IPCC Report.pdf - Adam Curry

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3from river discharge simulations. Nevertheless, as was argued byKundzewicz et al. (2007) and Bates et al. (2008), physical reasoningsuggests that projected increases in heavy rainfall in some catchmentsor regions would contribute to increases in rain-generated local floods(medium confidence). We note that heavy precipitation may be projectedto increase despite a projected decrease of total precipitation dependingon the regions considered (Section 3.3.2), and that changes in severalvariables (e.g., precipitation totals, frequency, and intensity, snow coverand snowmelt, soil moisture) are relevant for changes in floods.Confidence in change in one of these components alone may thus notbe sufficient to confidently project changes in flood occurrence. Hence,medium confidence is attached to the above statement based onphysical reasoning, although the link between increases in heavyrainfall and increases in local flooding seems apparent. The earlier shiftsof spring peak flows in snowmelt- and glacier-fed rivers are robustlyprojected (Kundzewicz et al., 2007; Bates et al., 2008); so these areassessed as very likely, though this may not necessarily be relevant forflood occurrence. There is low confidence (due to limited evidence) inthe projected magnitude of the earlier peak flows in snowmelt- andglacier-fed rivers.In summary, there is limited to medium evidence available toassess climate-driven observed changes in the magnitude andfrequency of floods at a regional scale because the availableinstrumental records of floods at gauge stations are limited inspace and time, and because of confounding effects of changesin land use and engineering. Furthermore, there is low agreementin this evidence, and thus overall low confidence at the globalscale regarding even the sign of these changes. There is lowconfidence (due to limited evidence) that anthropogenic climatechange has affected the magnitude or frequency of floods,though it has detectably influenced several components of thehydrological cycle such as precipitation and snowmelt (mediumconfidence to high confidence), which may impact flood trends.Projected precipitation and temperature changes imply possiblechanges in floods, although overall there is low confidence inprojections of changes in fluvial floods. Confidence is low due tolimited evidence and because the causes of regional changes arecomplex, although there are exceptions to this statement. There ismedium confidence (based on physical reasoning) that projectedincreases in heavy rainfall (Section 3.3.2) would contribute toincreases in rain-generated local flooding, in some catchments orregions. Earlier spring peak flows in snowmelt- and glacier-fedrivers are very likely,but there is low confidence in their projectedmagnitude.3.5.3. Extreme Sea LevelsTransient sea level extremes and extreme coastal high water are causedby severe weather events or tectonic disturbances that cause tsunamis.Since tsunamis are not climate-related, they are not addressed here. Thedrop in atmospheric pressure and strong winds that accompany severeweather events such as tropical or extratropical cyclones (Sections 3.4.4and 3.4.5) can produce storm surges at the coast, which may be furtherelevated by wave setup caused by an onshore flux of momentum due towave breaking in the surf zone. Various metrics are used to characterizeextreme sea levels including storm-related highest values, annualmaxima, or percentiles. Extreme sea levels may change in the future asa result of both changes in atmospheric storminess and mean sea levelrise. However, neither contribution will be spatially uniform across theglobe. For severe storm events such as tropical and extratropicalcyclones, changes may occur in the frequency, intensity, or genesisregions of severe storms and such changes may vary between oceanbasins (see Sections 3.4.4 and 3.4.5). Along some coastlines, landsubsidence due to glacial isostatic adjustment (e.g., Lambeck et al.,2010) is causing a relative fall in sea levels. Variations in the rate of sealevel rise can be large relative to mean sea level (Yin et al., 2010) andwill occur as a result of variations in wind change (e.g., Timmermann etal., 2010), changes in atmospheric pressure and oceanic circulation(e.g., Tsimplis et al., 2008), and associated differences in water densityand rates of thermal expansion (e.g., Bindoff et al., 2007; Church et al.,2010; Yin et al., 2010). In addition, if rapid melting of ice sheets occursit would lead to non-uniform rates of sea level rise across the globe dueto adjustments in the Earth’s gravitational field (e.g., Mitrovica et al.,2010). On some coastlines, higher mean sea levels may alter theastronomical tidal range and the evolution of storm surges, andincrease the wave height in the surf zones. As well as gradual increasesin mean sea level that contribute to extreme impacts from transientextreme sea levels, rapid changes in sea level arising from, for example,collapse of ice shelves could be considered to be an extreme event withthe potential to contribute to extreme impacts in the future. However,knowledge about the likelihood of such changes occurring is limitedand so does not allow an assessment at this time.Mean sea level has varied considerably over glacial time scales as theextent of ice caps and glaciers have fluctuated with global temperatures.Sea levels have risen around 120 to 130 m since the last glacial maximum19 to 23 ka before present to around 7,000 years ago, and reached alevel close to present at least 6,000 years ago (Lambeck et al., 2010). Aswell as the influence on sea level extremes caused by rapidly changingcoastal bathymetries (Clarke and Rendell, 2009) and large-scale circulationpatterns (Wanner et al., 2008), there is some evidence that changes inthe behavior of severe tropical cyclones has changed on centennial timescales, which points to non-stationarity in extreme sea level events(Nott et al., 2009). Woodworth et al. (2011) use tide gauge records datingback to the 18th century, and salt marsh data, to show that sea levelrise has accelerated over this time frame.The AR4 reported that there was high confidence that the rate of observedsea level rise increased from the 19th to the 20th century (Bindoff et al.,2007). It also reported that the global mean sea level rose at an averagerate of 1.7 (1.2 to 2.2) mm yr -1 over the 20th century, 1.8 (1.3 to 2.3)mm yr -1 over 1961 to 2003, and at a rate of 3.1 (2.4 to 3.8) mm yr -1over 1993 to 2003. With updated satellite data to 2010, Church andWhite (2011) show that satellite-measured sea levels continue to rise at178