Coastal Impacts, Adaptation, and Vulnerabilities - Climate ...

28 Coastal Impacts, Adaptation, and Vulnerabilitiesinterests because of the associated risk of damage and loss of life during landfall eventsas well as the significant role that tropical cyclones can play in maintaining regionalwater resources (Jiang & Zipser, 2010). Atlantic tropical cyclone variability is closely correlatedwith tropical Atlantic climate variability on a broad range of time-scales. A majorchallenge in detecting past trends in various measures of tropical cyclone activity is theneed to identify the causal factors underpinning the observed Atlantic climate variability,which is required to separate tropical cyclone variability into naturally and anthropogenicallyforced constituents. Even when regional climate variability can be attributed toanthropogenic causes, the question of how tropical cyclones respond to such variabilityremains. For example, studies have detected a tropical Atlantic sea surface temperaturewarming trend due to increasing greenhouse gases (Gillett et al., 2008; Karoly & Wu,2005; Knutson et al., 2006; Santer et al., 2006), but the question of how tropical cyclonesrespond to sea surface temperature changes under global warming remains (Johnson &Xie, 2010; Knutson et al., 2008; Kunkel et al., 2011).Detection of past trends in Atlantic tropical cyclone activity is also significantly constrainedby the quality of the historical data records (Knutson et al., 2010; Kunkel etal., 2011). Attempts to detect trends in landfalling tropical cyclone events are furtherconstrained by the reduced data sample size associated with parsing of the data and arealso substantially challenged by tropical cyclone track variability (Kossin & Camargo,2009). This variability is driven largely by random fluctuations in atmospheric steeringcurrents and introduces substantial noise into time series of U.S. landfalling tropicalcyclone activity, which show no statistically significant long-term trends (Landsea,2005; Vecchi & Knutson 2011). Atlantic tropical cyclone track variability is also driven bymore systematic climatic forcings such as the El Niño-Southern Oscillation, North AtlanticOscillation, Atlantic Meridional Mode, and Madden-Julian Oscillation (Kossin et al.,2010), but uncertainty still remains regarding how these modes of variability respond toclimate change (Collins et al., 2010). Even modest tropical cyclone track variability canlead to large differences in associated coastal impacts. When compounded by uncertaintiesin the historical data, this severely challenges detection-attribution studies as wellas disaster risk reduction in specific coastal regions. Only very low confidence can be offeredthat any trends in tropical cyclone activity reported within specific coastal regionsare detectable.Regarding global hurricane or tropical cyclone activity, an expert team of the WorldMeteorological Organization (Knutson et al., 2010) concluded that by the late 21st century,greenhouse warming would likely cause: 1) the global number of tropical cyclonesto remain at current levels or to decrease by up to one-third; 2) the average intensityof tropical cyclones to increase by up to 10 percent; and 3) near-storm rainfall rates toincrease by roughly 20 percent. For Atlantic basin tropical cyclone activity, the 21st centuryclimate model projections summarized in Knutson and colleagues’ work (2010; andupdated in Tables S1-S4 of this report) show a much larger range of uncertainty thanthe global projections. For example, the model projections from 14 studies for Atlantictropical storm frequency for the late 21st century range from about a 60 percent increaseto about a 60 percent decrease relative to current levels (Table S1). Thus, projections ofthe sign of Atlantic tropical storm frequency change are offered at very low confidence.Model or theoretical projections for Atlantic hurricane intensity range from an 8 percentdecrease to a 14 percent increase (Table S3) with a clear tendency for an increase