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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 E X T R E M E W E A T H E R G U I D E
Climate SignalsA Guide to Selected Extreme Weather and Climate Change
For informa:on contact:
Hunter Cu=ngClimate Nexus+1 415-‐420-‐[email protected]
This work is licensed under the Crea:ve Commons AOribu:on-‐NoDerivs 3.0 Unported License.
New York, NY 2012
Table of Contents
Overview . . . . . . . . . . . . . . . . . . . . . .
Heat Waves . . . . . . . . . . . . . . . . . . . .
Drought . . . . . . . . . . . . . . . . . . . . . . .
Rain and Snow . . . . . . . . . . . . . . . . . .
Flooding . . . . . . . . . . . . . . . . . . . . . . .
Tornadoes . . . . . . . . . . . . . . . . . . . . .
Hurricanes . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . .
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Climate change is already affec:ng extreme weather. The Na:onal Academy of Sciences reports that rain has become concentrated in heavier downpours and the hoOest days are now hoOer.1 And the fingerprint of global warming behind these changes has been firmly iden:fied.2
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 Na:onal Oceanic and Atmospheric Administra:on (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
Climate Change and Extreme Weather
Overview
4
Photo credit: Monika Sharma
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 drama:c 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
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. 20102 Min et al. 2011, Dai et al. 2011, Seneviratne 20123 Trenberth 20124 Dutzik and Wilcox, 20125 Hoeppe 20126 Meehl et al. 20097 Karl et al. 2008; Trenberth 1999; Gutowski et al. 2008
Number of Weather Related Disasters 2006-‐2001. Credit: Environment America
6
Heat Waves
There has been a remarkable run of record-‐shaOering 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 devasta:ng. 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 fold.4 In the United States, new record high temperatures now regularly outnumber new record lows by a ra:o 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
"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.
Summer in March. Unusual temperatures, March 13-‐19, 2012 Credit: NASA
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 varia:on 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 ra:o of 2:1.16 NOAA’s Na:onal Clima:c 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
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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 rela:vely 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 flaOens the curve, providing for a greater spread of events, an increase in varia:on. The combina:on provides for a drama:c 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 nega:ve temperature anomaly means they are cooler. Thus, the graph illustrates both the shic and flaOening 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
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1 NCDC, 20122 Trenberth et al 20073 Gutowski et al. 2008, Trenberth et al 20074 Hansen et al 20125 Meehl et al. 20096 Gutowski et al. 2008, StoO et al. 2010, Chris:dis et al. 2011, Seneviratne et al 2012, Hansen et al 20127 Hoerling et al. 20078 Duffy and Tebaldi 20129 Gutowski et al. 2008, StoO et al. 2010, Chris:dis et al. 2011, Seneviratne et al 2012, Hansen et al 201210 Chris:dis et al. 2009, and StoO et al. 201011 Trenberth et al 200712 Gutowski et al. 2008, Trenberth et al 200713 Zwiers et al. 201014 Hansen et al 201215 Gutowski et al. 2008, StoO et al. 2010, Chris:dis et al. 2011, Seneviratne et al 2012, Hansen et al 201216 Meehl et al. 200917 Hoerling et al. 200718 Wergen and Krug 201019 Rahmstorf and Coumou, 201220 Gutowski et al. 200821 Rahmstorf and Coumou, 201222 Hansen et al 2012
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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. Global warming is both drying out land and re-‐working regional weather paOerns to move rain even farther away from the dry areas of the subtropical belt.2
Rain is also becoming increasingly concentrated in heavy downpours due to global warming, even in regions experiencing less precipita:on overall. That causes greater runoff and reduced soil moisture, which contributes to agricultural drought.3
The historic Texas drought drama:cally illustrates drying driven by global warming and has incurred to date nearly $8 billion in agricultural losses alone.4 The role of global warming in driving record temperatures that helped to dry out the state has been fingerprinted by the Texas State Climatologist.5 And a study by scien:sts with NASA found that the extreme temperatures were so unusual that they would not have occurred in the absence of global warming.6
Rising temperatures have also led to earlier mel:ng of the snowpack in the western United States, more than 20 days earlier in some loca:ons.7 Early snow melt, along with an increased
Source: Texas State Climatologist
12
tendency for precipita:on to fall as rain rather than snow, is a driver of drought in regions that count on snowpack to supply water.
