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4 Attribution of Particular Types of Extreme Events
Pages 85-126

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From page 85...
... Existing attribution studies on single extreme events also are reviewed as part of this background. The number of studies varies widely; for some event types there are few or even no such studies.
From page 86...
... . The actual temperatures that characterize a cold event vary regionally and seasonally, but the temperatures during such an event will be in the cold tail of the probability distribution of temperatures for a location or region and time of year.
From page 87...
... have used self-organizing maps derived from atmospheric reanalyses to show that both factors have played roles in recent changes in extreme cold events. In particular, increasing trends in northerly flow have led to an increasing trend in winter cold extremes over central Asia.
From page 88...
... show that decreases in temperature variance account for generally less than 20% of the projected 21st-century decreases in extreme cold temperatures over North America; the mean warming accounts for most of the remainder. The fact that underlying warming has moderated cold extremes also has been shown using daily circulation analogs for the European cold events of 2010 (­ attiaux et al., 2010)
From page 89...
... find that ice loss is associated with decreased likelihood of extreme cold events (as well as decreased variability of temperature) over nearly the entire Northern Hemisphere land areas.
From page 90...
... Finally, impact-relevant metrics of extreme cold events need to be developed for use in attribution studies. In a climate with polar amplified warming, increased equatorward flow will likely be required if cold air advection is to cause any hypothetical increase of extreme cold events in middle latitudes.
From page 91...
... . They also note that minimum temperatures have increased more than maximum temperatures, and maps of changes show statistically significant increases in two indices of extreme temperatures in almost every land area since 1950: the 90th percentile of daily minimum temperatures and the 90th percentile of daily maximum temperatures.
From page 92...
... . This may reflect the greater likelihood of successful attribution of heat waves, compared to other event types, to human-induced climate change using existing models and data (see the discussion of selection bias in Chapter 2)
From page 93...
... do not include heat events among their examples of a highly conditioned approach, this approach clearly could be applied to heat events, starting perhaps with one of the most impactful events, like the Russian heat wave of 2010. Heat events and warm anomalies may be the best candidates for assessing the reliability and robustness of attribution methods because the direct thermodynamic effects on this type of extreme event are generally more straightforward than, for example, heavy rainfall.
From page 94...
... SOURCE: Black et al., 2015. DROUGHTS Event Type Definition Droughts are complex phenomena involving various combinations of atmospheric inputs (chiefly precipitation, but also temperature)
From page 95...
... Similar holistic considerations apply to other extreme event types, to varying degrees. One reason that attributing both extreme flooding and extreme droughts to anthropogenic climate change is particularly difficult is that changes in the hydrologic cycle are both causes of the event (a climatic driver)
From page 96...
... Because temperature plays a role in determining evaporation, snowpack, soil moisture, and -- indirectly -- streamflow, attribution of hydrological drought may be more robust than is strictly meteorological drought, which is more strongly influenced by precipitation. It also may be the case that attribution for some specific droughts may be more straightforward than reaching broad conclusions about the role of anthropogenic climate change in droughts globally because some of the specific regional factors that cause varying responses of drought to climate may be better understood in particular locations and times than others.
From page 97...
... use SST-conditioned runs: that is, atmosphere-only model simulations using observed SSTs, often compared with a counterfactual to compute FAR. A few also use an approach closer to seasonal forecasting, which somewhat resembles a highly conditioned approach: Hoerling and colleagues (2013)
From page 98...
... While drought is acknowledged to be a complex phenomenon due to the many physical processes involved and the broad range of societal factors that influence its occurrence and intensity, some aspects of drought are influenced by temperature in ways that are better understood, and thus more amenable to attribution, than ­ thers. o In particular, temperature exacerbates hydrological drought in some regions by increasing surface evaporation, so that increasing temperature causes an increasing risk of hydrological drought even if precipitation does not change (e.g., Diffenbaugh et al., 2015; Williams et al., 2015)
From page 99...
... EXTREME RAINFALL Event Type Definition An extreme rainfall event is defined as one in which precipitation over some specified time period exceeds some threshold, either at a point (i.e., as measured by a single rain gauge) or in an average over some spatial region.
From page 100...
... . Extreme precipitation is asso­ ciated with an array of meteorological processes, including tropical cyclones, extra­ tropical cyclones, monsoons, atmospheric rivers, and localized convection (­ unkel et K al., 2013)
From page 101...
... and note an upward trend in both the intensity and the frequency of extreme precipitation events in the United States. A number of other studies have noted statistically significant increases in the frequency of occurrence or intensity of extreme precipitation events with durations ranging from hours to several days in various parts of the world (Donat et al., 2013; Krishnamurthy et al., 2009; Mann and Emanuel, 2006; Westra et al., 2013)
From page 102...
... Their results are somewhat conflicting in terms of whether anthropogenic forcing contributed to extreme summer precipitation events. They find that the risk of an extreme rainfall event doubled in July because of anthropogenic 102
From page 103...
... EXTREME SNOW AND ICE STORMS Event Type Definition Severe winter weather includes snow and ice (freezing rain) storms, often accompanied by wind.
From page 104...
... For the entire Northern Hemisphere, the summary in the preceding section ("Extratropical Cyclones") showed that there is mixed evidence for trends in the frequency and intensity of cold-season storms, regardless of whether they produce snow and/or freezing rain.
From page 105...
... Southeast and South, (c) Upper Midwest and Northern Plains.
From page 106...
