I want to continue in the personal and spontaneous spirit of the last blog. I heard a lecture recently talking about climate change and farming. The speaker made the comment that the climate was changing fast enough that a family farmer could not count on the weather being the same as his father’s.
Many of the discussions I have heard about farming and climate change start with a discussion of drought and that we expect more frequent and more severe droughts in the future. Flood is also mentioned, but anecdotally at least, we think of flood as more localized than drought. We also hear about warmer and earlier springs and, hence, longer growing seasons. This potential opportunity is muted by concerns that even if there is more precipitation that a warmer climate will cause more water stress for crops. In general, the farmer will have to manage more variable and more extreme weather.
We are already in a time of rapidly changing climate. The first decade of this century was the warmest recorded, and it has been many years since the monthly average of the Earth’s surface was cooler than the 20th century average. For the northern hemisphere, this warming has led to a lengthening of the growing season, as defined by frost-free days. Farmers have already adapted by planting earlier with seed developed to take advantage of these changes or to survive despite them. The last thirty years have also been a time when the rhythm of precipitation has changed. We see more precipitation in intense storms and changes in the seasonal cycle of the availability of fresh water.
I was recently on a telecon with some scientists from the Department of Agriculture. I learned that in recent years, heavy spring rains had been inhibiting spring planting. There have been problems with getting heavy equipment into the field. The amount of time when the soil moisture is right for both holding up the equipment and providing a good seedbed is becoming shorter (news link). The likelihood of seedlings being washed out by intense rains is increasing. Curiously to me, one response to this has been to build still bigger equipment so that more can be planted in the shorter amount of time that is available.
What I described in the previous paragraph is not something that is projected for the future; it is already happening (Impacts of Climate Change on Illinois Agriculture). Farmers and manufacturers see what is happening, and they adapt. This adaptation to perceived changes is real, costly and much more concrete than the abstract threats of more drought and more flood. Another real issue that we already respond to is the warm spell in spring that causes budburst of orchards, followed by a freeze that wipes out a crop.
Events such as the wet spring, budburst and crop loss, flooding out a crop are not new to farmers. What is new is how often such events are happening. It is also new that the places where the events are occurring are changing.
I grew up in the South of the United States, which is a four-season climate. I remember throughout my childhood peach crops that were wiped out by a late frost. In fact, almost every year there was concern in some part of the South of a swath of peaches being wiped out. And that is an interesting fact of climate variability and farming: There is almost always weather-related damage some place. In a country and rich as the United States, other regions of plenty mostly balance out these places of loss. It is this balance of agricultural plenty and loss that leads some to say that when viewed as a global or national market, agriculture is resilient to climate change.
If this collective agriculture is, in fact, resilient to climate change, this assumes either 1) the future climate is, on average, like our father’s climate or 2) we effectively adapt to climate as it changes. A confidence in agricultural resilience assumes that what resilience we have built in the past transfers into the future. Even if agriculture is collectively resilient, locally there is boom and bust.
In the South, precipitation is spread out across all of the seasons. Irrigated farming is the exception, not the rule. Southerners do not worry about water being stored in snow and dribbling out to use as it melted in the spring and the summer. As I have grown older and traveled and moved, I found out that much of the world does not have four seasons with rain spread throughout the year. Much of the world has a wet season and a dry season. Many parts of the world rely on water being stored as snow on high mountains, lasting into spring and melting to be used for agriculture in the warm season.
Scientists call being able to rely on having our father’s climate “stationarity.” If the climate were stationary, then in the future the averages and the extremes would be the same. To describe stationarity, scientists often use figures that describe the statistical distribution of “climate” or perhaps more correctly of temperature and precipitation. We talk about the average temperature increasing. We talk about average precipitation increasing or decreasing, depending on the region. We often talk about the “extremes,” especially extremely hot temperatures increasing. Precipitation extremes might increase either as prolonged drought or as intense rain and snowstorms. The changes in the statistical distribution of parameters that measure climate describe the lack of stationarity.