Elsewhere in the U.S. drama:c swings between drought and flooding in the Southeast have been linked to changes in the North Atlan:c Subtropical High driven by global warming.8
Looking Ahead
The Na:onal Center for Atmospheric Research (NCAR) has found that within 20 to 30 years areas in the U.S. will face unprecedented drought at levels far beyond the worst of the Dust Bowl in the 1930s if carbon pollu:on con:nues at only a moderate pace.9 NCAR projected that drought levels in the U.S., as measured by the commonly used Palmer Drought Severity Index, could reach index readings of -‐4 to -‐8, while readings in the Great Plains during the Dust Bowl rarely exceeded -‐3.10
Drought and Climate Change: The Science
A drought is a period of unusually persistent dry weather that lasts long enough to cause serious problems such as crop damage and/or water supply shortages. The severity of the drought depends upon the degree of moisture deficiency, the dura:on, and the size of the affected area.11
Drought can be defined in several ways: a meteorological drought is caused by below normal precipita:on, a hydrological drought occurs when surface and subsurface water supplies are below normal, an agricultural drought refers to a situa:on in which the amount of moisture in the soil no longer meets the needs of a par:cular crop, and a socioeconomic drought refers to
Source: NCAR/Dai et al. 2011
13
the situa:on that occurs when physical water shortages begin to affect people. All types of droughts are expected to increase in frequency and severity as the climate warms.
Very dry areas across the globe have doubled in extent since the 1970s.12 This global trend has been linked directly to climate change. Global warming is both drying out land and re-‐working regional weather paOerns to move rain even farther away from the dry areas of the subtropical belt.13 In addi:on, rain is becoming increasingly concentrated in heavy downpours due to global warming, even in regions experiencing less precipita:on overall, contribu:ng to agricultural drought through greater runoff and reduced soil moisture.14
Precipita:on has increased in many regions of the world and decreased in others, with liOle or no net change in the total amount of precipita:on. Generally, over the past 40 years wet areas have become weOer, and dry areas have become drier. Rapid warming since the late 1970s has both evaporated large amounts of moisture from the land into the atmosphere and altered atmospheric circula:on paOerns, contribu:ng to the drying over land.15 In par:cular, a long-‐term drying trend (from 1900 to 2008) persists in Africa, East and South Asia, eastern Australia, southern Europe, northern South America, most of Alaska, and western Canada.16 Precipita:on decreases have been observed in the subtropics and the tropics outside the monsoon trough, namely the Sahel in sub-‐Saharan Africa, the Mediterranean, Southern Africa, and Southern Asia.17
14
Some areas have experienced widening swings between the two precipita:on extremes.18 For instance, the summer of 2002 in Europe brought widespread floods, but was followed a year later in 2003 by record-‐breaking heat waves and drought. In the summer of 2007, widespread flooding in central England (the weOest since records began in 1766) was accompanied by drought and record-‐breaking heat waves in southeast Europe.19
Apart from changes in precipita:on, earlier snow melt and increased evapora:on from soil and vegeta:on due to higher atmospheric temperatures also help to drive drought. Both of these factors are also worsened by climate change. For major droughts that last a month or longer, the absence of moisture means the loss of evapora:ve cooling and all hea:ng goes directly into raising temperatures, that in turn desiccates plants, and promotes heat waves and wild fires. This vicious circle creates a cumula:ve effect that works to intensify and prolong droughts. Because the number of heat waves worldwide has increased since 1950, and heat waves have become longer, it is more likely that droughts will last longer and become more severe due to increased heat.20
Rising temperatures have led to earlier mel:ng of the snowpack in the western United States, more than 20 days earlier in some loca:ons.21 Early snow melt, along with an increased tendency for precipita:on to fall as rain rather than snow, can be an important contributor to drought in regions that count on snowpack to supply water, such as the western U.S. and Canada. A recent study of water cycle changes in the western U.S. aOributes to human influence up to 60% of observed climate-‐related trends in river flow, winter air temperature, and snow pack in the region over the 1950–1999 period.22
Change in Snow Pack Melt. Source: United States Global Change Research Program.