... On the Horizon Attribution of extreme snow and ice events suffers from a similar challenge as do some other extreme event types in that the events are strongly governed by the atmospheric circulation, for which externally forced changes are uncertain. For this reason, attribution of extreme snow and ice storm events may benefit from an emphasis on the thermodynamic state during particular events, as argued by Trenberth and colleagues (2015)
From page 107...
... This apparent abrupt increase, as well as the need to distinguish changes in drivers from changes in impacts, makes clarification of the role of anthropogenic climate change in snowstorms affecting the northern United States a high priority. TROPICAL CYCLONES Event Type Definition The National Oceanic and Atmospheric Administration (NOAA)
From page 108...
... . To the committee's knowledge, purely observation-based methods have not been used to perform event attribution studies on tropical cyclones.
From page 109...
... There is, however, a large literature that addresses how tropical cyclones may change in future climates. Some of these studies use the same global climate models as used for overall climate change assessment (e.g., Camargo, 2013)
From page 110...
... state that the frequency of the most intense storms "will more likely than not increase substantially in some basins under projected 21st century warming." Precipitation in tropical cyclones is expected to increase because of the increased water vapor content of the atmosphere, similarly to other extreme precipitation events; Christensen and colleagues (2013) express ­ edium ­confidence in this projection.
From page 111...
... EXTRATROPICAL CYCLONES Event Type Definition The term "extratropical cyclone" refers to the migratory frontal cyclones of middle and high latitudes, which are embedded within the large-scale westerly flow and thus 111
From page 112...
... that used minimum surface pressure as the index; the overall expected decrease in surface pressure at higher latitudes thus induced a trend which was not actually related to cyclone intensity. In the IPCC Fifth Assessment Report, future projections of extratropical cyclones were found to be uncertain (Christensen et al., 2013)
From page 113...
... Because extratropical cyclones are defined as discrete events rather than extreme values of continuous time series, observation-based methods for attribution using extreme value theory may not apply as straightforwardly to extratropical cyclones as to some other event types. Nevertheless, both van Oldenborgh and colleagues (2015)
From page 114...
... suggest that although current global climate models generally underrepresent the intensity of extratropical cyclones due to insufficient latent heat release, once the horizontal resolution is finer than about 100 km they should be adequate, and that the systematic biases will then mainly involve storm track location. Seiler and Zwiers (2015a)
From page 115...
... The most common metric of wildfires is the area burned, either by a single wildfire or by all wildfires during a fire season in a particular region. Attribution of wildfire trends and extreme events is complicated by (1)
From page 116...
... Because climate models do not explicitly include lightning (or explicit formulations of convective storms) , atmospheric stability and rain rate have been used to construct indices of lightning activity derived from model output.
From page 117...
... Beginning in the 1990s, the latter part of the historical simulation, a clear separation emerges between fire risks driven by only natural variability (the counterfactual climate, a long preindustrial simulation) and those driven by anthropogenic climate forcing (Yoon et al., 2015; see Figure 2.2)
From page 118...
... Large fires are almost always smaller than the grid cells of today's earth system models, however, so sub-grid cell variability will need to be represented in land-surface modules that are either run offline or coupled to coarser-resolution atmospheric models. SEVERE CONVECTIVE STORMS Event Type Definition Severe convective storms (SCSs)
From page 119...
... Several studies have used climate model projections to estimate the effect of GHG increases on future SCS activity in the United States. Due to the impossibility of ade­ quately simulating severe convection in low-resolution climate models, these studies all focus on changes in large-scale environmental variables associated with SCS activity (e.g., CAPE and vertical wind shear)
From page 120...
... did consider hail distinctly from other precipitation extremes, finding that "confidence is still low for hail projections particularly due to a lack of hail-specific modelling studies, and a lack of agreement among the few available studies." Highly conditioned approaches are feasible for SCS today, using either environmental indices or small-domain, high-resolution models forced by environmental conditions derived from larger-scale ones, as has been done earlier for tropical cyclones (Knutson and Tuleya, 2004) ; a small number of studies have already been done using this methodology for future scenarios (Gensini and Mote, 2015; Trapp and Hoogewind, 2016)
From page 121...
... Tropical cyclones are among the more challenging event types, though they are somewhat more tractable than tornadoes due to their larger scales and better observations. Extratropical cyclones, extreme precipitation events, and snow and ice storms are in between these extremes.
From page 122...
... The vertical position of each event type indicates an assessment of scientific confidence in current capabilities for attribution of specific events to anthropogenic climate change for that event type (assuming the attribution is carried out following the recommendations in this report) , which draws on all three columns of Table 4.1.
From page 123...
... Understanding of Physical Mechanisms ˜ = high Capabilities of Climate That Lead to Changes in › = medium Models to Simulate Quality/Length of the Extremes as a Result of ™ = low Event Type Observational Record Climate Change Extreme cold events ˜ ˜ ˜ Extreme heat events ˜ ˜ ˜ Droughts › › › Extreme rainfall › › › Extreme snow and ice › ™ › storms Tropical cyclones ™ ™ › Extratropical cyclones › ™ ™ Wildfires ™ › ™ Severe convective ™ ™ ™ storms 123
From page 124...
... Extreme heat events: Climate models represent heat events well, compared to many other event types, and observations characterize events and trends similarly well. Long-term change in hot extremes has been attributed to human influence on the climate system.
From page 125...
... Extratropical cyclones: Climate models can simulate the events to some extent, though resolution and physics may still be limiting in many models, particularly in their ability to resolve the most extreme local manifestations of the storms, such as strong winds and heavy precipitation. Detection of trends in observations, robustness of projections, and physical understanding of climate change influences are all weak.


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