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Focus: Changes in Extreme Temperatures A thorough assessment of extreme events by Field et al. (2012) concludes that it is very likely that the length, frequency, and intensity of heat waves will increase over most land areas, with more warming resulting in more extremes. Zwiers and Kharin (1998) report, when examining simulations with doubled CO 2, (which typically results in about 3°C global mean warming), that the intensity of extremely hot days, with a return time of 20 years, increases between 5°C and 10°C over continents, with the larger values over North and South America and Eurasia, related to substantial decreases in regional soil moisture. Meehl and Tebaldi (2004) found significant increases in intensity, duration, and frequency of three-day heat events under a businessas-usual scenario. The intensity of such events increases by up to 3°C in the Mediterranean and the western and southern United States. Based on the SRES A2 transient greenhouse-gas scenario, Schär et al. (2004) predict that toward the end of the century about every second European summer could be as warm as or warmer than the summer of 2003. Likewise, Stott et al. (2004) show that under unmitigated emission scenarios, the European summer of 2003 would be classed as an anomalously cold summer relative to the new climate by the end of the century. Barnett et al. (2006) show that days exceeding the present-day 99 th percentile occur more than 20 times as frequently in a doubled CO 2 climate. In addition, extremely warm seasons are robustly predicted to become much more common in response to doubled CO 2 (Barnett et al. 2006). Based on the same ensemble of simulations, Clark, Brown, and Murphy (2006) conclude that the intensity, duration, and frequency of summer heat waves are expected to be substantially greater over all continents, with the largest increases over Europe, North and South America, and East Asia. These studies, which analyze extreme weather events in simulations with a doubling of CO 2 and those following a businessas-usual emissions path, can provide useful insights. Without exception, such studies show that heat extremes, whether on daily or seasonal time scales, greatly increase in climates more than 3°C warmer than today. To the authors’ knowledge, no single study has specifically analyzed the number of extremes in a world beyond 4°C warmer than preindustrial conditions. The authors address this gap in the science and provide statistical analysis of heat extremes in CMIP5 (Coupled Model Intercomparison Project) climate projections that reach a 4°C world by the end of the 21st century (Taylor et al. 2012). Methods are described in Appendix 2. A Substantial Increase in <strong>Heat</strong> Extremes The authors’ statistical analysis indicates that monthly heat extremes will increase dramatically in a world with global mean temperature more than 4°C warmer than preindustrial temperatures. Temperature anomalies that are associated with highly unusual heat extremes today (namely, 3-sigma events occurring only once in several hundreds of years in a stationary climate) 12 will have become the norm over most (greater than 50 percent) continental areas by the end of the 21st century. Five-sigma events, which are now essentially 12 In general, the standard deviation (sigma) shows how far a variable tends to deviate from its mean value. In the authors’ study it represents the possible year-to-year changes in local monthly temperature because of natural variability. For a normal distribution, events warmer than 3 sigma away from the mean have a return time of 740 years and events warmer than 5 sigma have a return time of several million years. Monthly temperature data do not necessarily follow a normal distribution (for example, the distribution can have “long” tails making warm events more likely) and the return times can be different. Nevertheless, 3-sigma events are extremely unlikely and 4-sigma events almost certainly have not occurred over the lifetime of key infrastructure. A warming of 5 sigma means that the average change in the climate is 5 times larger than the normal year-to-year variation experienced today. 37
Turn Down the <strong>Heat</strong>: Why a 4°C Warmer World Must Be Avoided absent, will become common, especially in the tropics and in the Northern Hemisphere (NH) mid-latitudes during summertime. According to the authors’ analysis, the most pronounced warming will occur over land (see Figure 33, top row). Monthly mean temperatures over oceans will increase between 0°C and 4°C and over continents between 4°C and 10°C. Warming over continental regions in the tropics and in the Southern Hemisphere (SH) is distributed rather evenly without strong spatial and seasonal variations. The only exception is Argentina, which is expected to see less wintertime (JJA) warming. In the NH, much stronger spatial and seasonal variations in continental warming patterns are observed. During the boreal winter, strong warming in the near Arctic region is observed due to the so-called “arctic amplification” effect, resulting in temperature anomalies of over 10°C. Two NH regions can be identified that are expected to see more warming in summertime than in wintertime: The subtropical region consisting of the Mediterranean, northern Africa, and the Middle East, as well as the contiguous United States, are likely to see monthly summer temperatures rise by more than 6°C. All land areas show a mean warming of at least 1-sigma above the present-day mean and most land areas (greater than 80 percent) show warming of at least 2-sigma. Roughly half of the land area will likely experience a mean warming of more than 3-sigma during the boreal winter and more than 4-sigma during the boreal summer. This seasonal difference is due to enhanced warming over NH mid-latitudinal land areas during the boreal summer. Shifts in Temperature by Region In the authors’ analysis, a 4°C warmer world will consistently cause temperatures in the tropics to shift by more than 6 standard deviations for all months of the year (Figure 33 bottom panels). Particularly, countries in tropical South America, Central Africa, and all tropical islands in the Pacific will see unprecedented extreme temperatures become the new norm in all months of the year. In fact, a temperature shift of 6 standard deviations or more implies a new climatic regime with the coolest months in 2080–2100 being substantially warmer than the warmest months in the end of the 20th century. In the SH mid-latitudes, monthly temperatures over the continents by the end of the 21st century lie in the range of 2- to 4-sigma above the present-day mean in both seasons. Over large regions of the NH mid-latitudes, the continental warming (in units of sigma) is much stronger in summer, Figure 33: Multimodel mean of monthly warming over the 21st century (2080–2100 relative to present day) for the months of JJA (left) and DJF (right) in units of degrees Celsius (top) and in units of local standard deviation of temperature (bottom). The intensity of the color scale has been reduced over the oceans for distinction. 38
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Turn Down the Heat Why a 4°C Warme
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© 2012 International Bank for Reco
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Turn Down the Heat: Why a 4°C Warm
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Page 10 and 11: Foreword It is my hope that this re
Page 13 and 14: Executive Summary
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Page 26 and 27: Observed Climate Changes and Impact
Page 42 and 43: 21st Century Projections This secti
Page 44 and 45: 21st Century Projections technologi
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Page 50 and 51: Focus: Sea-level Rise Projections P
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Page 54 and 55: Focus: Sea-level Rise Projections F
Page 57: Chapter 5
Page 63 and 64: Chapter 6
Page 80 and 81: System Interaction and Non-linearit
Page 87 and 88: Appendix 1
Page 91 and 92: Appendix 2
Page 93 and 94: Bibliography
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