The normal ways that we talk about extremes do not always convey the way we are feeling climate change. The seasonality, the rhythm, the ebb and flow this is changing and felt in those muddy fields that preclude farm equipment and endanger planting. The change in seasonality is felt in intense winter snowstorms, followed by winter rains and early spring causing water to run through the ditches, rivers and reservoirs and to be unavailable for summer growing. The changes in seasonality are felt in an increasing number of early budbursts followed by the killing frost. This change in seasonality is as much a change in stationarity as any change in the average and mean temperature. In fact, the change of the rhythm of seasons can occur with very little change to the statistical description of averages and extremes. It might not even seem hotter.
How to cope with a climate that is not stationary is a major challenge for agriculture (and engineering). Deep within our planning for the future is the assumption that weather will remain the same – it will be like our father’s and mother’s weather. This is no longer the case.
A historic multi-billion dollar flood disaster has killed at least eighteen people in Central Europe after record flooding unprecedented since the Middle Ages hit major rivers in Austria, the Czech Republic, Germany, Poland and Slovakia over the past two weeks. The Danube River in Passau, Germany hit the highest level since 1501, and the Saale River in Halle, Germany was the highest in its 400-year period of record. Numerous cities recorded their highest flood waters in more than a century, although in some locations the great flood of 2002 was higher. The Danube is expected to crest in Hungary's capital city of Budapest on June 10 at the highest flood level on record, 35 cm higher than the record set in 2006. The flooding was caused by torrential rains that fell on already wet soils. In a 2-day period from May 30 - June 1, portions of Austria received the amount of rain that normally falls in two-and-half months: 150 to 200 mm (5.9 to 7.9"), with isolated regions experiencing 250 mm (9.8"). This two-day rain event had a greater than 1-in-100 year recurrence interval, according to the Austrian Meteorological Agency, ZAMG. Prior to the late May rains, Austria had its seventh wettest spring in 150 years, which had resulted in the ground in the region becoming saturated, leading to greater runoff when the rains began.
Figure 1. Aerial view of the flooded Danube River in Deggendorf, Germany on Friday, June 7, 2013. (AP Photo/Armin Wegel)
Floods caused by a blocking high pressure system The primary cause of the torrential rains over Central Europe during late May and early June was large loop in the jet stream that developed over Europe and got stuck in place. A "blocking high" set up over Northern Europe, forcing two low pressure systems, "Frederik" and "Günther", to avoid Northern Europe and instead track over Central Europe. The extreme kink in the jet stream ushered in a strong southerly flow of moisture-laden air from the Mediterranean Sea over Central Europe, which met up with colder air flowing from the north due to the stuck jet stream pattern, allowing "Frederik" and "Günther" to dump 1-in-100 year rains. The stuck jet stream pattern also caused record May heat in northern Finland and surrounding regions of Russia and Sweden, where temperatures averaged an astonishing 12°C (21°F) above average for a week at the end of May. All-time May heat records--as high as 87°F--were set at stations north of the Arctic Circle in Finland.
Figure 3. Nine-day rainfall amounts in portions of Southern Germany and Western Austria exceeded 12" (305 mm.) Image credit: ZAMG.
If it seems like getting two 1-in-100 to 1-in-500 year floods in eleven years is a bit suspicious--well, it is. Those recurrence intervals are based on weather statistics from Earth's former climate. We are now in a new climate regime with more heat and moisture in the atmosphere, combined with altered jet stream patterns, which makes major flooding disasters more likely in certain parts of the world, like Central Europe. As I discussed in a March 2013 post, "Are atmospheric flow patterns favorable for summer extreme weather increasing?", research published this year by scientists at the Potsdam Institute for Climate Impact Research (PIK) in German found that extreme summertime jet stream patterns had become twice as common during 2001 - 2012 compared to the previous 22 years. One of these extreme patterns occurred in August 2002, during Central Europe's last 1-in-100 to 1-in-500 year flood. When the jet stream goes into one of these extreme configurations, it freezes in its tracks for weeks, resulting in an extended period of extreme heat or flooding, depending upon where the high-amplitude part of the jet stream lies. The scientists found that because human-caused global warming is causing the Arctic to heat up more than twice as rapidly as the rest of the planet, a unique resonance pattern capable of causing this behavior was resulting. According to German climate scientist Stefan Rahmstorf, "Planetary wave [jet stream] amplitudes have been very high in the last few weeks; we think this plays a role in the current German flooding event." More rains are in store for the flood area through Monday, then the blocking pattern responsible for the great 2013 Central European flood is expected to disintegrate, resulting in a return to more typical June weather for the next two weeks.
