IPCC Report.pdf - Adam Curry

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3a tendency toward drier conditions, with some regions displaying a shiftin climate regimes (e.g., from humid to transitional or transitional to dry).Some of these regional changes will depend on how forcing changes mayalter the regional atmospheric circulation, especially in coastal regionsand regions with substantial orography. Hence for certain extremes suchas floods and droughts, regional projections might indicate largerchanges than is the case for projections of global averages (whichwould average the regional signals exhibiting changes of oppositesigns). This also means that signals at the regional scale may be morereliable (and meaningful) in some cases than assessments at the globalscale. On the other hand, temperature extremes projections, which areconsistent across most regions, are thus more reliable at the globalscale (‘virtually certain’) than at the regional scale (at most ‘very likely’).3.1.7. Surprises / Abrupt Climate ChangeThis report focuses on the most probable changes in extremes based oncurrent knowledge. However, the possible future occurrence of lowprobability,high-impact scenarios associated with the crossing of poorlyunderstood climate thresholds cannot be excluded, given the transientand complex nature of the climate system. Such scenarios have importantimplications for society as highlighted in Section 8.5.1. So, an assessmentthat we have low confidence in projections of a specific extreme, or evenlack of consideration of given climate changes under the categoriescovered in this chapter (e.g., shutdown of the meridional overturningcirculation), should not be interpreted as meaning that no change isexpected in this extreme or climate element (see also Section 3.1.5).Feedbacks play an important role in either damping or enhancingextremes in several climate variables (Section 3.1.4), and this can alsolead to ‘surprises,’ that is, changes in extremes greater (or less) thanmight be expected with a gradual warming of the climate system.Similarly, as discussed in 3.1.3, contrasting or multiple extremes canoccur but our understanding of these is insufficient to provide crediblecomprehensive projections of risks associated with such combinations.One aspect that we do not address in this chapter is the existence ofpossible tipping points in the climate system (e.g., Meehl et al., 2007b;Lenton et al., 2008; Scheffer et al., 2009), that is, the risks of abrupt,possibly irreversible changes in the climate system. Abrupt climatechange is defined as follows in the Glossary: “The nonlinearity of theclimate system may lead to abrupt climate change, sometimes calledrapid climate change, abrupt events, or even surprises. The term abruptoften refers to time scales faster than the typical time scale of theresponsible forcing. However, not all abrupt climate changes need beexternally forced. Some changes may be truly unexpected, resultingfrom a strong, rapidly changing forcing of a nonlinear system.”Thresholds associated with tipping points may be termed ‘criticalthresholds,’ or, in the case of the climate system, ‘climate thresholds’.Scheffer et al. (2009) illustrate the possible equilibrium responses of asystem to forcing. In the case of a linear response, only a large forcingcan lead to a major state change in the system. However, in the presenceof a critical threshold even a small change in forcing can lead to a similarmajor change in the system. For systems with critical bifurcations in theequilibrium state function two alternative stable conditions may exist,whereby an induced change may be irreversible. Such critical transitionswithin the climate system represent typical low-probability, high-impactscenarios, which were also noted in the AR4 (Meehl et al., 2007b).Lenton et al. (2008) provided a recent review on potential tipping elementswithin the climate system, that is, subsystems of the Earth system thatare at least subcontinental in scale and which may entail a tippingpoint. Some of these would be especially relevant to certain extremes[e.g., El Niño-Southern Oscillation (ENSO), the Indian summer monsoon,and the Sahara/Sahel and West African monsoon for drought and heavyprecipitation, and the Greenland and West Antarctic ice sheets for sealevel extremes], or are induced by changes in extremes (e.g., Amazonrainforest die-back induced by drought). For some of the identifiedtipping elements, the existence of bistability has been suggested bypaleoclimate records, but is still debated in some cases (e.g., Brovkin etal., 2009). There is often a lack of agreement between models regardingthese low-probability, high-impact scenarios, for instance, regarding apossible increased drought and consequent die-back of the Amazonrainforest (e.g., Friedlingstein et al., 2006; Poulter et al., 2010; seeTable 3-3 for dryness projections in this region), the risk of an actualshutdown of the Atlantic thermohaline circulation (e.g., Rahmstorf etal., 2005; Lenton et al., 2008), or the potential irreversibility of thedecrease in Arctic sea ice (Tietsche et al., 2011). For this reason,confidence in these scenarios is assessed as low.3.2. Requirements and Methodsfor Analyzing Changes in Extremes3.2.1. Observed ChangesSections 3.3 to 3.5 of this chapter provide assessments of the literatureregarding changes in extremes in the observed record published mainlysince the AR4 and building on the AR4 assessment. Summaries of theseassessments are provided in Table 3-1. Overviews of observed regionalchanges in temperature and precipitation extremes are provided inTable 3-2. In this section issues are discussed related to the data andobservations used to examine observed changes in extremes.Issues with data availability are especially critical when examiningchanges in extremes of given climate variables (Nicholls, 1995). Indeed,the more rare the event, the more difficult it is to identify long-termchanges, simply because there are fewer cases to evaluate (Frei andSchär, 2001; Klein Tank and Können, 2003). Identification of changes inextremes is also dependent on the analysis technique employed (X.Zhang et al., 2004; Trömel and Schönwiese, 2005). Another importantcriterion constraining data availability for the analysis of extremes is therespective time scale on which they occur (Section 3.1.2), since thisdetermines the required temporal resolution for their assessment (e.g.,heavy hourly or daily precipitation versus multi-year drought). Longertime resolution data (e.g., monthly, seasonal, and annual values) fortemperature and precipitation are available for most parts of the world122