15
The global increase in drier, hoOer areas and the trend in which dry areas are becoming drier can both be traced to the human influence.23 Drying trends have been observed in both the Northern and Southern hemispheres since the 1950s.24 These trends cannot be explained by natural varia:ons but do fit well with computer models of the climate when global warming is added to these models.25 In par:cular, greenhouse gas emissions have contributed significantly to recent drying by driving warming over land and ocean.26
Individual droughts have been linked to climate change, such as the drought that hit central India in 2008 when the north-‐south paOern of precipita:on was disrupted by unusual weather driven by abnormally high sea surface temperatures due in part to global warming.27 The role of global warming in helping to drive the Texas drought of 2011/2012 has also been fingerprinted.28
Local weather is ocen determined by fluctua:ons in large paOerns of regional atmospheric pressure and sea surface temperatures, such as the Arc:c and North Atlan:c Oscilla:ons and the El Niño-‐Southern Oscilla:on (ENSO). Global warming may alter these recognizable paOerns, which occur over a period of months to years. For example, the drama:c swings between drought and flooding in the southeastern United States during the early 2000s have been linked to changes in the North Atlan:c Subtropical High, and these changes have been linked to global warming.29
Looking Ahead
The Na:onal Center for Atmospheric Research has found that within 20 to 30 years areas in the U.S. will face unprecedented drought at levels far beyond the worst of the Dust Bowl in the 1930s if carbon pollu:on con:nues at only a moderate pace.30 NCAR projected that drought levels in the U.S., as measured by the commonly used Palmer Drought Severity Index, could reach index readings of -‐4 to -‐8, while readings in the Great Plains during the Dust Bowl rarely exceeded -‐3.31
16
1 Dai 20112 Dai 2011, Trenberth 20113 Dai 20114 USA Today, March 22, 20125 Nielsen-‐Gammon 20116 Hansen et al 20127 Karl et al. 20098 Li et al. 20109 Dai 201110 Guan 200511 NOAA Drought FAQ12 Dai 201113 Dai 2011; Trenberth 201114 Dai 201115 Trenberth 2011; Durack 201216 Dai 201117 Trenberth et al. 2007; Trenberth 201118 Trenberth et al. 200719 Trenberth 201120 Trenberth et al. 200721 Karl et al. 200922 StoO el al. 201023 StoO 201024 Gutowski et al. 200825 Gutowski et al. 200826 Dai 201127 Rao et al. 201028 Hansen et al. 2012; Nielsen-‐Gammon 201129 Li et al. 201030 Dai 201131 Guan 2005
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Rain and SnowOne of the clearest changes in the weather across the U.S. is the increasing frequency and intensity of heavy rain and snow.1 In the Northeast, especially, the amount of precipita:on falling in the heaviest 1% of events has increased 67% over the last 50 years.2
As the atmosphere warms it holds more moisture. So when it rains, it pours out of the sky as if emptying out of a larger bucket. Snowfall, too, is heavier as a result.3
Storms supplied with increasing moisture are widely observed to be producing heavier rain and snow.4 The Na:onal Oceanic and Atmospheric Administra:on (NOAA) reports that the record-‐breaking rainfall dumped by Hurricane Irene was the main impact of the storm in the United States, where flooding and other damage totaled over $15 billion.5
The fingerprint of global warming has been firmly documented in the shic toward extreme precipita:on observed in the northern hemisphere.6 And the regional trend here in the U.S. follows the larger trend; we have witnessed a 20% increase in the amount of precipita:on falling in the heaviest downpours over the past century.7
In addi:on to concentra:ng rain and snowfall into heavier events, climate change also has drama:cally reworked the paOern of wet and dry areas around the world. Dry areas are becoming drier and wet areas weOer.8 Mid-‐la:tude areas, such as the U.S. Midwest and Northeast, have experienced an increase in total precipita:on, while sub-‐tropical areas, such as the U.S. Southeast and Southwest, have experienced a sharp decrease.9 As a result the risk of both drought and flooding is increasing with global warming.10
Looking Ahead
If global warming con:nues, the intensity of the heaviest rain and snow in the United States is expected to increase even further, by another 40% over the coming years.11
Flooding: Hurricane Irene. Credit: The Wilmington.