Figure 4. The northward wind speed (negative values, blue on the map, indicate southward flow) at an altitude of 300 mb in the mid-latitudes of the Northern Hemisphere during July 1980, July 2011, and the last twelve days of May 2013. July of 2011 featured an unusually intense and long-lasting heat wave in the U.S. (the 4th warmest month in U.S. history), and the normally weak and irregular waves (like observed during the relatively normal July of 1980) were replaced by a strong and regular wave pattern. Late May 2013 was also very extreme, resulting the great Central European floods of 2013. Image credit: Vladimir Petoukhov and Stefan Rahmstorf.
Links Stefan Rahmstorf's blog (translated from German) on the unusual jet stream patterns that caused the Central European floods of 2013.
NASA has high-resolution MODIS satellite images showing the flooding of the Elbe River in Germany.
Video 1. Climate, Ice, and Weather Whiplash: In this June 3, 2013 video by the Yale Climate Forum's Peter Sinclair, Rutgers' Jennifer Francis and Weather Underground's Jeff Masters explore the 'Why?' of two years of mirror images of weather across North America.
I'm in Granby, Colorado this week for the American Geophysical Union's Chapman Conference on Climate Change Communication. Many of the talks will be webcast live; you can see a list of the talks (times in MDT) here. My talk, "The Weather Underground Experience," is scheduled for Monday at 4:30 pm MDT. I'll give a 15-minute overview of the history of wunderground, and what I've learned about communicating weather and climate change information along the way. There is live tweeting going on from the conference, #climatechapman. My blog updates this week may be somewhat random as a result of the conference, but I'm not seeing anything in the tropics worthy of discussion at this point.
I have disappeared for a while because of technological failures. Honestly, I embraced them for a couple of days, but it's hard for me to remain in denial for more than a day or two. So I have sought out the computer at the public library and remembered my WU login. Here is a personal reflection on how local weather conditions might impact how one thinks about climate change.
I am currently residing in Boulder, Colo., where I try to grow a pretty large garden. Last year, 2012, was exceedingly hot in the spring and very dry. The dryness continued into the winter of 2013.
Water is in short supply in the West. This is not news. In fact, when John Wesley Powell explored the West he was pessimistic about its habitability because of scarcity of water (an old NPR story). He laid out a vision of a West of small settlements anchored in reliable water sources. Earlier, when Stephen Long explored the Midwest and the Front Range of the Rockies, he labeled the area the "Great Desert." (some cool maps from University of Tulsa).
Of course, the Great Plains and the West have now been populated with large cities. Water is managed in a fragmented way on an enormous spatial scale. There is huge contention for water between cities, agricultural and conservation management, and energy production. This is one area of the country where there is concern shared amongst the governors about drought and climate change.
In March 2013 as the local drought persisted, I was downright depressed about the coming spring and summer. The snowpack in the mountains was low. In the previous year, the spring had been so warm that much of the snow melted well before the normal spring runoff. I remember in June 2012 putting pumpkins into soil that was well over 110 degrees F and dry down to the underlying clay bed. With the low humidity and heat, I could not water most of them enough to keep them alive. In March 2013, we seemed to be looking at even less water.
Spring 2013 was just plain odd in the U.S. Largely, it was cold, with many record cold temperatures. The cold waves were interspersed with sometimes record heat. The variability was enormous. In my part of Colorado during April, at just about exactly seven-day intervals, there was one record snow a week. On the flat lands east of the mountains, these snows were followed by extraordinary seasonal cold, then a rapid melt. Virtually all blossoming trees did not blossom; the bees are not happy. In the mountains, the snowpack built up to be higher than average. Some ski resorts reopened for Memorial Day because of fresh May snow.
