IPCC Report.pdf - Adam Curry

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical Environment3.1. Weather and Climate EventsRelated to DisastersA changing climate leads to changes in the frequency, intensity, spatialextent, duration, and timing of weather and climate extremes, and canresult in unprecedented extremes (Sections 3.1.7, 3.3, 3.4, and 3.5). Aswell, weather or climate events, even if not extreme in a statistical sense,can still lead to extreme conditions or impacts, either by crossing a criticalthreshold in a social, ecological, or physical system, or by occurringsimultaneously with other events (Sections 3.1.2, 3.1.3, 3.1.4, 3.3, 3.4,and 3.5). Some climate extremes (e.g., droughts, floods) may be the resultof an accumulation of weather or climate events that are, individually,not extreme themselves (though their accumulation is extreme, e.g.,Section 3.1.2). A weather system such as a tropical cyclone can have anextreme impact, depending on where and when it approaches landfall,even if the specific cyclone is not extreme relative to other tropicalcyclones. Conversely, not all extremes necessarily lead to serious impacts.Changes in extremes can also be directly related to changes in meanclimate, because mean future conditions in some variables are projectedto lie within the tails of present-day conditions (Section 3.1.6). Hence,the definition of extreme weather and climate events is complex(Section 3.1.2 and Box 3-1) and the assessment of changes in climatethat are relevant to extreme impacts and disasters needs to considerseveral aspects. Those related to vulnerability and exposure areaddressed in Chapters 2 and 4 of this report, while we focus here on thephysical dimension of these events.Many weather and climate extremes are the result of natural climatevariability (including phenomena such as El Niño), and natural decadalor multi-decadal variations in the climate provide the backdrop foranthropogenic climate changes. Even if there were no anthropogenicchanges in climate, a wide variety of natural weather and climateextremes would still occur.3.1.1. Categories of Weather and Climate EventsDiscussed in this ChapterThis chapter addresses changes in weather and climate events relevantto extreme impacts and disasters grouped into the following categories:1) Extremes of atmospheric weather and climate variables (temperature,precipitation, wind)2) Weather and climate phenomena that influence the occurrence ofextremes in weather or climate variables or are extremes themselves(monsoons, El Niño and other modes of variability, tropical andextratropical cyclones)3) Impacts on the natural physical environment (droughts, floods,extreme sea level, waves, and coastal impacts, as well as otherphysical impacts, including cryosphere-related impacts, landslides,and sand and dust storms).The distinction between these three categories is somewhat arbitrary, andthe categories are also related. In the case of the third category, ‘impactson the natural physical environment,’ a specific distinction betweenthese events and those considered under ‘extremes of atmosphericweather and climate variables’ is that they are not caused by variationsin a single atmospheric weather and climate variable, but are generallythe result of specific conditions in several variables, as well as of somesurface properties or states. For instance, both floods and droughts arerelated to precipitation extremes, but are also impacted by otheratmospheric and surface conditions (and are thus often better viewed ascompound events, see Section 3.1.3). Most of the impacts on the naturalphysical environment discussed in the third category are extremesthemselves, as well as often being caused or affected by atmosphericweather or climate extremes. Another arbitrary choice made here is theseparate category for phenomena (or climate or weather systems) thatare related to weather and climate extremes, such as monsoons, El Niño,and other modes of variability. These phenomena affect the large-scaleenvironment that, in turn, influences extremes. For instance, El Niñoepisodes typically lead to droughts in some regions with, simultaneously,heavy rains and floods occurring elsewhere. This means that alloccurrences of El Niño are relevant to extremes and not only extremeEl Niño episodes. A change in the frequency or nature of El Niño episodes(or in their relationships with climate in specific regions) would affectextremes in many locations simultaneously. Similarly, changes in monsoonpatterns could affect several countries simultaneously. This is especiallyimportant from an international disaster perspective because copingwith disasters in several regions simultaneously may be challenging(see also Section 3.1.3 and Chapters 7 and 8).This section provides background material on the characterization anddefinition of extreme events, the definition and analysis of compoundevents, the relevance of feedbacks for extremes, the approach used forthe assignment of confidence and likelihood assessments in this chapter,and the possibility of ‘surprises’ regarding future changes in extremes.Requirements and methods for analyzing changes in climate extremesare addressed in Section 3.2. Assessments regarding changes in theclimate variables, phenomena, and impacts considered in this chapterare provided in Sections 3.3 to 3.5. Table 3-1 provides summaries ofthese assessments for changes at the global scale. Tables 3-2 and 3-3(found on pages 191-202) provide more regional detail on observed andprojected changes in temperature extremes, heavy precipitation, anddryness (with regions as defined in Figure 3-1). Note that impacts onecosystems (e.g., bushfires) and human systems (e.g., urban flooding)are addressed in Chapter 4.3.1.2. Characteristics of Weather and Climate EventsRelevant to DisastersThe identification and definition of weather and climate events that arerelevant from a risk management perspective are complex and dependon the stakeholders involved (Chapters 1 and 2). In this chapter, we focuson the assessment of changes in ‘extreme climate or weather events’(also referred to herein as ‘climate extremes’ see below and Glossary),which generally correspond to the ‘hazards’ discussed in Chapter 1.115