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Rain, Snow and Climate Change: The Science
The water holding capacity of the atmosphere increases in a warmer world. And a 4% increase in atmospheric moisture has been observed, consistent with a warming climate.12
Storms reach out to gather water vapor over regions that are 10 to 25 :mes as large as the precipita:on area, mul:plying the effect of increased atmospheric moisture. As water vapor condenses to form clouds and rain, it releases heat energy that adds buoyancy to the air and fuels the storm. This increases the gathering of moisture into storm clouds and further intensifies precipita:on. As a result, storms are producing more intense precipita:on, both rain and snow.13
The increased moisture in the atmosphere is driving the shic to heavier but less frequent rains — “when it rains, it pours.” While an atmosphere that holds more moisture has greater poten:al to produce heavier precipita:on, precipita:on events also become less frequent, as it takes longer to recharge the atmosphere with moisture.14 By analogy, a larger bucket holds and dumps more water, but takes longer to refill. Even in areas where the total precipita:on has decreased, increases in heavy precipita:on have been observed.15
Total precipita:on has increased in many regions of the world and decreased in others, with liOle or no net change in the total amount of global precipita:on. Drought has increased, consistent with expecta:ons for a warming climate. Generally, wet areas have become weOer, and dry areas have become drier in the past 40 years.16
Increasing Heavy Rain and Snow. Source: Global Climate Change Impacts in the United States. U.S. Global Climate Change Research Program.
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Extreme PrecipitaFon Trend, USA. Source: NaFonal ClimaFc Data Center
Increasing 90-‐day Extreme PrecipitaFon. Source: Weather and Climate Extremes in a Changing Climate, U.S. Climate Change Science Program
20
Contiguous U.S. Extremes in 1-Day Precipitation (Step 4*) Annual (January-December) 1910-2011
The higher la:tudes have become weOer in recent years, due mainly to the warmer air holding more moisture and in part to altera:ons in atmospheric circula:on driven by climate change. The subtropics and parts of the tropics have become drier as winds carry the moisture away to
the monsoon rain areas or to mid-‐la:tude storms.17
Some areas have experienced widening swings between the two precipita:on extremes.18 For instance, the summer of 2002 in Europe brought widespread floods, but was followed a year later in 2003 by record-‐breaking heat waves and drought. In the summer of 2007, widespread flooding in central England (the weOest since records began in
1766) was accompanied by drought and record-‐breaking heat waves in southeast Europe.19
In the more northern regions, more precipita:on falls as rain rather than snow. The liquid-‐precipita:on season has become longer by up to three weeks in some regions of the boreal high la:tudes over the last 50 years owing, in par:cular, to an earlier onset of spring.20
Natural variability cannot explain the observed changes in the intensity or geographic distribu:on of precipita:on. The observed changes follow from basic physical principles of global warming and are consistent with a combina:on of natural factors and human influence.21 Human-‐induced increases in greenhouse gases have contributed to the observed intensifica:on of heavy precipita:on events found over approximately two-‐thirds of Northern Hemisphere land areas.22
In the United States, total average precipita:on has increased by about 7% in the past century, while the amount of precipita:on falling in the heaviest 1% of rain events has increased 20%. Regions such as the Northeast and Midwest have seen considerably larger increases in the heaviest rains.23
Drought Wet Extremes Trend, USA. Source: NaFonal ClimaFc Data Center
21
1 Karl et al. 20092 Karl et al. 20093 Trenberth et al. 2007; Trenberth 2011; Seneviratne et al. 20124 Trenberth 2011 5 Lixion and Cangialosi 2011.6 StoO et al. 2010, Minn et al. 2011, Seneviratne et al. 20127 Karl et al. 20098 Trenberth 2011, Seneviratne et al. 20129 Trenberth 201110 Trenberth 2011, Seneviratne et al. 201211 Karl et al. 200912 Trenberth et al. 200713 Trenberth 201114 Trenberth 201115 Trenberth 201116 Trenberth et al. 2007, Trenberth 201117 Trenberth 201118 Trenberth et. al. 200719 Trenberth 201120 Trenberth et al. 200721 Trenberth et al. 2007, Trenberth 201122 Min, et al. 2011, Seneviratne et al. 201223 Karl et al. 2009