Here at the end of May, I look at the mountains and there is a lot of snow. The farm irrigation ditches run full of water. The cities are reconsidering the water restrictions they imposed in February and March. The hay fields are green and tall. I look around, and I feel pretty good about the summer.
I remember when I was quite young there was a drought in my home state of North Carolina. I was only a bit more naive then, perhaps more prone to the mystical, and I worried about the weather being broken in some way. At that age, weather was itself a mystery. I had no idea how to describe the motion of air and how to turn humidity into rain. I imagined that there had been a divine intervention into how the weather worked--it was the opposite of the biblical flood. I was a young boy with a narrow view of the world, so I assumed the whole world was in drought. I am sure that a few hundred miles away, however, the weather was still working; it was raining. I probably even checked to make sure that was the case. As I now sit in a world with what looks like enough snow for a good season in the garden, that childhood comfort of the weather working comes back.
This little vision I have into the world, that my weather has been beneficent, really has little relevance to whether or not the climate is changing. My little vision is no different than that of all of the people who have looked at the cold U.S. spring of 2013 and stated that as evidence or proof that the Earth was not warming. You have to look at all of the Earth and look at what is happening in the oceans and look at all that is melting.
Rather than looking out your window and saying that the weather is working and that our climate is like it has always been, better to take a broader look--a global perspective. For a national perspective on drought, here is the outlook from NIDIS on May 15, 2013.
Hope to get my computer and files back early next week. Don't forget me.
In 2011, a series of violent severe storms swept across the Plains and Southeast U.S., bringing an astonishing six billion-dollar disasters in a three-month period. The epic tornado onslaught killed 552 people, caused $25 billion in damage, and brought three of the five largest tornado outbreaks since record keeping began in 1950. In May 2011, the Joplin, Missouri tornado did $3 billion in damage--the most expensive tornado in world history--and killed 158 people, the largest death toll from a U.S. tornado since 1947. An astounding 1050 EF-1 and stronger tornadoes ripped though the U.S. for the one-year period ending that month. This was the greatest 12-month total for these stronger tornadoes in the historical record, and an event so rare that we might expect it to occur only once every 62,500 years. Fast forward now to May 2012 - April 2013. Top-ten coldest temperatures on record across the Midwest during March and April of 2013, coming after a summer of near-record heat and drought in 2012, brought about a remarkable reversal in our tornado tally--the lowest 12-month total of EF-1 and stronger tornadoes on record--just 197. This was an event so rare we might expect it to occur only once every 3,000 - 4,000 years. And now, in May 2013, after another shattering EF-5 tornado in Moore, Oklahoma, residents of the Midwest must be wondering, are we back to the 2011 pattern? Which of these extremes is climate change most likely to bring about? Is climate change already affecting these storms? These are hugely important questions, but ones we don't have good answers for. Climate change is significantly impacting the environment that storms form in, giving them more moisture and energy to draw upon, and altering large-scale jet stream patterns. We should expect that this will potentially cause major changes in tornadoes and severe thunderstorms. Unfortunately, tornadoes and severe thunderstorms are the extreme weather phenomena we have the least understanding on with respect to climate change. We don't have a good enough database to determine how tornadoes may have changed in recent decades, and our computer models are currently not able to tell us if tornadoes are more likely to increase or decrease in a future warmer climate.
Video 1. Remarkable video of the tornado that hit Tuscaloosa, Alabama on April 27, 2011, part of the largest and most expensive tornado outbreak in U.S. history--the $10.2 billion dollar Southeast U.S. Super Outbreak of April 25 - 28, 2011. With damage estimated at $2.2 billion, the Tuscaloosa tornado was the 2nd most expensive tornado in world history, behind the 2011 Joplin, Missouri tornado. Fast forward to minute four to see the worst of the storm.
