IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3bottom drainage (Smith et al., 2005). Overall, increased air temperatureover high latitudes is primarily responsible for the development ofthermokarst terrains and thaw lakes.In summary, it is likely that there has been warming of permafrostin recent decades. There is high confidence that permafrosttemperatures will continue to increase, and that there will beincreases in active layer thickness and reductions in the area ofpermafrost in the Arctic and subarctic.3.5.8. Sand and Dust StormsSand and dust storms are widespread natural phenomena in many partsof the world. Heavy dust storms disrupt human activities. Dust aerosols inthe atmosphere can cause a suite of health impacts including respiratoryproblems (Small et al., 2001). The long-range transport of dust canaffect conditions at long distances from the dust sources, linking thebiogeochemical cycles of land, atmosphere, and ocean (Martin andGordon, 1988; Bergametti and Dulac, 1998; Kellogg and Griffin, 2006).For example, dust from the Saharan region and from Asia may reachNorth America and South America (McKendry et al., 2007). Some climatemodels have representations of dust aerosols (Textor et al., 2006).Climate variables that are most important to dust emission and transportsuch as soil moisture (see also Section 3.5.1), precipitation, wind, andvegetation cover are still subject to large uncertainties in climate modelsimulations. As a result, the sand and dust storm simulations have largeuncertainties as well.The Sahara (especially the Bodélé Depression in Chad) and east Asiahave been recognized as the largest dust sources globally (Goudie,2009). Over the few decades before the 1990s, the frequency of dustevents increased in some regions such as the Sahel zone of Africa(Goudie and Middleton, 1992), and decreased in some other regionssuch as China (Zhang et al., 2003). There seems to be an increase in morerecent years in China (Shao and Dong, 2006). Despite the importance ofAfrican dust, studies on long-term change in Sahel dust are limited.However, dust transported far away from the source region may providesome evidence of long-term changes in the Sahel region. The Africandust transported to Barbados began to increase in the late 1960s andthrough the 1970s; transported dust reached a peak in the early 1980sbut remains high into the present (Prospero and Lamb, 2003; Prosperoet al., 2009).Surface soil dust concentration during a sand and dust storm iscontrolled by a number of factors. The driving force for the productionof dust storms is the surface wind associated with cold frontal systemssweeping across arid and semi-arid regions and lifting soil particles inthe atmosphere. Dust emissions are also controlled by the surfaceconditions in source regions such as the desert coverage distributions,snow cover, and soil moisture. For example, in the Sahel region, theelevated high level of dust emission prior to the 1990s was related tothe persistent drought during that time, and to long-term changes in theNAO (Ginoux et al., 2004; Chiapello et al., 2005; Engelstaedter et al.,2006), and perhaps to North Atlantic SST as well (Wong et al., 2008).Further evidence of the importance of climate on dust emission is thatdespite an increase of approximately 2 to 7% in desert areas in Chinaover the four decades since 1960, dust storm frequency decreased inthat period (Zhong, 1999). Studies on Asian soil dust production from1960 to 2003 suggest that climatic variations have played a major rolein the declining trends in dust emission and storm frequencies in China(Zhang et al., 2003; Zhou and Zhang, 2003; Zhao et al., 2004; Gong etal., 2006). Overall, changes in dust activity are affected by changes inthe climate, such as wind and moisture conditions in the dust sourceregions. Changes in large-scale circulation play an additional role in thelong-distance transport of dust. However, understanding of the physicalmechanisms of the long-term trends in dust activity is not complete; forexample, the relative importance of the various factors affecting dustfrequency as outlined above is uncertain.Future dust activity depends on two main factors: land use in the dustsource regions, and climate both in the dust source region and largescalecirculation that affects long distance dust transport. Studies onprojected future dust activity are very limited. It is difficult to projectfuture land use. Precipitation, soil moisture, and runoff have beenprojected to decrease in major dust source regions (Figure 10.12 in Meehlet al., 2007b). Thomas et al. (2005) suggest that dune fields in southernAfrica can become active again, and sand will become significantlyexposed and move, as a consequence of 21st-century warming. A studybased on simulations from two climate models also suggests increaseddesertification in arid and semi-arid China, especially in the second halfof the 21st century (X.M. Wang et al., 2009). However, confident projectedchanges in wind are lacking (see Section 3.3.3).In summary, there is low confidence in projecting future duststorm changes, although an increase could be expected wherearidity increases. There is a lack of data and studies on pastchanges. There is also a lack of understanding of processes suchas the relative importance of different climate variables affectingdust storms, as well as a high uncertainty in simulating importantclimate variables such as soil moisture, precipitation, and windthat affect dust storms.190