22
Flooding
Globally, an increase in heavy precipita:on has contributed to flooding, a paOern that has been observed around the world.1
Several catastrophic floods have hit the U.S. in recent years, including the record-‐breaking flood in Nashville, Tennessee, in 2010 and the devasta:ng floods spawned by Hurricane Irene in Connec:cut, New York, Vermont and elsewhere in 2011. The Nashville deluge was off the charts, described by the Army Corp of Engineers as a “thousand-‐year flood.”2 The two-‐day rainfall total at Nashville Interna:onal Airport exceeded the monthly rainfall record for that en:re month. The heaviest rainfall occurred in a swath across several coun:es where the equivalent of 420 billion gallons of water fell in just two days.3
Flooding occurs for a host of reasons, many of which can be aOributed to human influence and ac:vity. Deforesta:on, changes in land use, and heavy precipita:on events linked to a changing climate are all causes of exacerbated flooding around the world.4
Flooding in the District in Nashville, TN. May 3, 2010. Photo credit: Stephen Yeargin.
As the atmosphere warms, it holds more moisture. So when it rains, it pours out of the sky as if emptying out of a larger bucket. Heavy downpours ocen saturate drainage systems and excess water cannot be absorbed, promp:ng an increase in runoff and therefore, flooding.5 Regional shics in precipita:on can also increase the risk of flooding by raising water table levels, as has been seen in the northeastern United States.6
The fingerprint of global warming has been firmly documented in the shic toward extreme precipita:on observed in the Northern Hemisphere.7 And the regional trend here in the U.S. follows the larger trend; we have witnessed a 20% increase in the amount of precipita:on falling in the heaviest downpours over the past century.8
Flooding in large river basins, such as the Mississippi, are caused by extreme precipita:on events persis:ng for weeks or even months. Record-‐breaking Mississippi flooding occurred in 2011 in associa:on with very heavy sustained rains and was followed by extensive flooding in the Missouri River basin due to heavy rain and snowmelt. Such long-‐term heavy precipita:on events are becoming more common. In the U.S., 90-‐day periods of heavy rainfall were 20% more common from 1981 to 2005 than in any 25-‐year period on record.9 The long periods of sustained rain in the upper Midwest are also consistent with the shic of the mid-‐la:tude rain belt, pushed northward by changes in atmospheric circula:on driven by global warming.10
Similar to the Mississippi and Missouri River basin flooding events, the record floods in Nashville (2010), Pakistan (2010), Australia (2010), and Vermont (2011) were all consistent with human-‐influenced changes in global precipita:on paOerns and trends.11
Flooding and Climate Change: The Science
Heavy precipita:on contributes to increased flooding, a paOern that has been observed around the world,12 and the frequency of great floods (100-‐year floods in large basins) has increased over the course of the 20th century.13
A 4% increase in atmospheric moisture has been observed, consistent with a warming climate.14 The increased moisture in the atmosphere is driving the shic to heavier but less frequent rains — “when it rains, it pours.” In turn, this increases the risk of flooding.15
Storms reach out to gather water vapor over regions that are 10 to 25 :mes as large as the precipita:on area, mul:plying the effect of increased atmospheric moisture. As water vapor condenses to form clouds and rain, it releases heat energy that adds buoyancy to the air and fuels the storm. This increases the gathering of moisture into storm clouds and further intensifies precipita:on.16
While an atmosphere that holds more moisture has greater poten:al to produce heavier precipita:on, precipita:on events also become less frequent, as it takes longer to recharge the atmosphere with moisture.17 By analogy, a larger bucket holds and dumps more water, but takes longer to refill. Even in areas where the total precipita:on has decreased, increases in heavy precipita:on have been observed.18