Figure 1. Will climate change increase the incidence of these sorts of frightening radar images? Multiple hook echoes from at least ten supercell thunderstorms cover Mississippi, Alabama, and Tennessee in this radar image taken during the height of the April 27, 2011 Super Outbreak, the largest and most expensive tornado outbreak in U.S. history. A multi-hour animation is available here.
Changes in past tornado activity difficult to assess due to a poor database It's tough to tell if tornadoes may have changed due to a changing climate, since the tornado database is of poor quality for climate research. We cannot measure the wind speeds of a tornado directly, except in very rare cases when researchers happen to be present with sophisticated research equipment. A tornado has to run over a building and cause damage before an intensity rating can be assigned. The National Weather Service did not begin doing systematic tornado damage surveys until 1976, so all tornadoes from 1950 - 1975 were assigned a rating on the Fujita Scale (F-scale) based on old newspaper accounts and photos. An improved Enhanced Fujita (EF) scale to rate tornadoes was adopted in 2007. The transition to the new EF scale still allows valid comparisons of tornadoes rated, for example, EF-5 on the new scale and F5 on the old scale, but does create some problems for tornado researchers studying long-term changes in tornado activity. More problematic is the major changes in the Fujita-scale rating process that occurred in the mid-1970s (when damage surveys began), and again in 2001, when scientists began rating tornadoes lower because of engineering concerns and unintended consequences of National Weather Service policy changes. According to Brooks (2013), "Tornadoes in the early part of the official National Weather Service record (1950 - approximately 1975) are rated with higher ratings than the 1975 - 2000 period, which, in turn, had higher ratings than 2001 - 2007." All of these factors cause considerable uncertainty when attempting to assess if tornadoes are changing over time. At a first glance, it appears that tornado frequency has increased in recent decades (Figure 2). However, this increase may be entirely caused by factors unrelated to climate change:
1) Population growth has resulted in more tornadoes being reported. Heightened awareness of tornadoes has also helped; the 1996 Hollywood blockbuster movie Twister "played no small part" in a boom in reported tornadoes, according to tornado scientist Dr. Nikolai Dotzek.
2) Advances in weather radar, particularly the deployment of about 100 Doppler radars across the U.S. in the mid-1990s, has resulted in a much higher tornado detection rate.
3) Tornado damage surveys have grown more sophisticated over the years. For example, we now commonly classify multiple tornadoes along a damage path that might have been attributed to just one twister in the past.
Figure 2. The total number of U.S. tornadoes since 1950 has shown a substantial increase. Image credit: NOAA/NCDC.
Figure 3. The number of EF-0 (blue line) and EF-1 and stronger tornadoes (maroon squares) reported in the U.S. since 1950. The rise in number of tornadoes in recent decades is seen to be primarily in the weakest EF-0 twisters. As far as we can tell (which isn't very well, since the historical database of tornadoes is of poor quality), there is not a decades-long increasing trend in the numbers of tornadoes stronger than EF-0. Since these stronger tornadoes are the ones most likely to be detected, this implies that climate change, as yet, is not having a noticeable impact on U.S. tornadoes. Image credit: Kunkel, Kenneth E., et al., 2013, "Monitoring and Understanding Trends in Extreme Storms: State of Knowledge," Bull. Amer. Meteor. Soc., 94, 499–514, doi: http://dx.doi.org/10.1175/BAMS-D-11-00262.1
Figure 4. Insured damage losses in the U.S. due to thunderstorms and tornadoes, as compiled by Munich Re. Damages have increased sharply in the past decade, but not enough to say that an increase in tornadoes and severe thunderstorms may be to blame.
