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CLIMATE SIGNALS E X T R E M E W E A T H E R G U I D E Climate Signals A Guide to Selected Extreme Weather and Climate Change For informaon contact: Hunter Cung Climate Nexus +1 415-420-7498 [email protected] This work is licensed under the Creave Commons AOribu:on-NoDerivs 3.0 Unported License. New York, NY 2012 Table of Contents Overview . 4 Heat Waves . 7 Drought . 12 Rain and Snow . 18 Flooding . 23 Tornadoes . 28 Hurricanes . 33 References . 36 Climate Change and Extreme Weather Overview Climate change is already affec:ng extreme weather. The Naonal Academy of Sciences reports that rain has become concentrated in heavier downpours and the hoOest days are now hoer.1 And the fingerprint of global warming behind these changes has been firmly iden:fied.2 Photo credit: Monika Sharma In the strictest sense all weather events are now affected by climate change because the environment in which they occur (the atmosphere) is significantly warmer and weOer than it used to be.3 The Naonal Oceanic and Atmospheric Administraon (NOAA) reports an increase in billion- dollar weather disasters across the U.S. in recent years with an astonishing 14 weather disasters totaling over $50 billion in damages occurring in 2011 alone. Four out of five Americans live in coun:es where natural disasters have been declared since 2006.4 The insurance giant Munich RE reports that the number of weather catastrophes across the world has tripled since 1980 and that climate change is helping to drive this trend.5 4 While natural variability con:nues to play a key role in extreme weather, climate change has shiced the odds and changed the natural limits, making certain types of extreme weather much more frequent and more intense. Sixty years ago in the con:nental United States, the number of new record high temperatures recorded around the country each year was roughly equal to the number of new record lows. Now, the number of new record highs recorded each year is twice the number of new record lows, a signature of a warming climate, and a clear example of its impact on extreme weather.6 A small change in average global temperature leads to a dramac change in the frequency of extreme events,7 as witnessed in recent years by the 50-fold increase in the global areas experiencing the most extreme global temperatures.8 5 Number of Weather Related Disasters 2006-2001. Credit: Environment America While our understanding of how climate change affects extreme weather is s:ll developing, evidence suggests that extreme weather may be altered even more than an:cipated. Recent changes in extreme weather have been even greater than the changes projected by climate models.9 1 Matson et al. 2010 2 Min et al. 2011, Dai et al. 2011, Seneviratne 2012 3 Trenberth 2012 4 Dutzik and Wilcox, 2012 5 Hoeppe 2012 6 Meehl et al. 2009 7 Karl et al. 2008; Trenberth 1999; Gutowski et al. 2008 6 Heat Waves "The duraFon, size, and intensity of the 'summer in March' heat wave are simply off-scale. The event ranks as one of North America's most extraordinary weather events in recorded history." – Dr. Jeff Masters, Weather Underground. There has been a remarkable run of record-shaering heat waves in recent years, from the Russian heat wave of 2010 that set forests ablaze to last year’s historic heat wave in Texas and this year’s Summer in March for the Midwest. And this stretch fits the on-going trend driven by climate change. The impacts of these events are devastang. The drought and heat wave that hit Texas and the Southern plains in the summer of 2012 cost $10 billion.1 Since 1950 the number of heat waves worldwide has increased, and heat waves have become longer.2 The hoOest days and nights have become hoOer and more frequent.3 In the past several years, the global area hit by extremely unusual hot temperatures has increased 50 Summer in March. Unusual temperatures, March 13-19, fold.4 In the United States, new 2012 Credit: NASA record high temperatures now regularly outnumber new record lows by a rao of 2:1.5 The fingerprint of global warming has been firmly iden:fied in this trend.6 And for the U.S., the rise in heat-trapping gases in the atmosphere has increased the probability of record-breaking temperatures 15-fold.7 Looking Forward If we con:nue business as usual, the same summer:me temperatures that ranked among the top 5% in 1950–1979 will occur at least 70% of the :me by 2035–2064 in the U.S. The South, Southwest, and Northeast will be especially prone to large increases in unusually hot summers. 