Natural variability cannot explain the observed changes in the intensity or geographic distribu:on of precipita:on. The observed changes follow from basic physical principles of global warming and are consistent with a combina:on of natural factors and human influence.19 Human-‐induced increases in greenhouse gases have contributed to the observed intensifica:on of heavy precipita:on events found over approximately two-‐thirds of Northern Hemisphere land areas.20
Steady moderate rains soak into the soil, while the same rainfall amounts in a short period of :me may cause local flooding and runoff. Runoff, or the surface water lec over when the land cannot soak up any more, has also increased in many parts of the world, consistent with changes in precipita:on. Regional shics in precipita:on can also increase the risk of flooding by raising water table levels, as has been seen in the northeastern United States.21
Source: NRDC
The warming climate is increasing the risks of both flood and drought, but at different :mes or in different places.22 For instance, the summer of 2002 in Europe brought widespread floods, but was followed a year later in 2003 by record-‐breaking heat waves and drought. Similarly, the summer of 2007 in England saw widespread flooding while southeast Europe experienced record-‐breaking heat.23 Because more precipita:on occurs as rain instead of snow with warming, and snow melts earlier, there is increased runoff and risk of flooding in early spring, but increased risk of drought in deep summer, especially over con:nental areas.24
Flooding in large river basins is ocen aOributed to extreme precipita:on events sustained for weeks or even months. In spring, heavy rains on top of snow can contribute to flooding in northern regions. In the more northern regions, more precipita:on falls as rain rather than snow. The liquid-‐precipita:on season has become longer by up to 3 weeks in some regions of the boreal high la:tudes over the last 50 years owing, in par:cular, to an earlier onset of spring.25
Long-‐term, heavy precipita:on episodes are becoming more common in some areas. In the U.S., 90-‐day periods of heavy rainfall were 20% more common from 1981 to 2005 than in any earlier 25-‐year period on record. 26 The long periods of sustained rain in the upper Midwest in 2011, for example, are also consistent with the shic of the mid-‐la:tude rain belt, pushed northward by changes in atmospheric circula:on driven by global warming.27
1 Parry et al. 20072 KNOX News, May 6, 2010. 3 NOAA, 2010 “Epic Flood Event for Western and Middle Tennessee”4 Trenberth 20115 Trenberth 20116 Weidner and BouO, 20107 StoO et al. 2010, Minn et al. 2011, Seneviratne et al. 20128 Karl et al. 20099 Kunkel et al. 200810 Trenberth 201111 Ash 2011, Asrar 2010, and Trenberth 201012 Parry et al. 200713 Milly 200214 Trenberth et al. 200715 Trenberth 201116 Trenberth 201117 Trenberth 201118 Trenberth 201119 Trenberth et al. 2007, Trenberth 201120 Min, et al. 2011, Seneviratne et al. 201221 Weider and BouO 201022 Trenberth 201123 Trenberth 201124 Trenberth 201125 Trenberth et al. 200726 Kunkel el al. 200827 Trenberth 2011
Tornadoes
Tornado ac:vity in the U.S. has spiked in recent years, sparking a debate about the connec:on to climate change.1
The 2011 Dixie outbreak produced the largest swarm of tornadoes on record (175) and ranked as the deadliest outbreak of the modern era.2 Seven tornado outbreaks in 2011 each incurred over a billion dollars in damages for a total of $28.7 billion.3
Early-‐season tornado ac:vity in 2012 ran well ahead of average.4 Last year (2011) was the second most ac:ve year on record, while 2004 ranked as the all-‐:me most ac:ve year.5
Meteorologists have noted that in recent years tornadoes have appeared well north of their usual zones and have also been unusually intense early in the calendar year.6 "This year's early start to tornado season is consistent with what we would expect from a warming climate," notes Dr. Jeff Masters.7
This year Nebraska reported its first ever tornadoes in February.8 The powerful twister that hit Dexter, Michigan, was the earliest EF3 tornado in the northern state’s history,9 and in 2007 Canada recorded its first F5 tornado, the most powerful under the Fujita scale, ever.