Stronger tornadoes do not appear to be increasing Tornadoes stronger than EF-0 on the Enhanced Fujita Scale (or F0 on the pre-2007 Fujita Scale) are more likely to get counted, since they tend to cause significant damage along a long track. Thus, the climatology of these tornadoes may offer a clue as to how climate change may be affecting severe weather. If the number of strong tornadoes has actually remained constant over the years, we should expect to see some increase in these twisters over the decades, since more buildings have been erected in the paths of tornadoes. However, if we look at the statistics of U.S. tornadoes stronger than EF-0 or F-0 since 1950, there does not appear to be any increase in their number (Figure 3.) Damages from thunderstorms and tornadoes have shown a significant increase in recent decades (Figure 4), but looking at damages is a poor way to determine if climate change is affecting severe weather, since there are so many human factors involved. A study in Environmental Hazards (Simmons et al., 2012) found no increase in tornado damages from 1950 - 2011, after normalizing the data for increases in wealth and property. Also, Bouwer (BAMS, 2010) reviewed 22 disaster loss studies world-wide, published 2001 - 2010; in all 22 studies, increases in wealth and population were the "most important drivers for growing disaster losses." His conclusion: human-caused climate change "so far has not had a significant impact on losses from natural disasters." Studies that normalize disaster data are prone to error, as revealed by a 2012 study looking at storm surge heights and damages. Given the high amount of uncertainty in the tornado and tornado damage databases, the conclusion of the "official word" on climate science, the 2007 United Nations IPCC report, pretty much sums things up: "There is insufficient evidence to determine whether trends exist in small scale phenomena such as tornadoes, hail, lighting, and dust storms." Until a technology is developed that can reliably detect all tornadoes, there is no hope of determining how tornadoes might be changing in response to a changing climate. According to Doswell (2007): "I see no near-term solution to the problem of detecting detailed spatial and temporal trends in the occurrence of tornadoes by using the observed data in its current form or in any form likely to evolve in the near future."
Figure 6. Six-hourly periods per year with environments supportive of significant severe thunderstorms in the U.S. east of the Rocky Mountains. The line is a local least-squares regression fit to the series, and shows no significant change in environments supportive of significant severe thunderstorms in recent decades. Image credit: Brooks, H.E., and N. Dotek, 2008, "The spatial distribution of severe convective storms and an analysis of their secular changes", Climate Extremes and Society
How are the background conditions that spawn tornadoes changing? An alternate technique to study how climate change may be affecting tornadoes is look at how the large-scale environmental conditions favorable for tornado formation have changed through time. Moisture, instability, lift, and wind shear are needed for tornadic thunderstorms to form. The exact mix required varies considerably depending upon the situation, and is not well understood. However, Brooks (2003) attempted to develop a climatology of weather conditions conducive for tornado formation by looking at atmospheric instability (as measured by the Convective Available Potential Energy, or CAPE), and the amount of wind shear between the surface and 6 km altitude. High values of CAPE and surface to 6 km wind shear are conducive to formation of tornadic thunderstorms. The regions they analyzed with high CAPE and high shear for the period 1997-1999 did correspond pretty well with regions where significant (F2 and stronger) tornadoes occurred. Riemann-Campe et al. (2009) found that globally, CAPE increased significantly between 1958 - 2001. However, little change in CAPE was found over the Central and Eastern U.S. during spring and summer during the most recent period they studied, 1979 - 2001. Brooks (2013) found no significant trends in wind shear over the U.S. from 1950 - 2010 (Figure 5.) A preliminary report issued by NOAA’s Climate Attribution Rapid Response Team in July 2011 found no trends in CAPE or wind shear over the lower Mississippi Valley over the past 30 years.
Figure 7. Change in the number of days per year with a high severe thunderstorm potential as predicted by the climate model (A2 scenario) of Trapp et al. 2007, due to predicted changes in wind shear and Convective Available Potential Energy (CAPE). Most of the U.S. east of the Rocky Mountains is expected to see 1 - 2 additional days per year with the potential for severe thunderstorms. The greatest increase in potential severe thunderstorm days (three) is expected along the North and South Carolina coast. Image credit: NASA.