8 7 Heat Waves and Climate Change: The Science Numerous studies have documented that human-induced climate change has increased the frequency and severity of heat waves across the globe.9 Human influence is es:mated to have more than doubled the likelihood of the warming trends experienced recently in virtually every region of the globe.10 Since 1950 the number of heat waves worldwide has increased, and heat waves have become longer.11 The hoOest days and nights have become hoOer and more frequent.12 Globally, extremely warm nights that used to come once in 20 years now occur every 10 years.13 Extremely hot summers are now observed in about 10% of the global land area, compared to 0.1-0.2% for the period 1951-1980.14 These trends cannot be explained by natural variaon alone. Only with the inclusion of human influences can computer models of the climate reproduce the observed changes in the number of warm nights in a year, warming on the warmest night of the year, warming on the coldest nights and days of the year, warming on the hoOest day of the year, unusually hot days throughout the year, and heat waves.15 In the United States, new record high temperatures now regularly outnumber new record lows by a rao of 2:1.16 NOAA’s Naonal Climac Data Center reports that during January-March of 2012 warm weather records outnumbered cold records across the United States by a factor of 12. The raFo of record daily temperature highs to record daily lows observed at about 1,800 weather staFons in the 48 conFguous United States from January 1950 through September 2009. Source: Meehl et al. 2009 8 For the U.S., the rise in heat-trapping gases in the atmosphere has increased the probability of record-breaking temperatures 15-fold.17 In Europe, global warming is now responsible for an es:mated 29% of the new record highs set each year.18 The significant increase in heat waves we have witnessed arising from a small shic in the global average temperature is expected. Global warming boosts the probability of very extreme events, like the recent Summer in March for the U.S., far more than it changes the likelihood of more moderate events.19 Weather events tend to strongly cluster around the average. So a substan:al change can result from a relavely small shic in the average temperature. A small shic in temperature will move some extreme events across the threshold near the edge of the cluster, and as result they become much more common.20 The following graphs help to illustrate this point. The change in probability for extreme events can be visualized like a tradi:onal bell curve. Climate change, however, changes the shape of the curve. Climate change shics the curve to one side, moving the mean average over. Climate change also flaens the curve, providing for a greater spread of events, an increase in variaon. The combinaon provides for a dramac increase in record hot weather.21 IPCC (2001) graph illustraFng how a shi[ and/or widening of a probability distribuFon of temperatures affects the probability of extremes. 9 The graph below plots historical temperature data from the Northern Hemisphere, with each colored line represen:ng a different decade. A posi:ve temperature anomaly means temperatures are warmer than average, while a negave temperature anomaly means they are cooler. Thus, the graph illustrates both the shic and flaening of the curve represen:ng the distribu:on of unusual temperatures.22 The overall effect corresponds with graph (c) on the previous page. Frequency of summer temperature anomalies (how o[en they deviated from the historical normal of 1951-1980) over the summer months in the northern hemisphere. Source: NASA/ Hansen et al. 2012 10 1 NCDC, 2012 2 Trenberth et al 2007 3 Gutowski et al. 2008, Trenberth et al 2007 4 Hansen et al 2012 5 Meehl et al. 2009 6 Gutowski et al. 2008, StoO et al. 2010, Chris:dis et al. 2011, Seneviratne et al 2012, Hansen et al 2012 7 Hoerling et al. 2007 8 Duffy and Tebaldi 2012 9 Gutowski et al. 2008, StoO et al. 2010, Chris:dis et al. 2011, Seneviratne et al 2012, Hansen et al 2012 10 Chris:dis et al. 2009, and StoO et al. 2010 11 Trenberth et al 2007 12 Gutowski et al. 2008, Trenberth et al 2007 13 Zwiers et al. 2010 14 Hansen et al 2012 15 Gutowski et al. 2008, StoO et al. 2010, Chris:dis et al. 2011, Seneviratne et al 2012, Hansen et al 2012 16 Meehl et al. 2009 17 Hoerling et al. 2007 18 Wergen and Krug 2010 19 Rahmstorf and Coumou, 2012 20 Gutowski et al. 2008 21 Rahmstorf and Coumou, 2012 22 Hansen et al 2012 11 Drought Very dry areas across the globe have doubled in extent since the 1970s.1 This global trend has been linked directly to climate change.