February of 2012 was the fich most ac:ve February in the modern record, while February of 2008 was the most ac:ve and February of 2010 the fourth most ac:ve.10
Tornado Damage, Birmingham, AL, April 27, 2011. Image Credit: Mark Schnackenberg
28
However, extremely uneven records from prior decades make it difficult to draw any conclusions about long-‐term trends in tornado ac:vity. The Na:onal Science and Technology Council notes that trends “cannot be determined at the present Fme due to insufficient evidence."11
Is global warming influencing tornadoes? According to the Na:onal Oceanic and Atmospheric Administra:on (NOAA), the best answer is: “We don't know.”12
However some scien:sts are poin:ng out that the recent spike in tornado forma:on is consistent with the warmer, weOer world brought forward by climate change.13
Looking Ahead
The most recent study on tornadoes and climate change, published in Natural Hazards, found that F2 and stronger US tornado days – a day with at least one recorded F2 tornado -‐-‐ will increase under global warming and that majority of this increase is likely to be manifested in the earlier part of the tornado season.14
Tornadoes and Climate Change: The Science
Tornado ac:vity in the U.S. has spiked in recent years. Last year (2011) was the second most ac:ve year on record, while 2004 ranked as the all-‐:me most ac:ve year.15
Meteorologists have also noted that tornadoes in recent years have appeared well north of usual and have been unusually intense early in the calendar year.16
February of 2012 was the fich most ac:ve February in the modern record while February of 2008 was the most ac:ve and February of 2010 the fourth most ac:ve.17 The five largest early-‐season two-‐day outbreaks have all occurred since 1997, and three of the top five outbreaks occurred in the last four years.18
According to some climate scien:sts, such earlier-‐than-‐normal outbreaks of tornadoes, which typically peak in the spring, will become the norm as the planet warms. "As spring moves up a week or two, tornado season will start in February instead of waiFng for April," reports climatologist Kevin Trenberth of the Na:onal Center for Atmospheric Research.19
Image Credit: NOAA
29
Is global warming currently influencing tornadoes? According to NOAA, the best answer is: “We don't know.”20
Changes in observa:on techniques and extremely uneven record keeping in prior decades makes it difficult to draw conclusions about the current long-‐term trends in tornado ac:vity. The Na:onal Science and Technology Council notes that trends “cannot be determined at the present Fme due to insufficient evidence."21 There is low confidence in observed trends because of apples-‐to-‐oranges comparisons in the data and inadequacies in monitoring systems.22 The tornado record in the U.S. displays an increasing trend that mainly reflects increased popula:on density and increased numbers of people in remote areas. Such trends increase the likelihood that tornadoes are observed and reported.23
While we cannot say for certain that global warming is helping to fuel early-‐season outbreaks of tornadoes, we can see how the recent spike in tornado forma:on is consistent with the warmer, weOer world that climate change has already brought. Trenberth notes:
“Tornadoes come from thunderstorms in a wind shear environment. This occurs east of the Rockies more than anywhere else in the world. The wind shear is from southerly flow from the Gulf overlaid by westerlies alo[ that have come over the Rockies. That wind shear can be converted to rotaFon. The basic driver of thunderstorms is the instability in the atmosphere: warm moist air at low levels with drier air alo[. With global warming the low level air is warm and moister and there is more energy available to fuel all of these storms and increase the buoyancy of the air so that thunderstorms are strong. There is no clear research on changes in
30
shear related to global warming. On average the low level air is 1 deg F and 4 percent moister than in the 1970s.”24
Looking Ahead
There is low confidence in projec:ons of changes in tornadoes because of limited studies and the inability of climate models to simulate tornadoes. In addi:on, it is not known which of the different factors that control tornado forma:on will predominate in the future.25
