extreme wind speeds in different parts of the world based onwind observations and reanalyses.149