However, while climate models cannot simulate tornadoes or individual thunderstorms, they can indicate broad-‐scale shics in three of the four favorable ingredients for severe thunderstorms (moisture, atmospheric instability and wind shear).26 Con:nued growth in atmospheric greenhouse gas concentra:ons may cause some of the atmospheric condi:ons conducive to tornadoes (moisture and atmospheric instability) to increase even further due to rising temperature and humidity, while others such as ver:cal shear may decrease due to a reduced pole-‐to-‐equator temperature gradient.27 The other key ingredient (storm-‐scale lic) depends mostly on day-‐to-‐day paOerns, and ocen, even minute-‐to-‐minute local weather.28However, over most of the United States, the increase in the power of thunderstorms is expected to more than compensate for the rela:ve decreases in shear.29 Moreover, while shear may decrease, it is expected to ocen remain above the threshold cri:cal for tornado forma:on.30 As a result, the environment would s:ll be considered favorable for severe convec:on of the kind that creates tornadoes.31 The number of days when condi:ons exist to form tornadoes is expected to increase as the world warms. Regions near the Gulf of Mexico and Atlan:c coasts not normally associated with tornadoes will experience tornado-‐making weather more frequently. For instance, a doubling in the number of days with such condi:ons in Atlanta and New York City is projected.32
The most recent study on tornadoes and climate change, published in Natural Hazards, finds that F2 and stronger US tornado days – a day when at least one F2 tornado is recorded -‐-‐ will increase under global warming and that the majority of this rise is likely to be manifested in the earlier part of the tornado season.33
31
1 Romm 20122 NOAA Tornado FAQ 3 NCDC Billion Dollar U.S. Weather/Climate Disasters4 NOAA Storm Predic:on Center5 NOAA Storm Predic:on Center6 Ostro 20117 USA Today, March 5, 20128 WWOT News, February 29, 20129 Masters 201210 NOAA Na:onal Clima:c Data Center Tornado Counts11 Na:onal Science and Technology Council, 200812 NOAA Tornado FAQ13 Romm 201214 Cameron 201115 NOAA Storm Predic:on Center16 Ostro 201117 NCDC U.S. February Tornadoes18 Masters 201219 Trenberth 2012a20 NOAA Tornado FAQ 21 Na:onal Science and Technology Council 200822 Seneviratne et al. 201223 Trenberth et al., 2007; Kunkel et al., 200824 Trenberth 2011a25 Seneviratne et al. 201226 NOAA Fact Sheet27 Seneviratne 201228 NOAA Fact Sheet29 Trapp 200730 Trenberth 2012a31 Trapp 200732 Trapp 200733 Lee 2011
32
Hurricanes
Global warming is already affec:ng hurricanes, loading them with addi:onal moisture, making for more intense rainfall.1 Hurricanes Katrina and Ivan, for example, carried significant increases in rainfall due to climate warming, and in the case of Katrina the increase may have contributed to the breach of the levees in New Orleans.2
NOAA reports that the record-‐breaking rainfall dumped by Hurricane Irene was the main impact of the storm in the United States, where flooding and other damage totaled over $15 billion.3
Substan:al evidence also indicates that global warming may be responsible for the recent increasing intensity of Atlan:c hurricanes,4 increasing their size5 and contribu:ng to a lengthening hurricane season.6 Out of the 11 most intense North Atlan:c hurricanes ever recorded, five have occurred in the last eight years (Wilma, Rita, Katrina, Dean and Ivan).7 However, the incomplete historical record makes it difficult to confidently assess the nature of these trends.8
Beyond these changes, hurricanes storm surges now ride higher upon coastal seas that have risen over the last century due to global warming, amplifying losses where the surge strikes.9
Looking Ahead
There is a strong scien:fic consensus that the most intense Atlan:c hurricanes will become more frequent in the coming decades if carbon pollu:on con:nues to grow at a moderate rate.10 The increase in damages to due climate change will rise to an average of over $40 billion per year, as stronger hurricanes are exponen:ally more destruc:ve than weaker storms.11
Hurricane Irene. Image Credit: NASA
33
Observed Fme series of Cat 4-‐5 hurricane counts from the 1944 through the 2008 hurricane seasons as contained in the AtlanFc HURDAT database (S6). Source: Bender et al. 2010
Source: Bender et al. 2010
34
1 Trenberth 20112 Trenberth, Davis, and Fasullo 20073 Lixion and Cangialosi 20114 Karl et al. 2009; Knutsen et al. 2010; Evan 20125 Trenberth, Davis, and Fasullo 20076 Kossin 20087 Na:onal Hurricane Center 20128 Knutsen et al. 20109 Hoffman et al. 201010 Knutsen et al. 201011 Mendelsohn et al. 2012
35
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