How will tornadoes and severe thunderstorms change in the future? Using a high-resolution regional climate model (25 km grid size) zoomed in on the U.S., Trapp et al. (2007) and Trapp et al. (2009) found that the decrease in 0-6 km wind shear in the late 21st century would more than be made up for by an increase in instability (CAPE). Their model predicted an increase in the number of days with high severe storm potential for most of the U.S. by the end of the 21st century, particularly for locations east of the Rocky Mountains (Figure 7.) Brooks (2013) also found that severe thunderstorms would likely increase over the U.S. by the end of the century, but theorized that the severe thunderstorms of the future might have a higher proportion causing straight-line wind damage, and slightly lower proportion spawning tornadoes and large hail. For example, a plausible typical future severe thunderstorm day many decades from now might have wind shear lower by 1 m/s, but a 2 m/s increase in maximum thunderstorm updraft speed. This might cause a 5% reduction in the fraction of severe thunderstorms spawning tornadoes, but a 5% increase in the fraction of severe thunderstorms with damaging straight-line winds. He comments: "However, if the number of overall favorable environments increases, there may be little change, if any, in the number of tornadoes or hailstorms in the US, even if the relative fraction decreases. The signals in the climate models and our physical understanding of the details of storm-scale processes are sufficiently limited to make it extremely hazardous to make predictions of large changes or to focus on small regions. Projected changes would be well within error estimates."
Figure 8. From 1995 (the first year we have wind death data) through 2012, deaths from high winds associated with severe thunderstorms accounted for 8% of all U.S. weather fatalities, while tornadoes accounted for 13%. Data from NOAA.
Severe thunderstorms are capable of killing more people than tornadoes If the future climate does cause fewer tornadoes but more severe thunderstorms, this may not end up reducing the overall deaths and damages from these dangerous weather phenomena. In 2012, the warmest year in U.S. history, the death toll from severe thunderstorms hit 104--higher than the 70 people killed by tornadoes that year. Severe thunderstorms occur preferentially during the hottest months of the year, June July and August, and are energized by record heat. For example, wunderground weather historian Christopher C. Burt called the number of all-time heat records set on June 29, 2012 “especially extraordinary,” and on that day, an organized thunderstorm complex called a derecho swept across a 700-mile swath of the Ohio Valley and Mid-Atlantic, killing thirteen people and causing more than $1 billion in damage. The amount of energy available to the derecho was extreme, due to the record heat. The derecho knocked out power to 4 million people for up to a week, in areas where the record heat wave was causing high heat stress. Heat claimed 34 lives in areas without power in the week following the derecho. Excessive heat has been the number one cause of weather-related deaths in the U.S since 1995, killing more than twice as many people as tornadoes have. Climate models are not detailed enough to predict how organized severe thunderstorm events such as derechos might change in a future warmer climate. But a warmer atmosphere certainly contributed to the intensity of the 2012 derecho, and we will be seeing a lot more summers like 2012 in coming decades. A future with sharply increased damages and deaths due to more intense severe thunderstorms and derechos is one nasty climate change surprise that may lurk ahead.
Figure 9. Lightning over Tucson, Arizona on August 14, 2012. A modeling study by Del Genio et al.(2007) predicts that lighting will increase by 6% by the end of the century, potentially leading to an increase in lightning-triggered wildfires. Image credit: wunderphotographer ChandlerMike.
Lightning may increase in a warmer climate Del Genio et al.(2007) used a climate model with doubled CO2 to show that a warming climate would make the atmosphere more unstable (higher CAPE) and thus prone to more severe weather. However, decreases in wind shear offset this effect, resulting in little change in the amount of severe weather in the Central and Eastern U.S. late this century. However, they found that there would likely be an increase in the very strongest thunderstorms. The speed of updrafts in thunderstorms over land increased by about 1 m/s in their simulation, since upward moving air needed to travel 50 - 70 mb higher to reach the freezing level, resulting in stronger thunderstorms. In the Western U.S., the simulation showed that drying led lead to fewer thunderstorms overall, but the strongest thunderstorms increased in number by 26%, leading to a 6% increase in the total amount of lighting hitting the ground each year. If these results are correct, we might expect more lightning-caused fires in the Western U.S. late this century, due to increased drying and more lightning. Only 12% of U.S. wildfires are ignited by natural causes, but these account for 52% of the acres burned (U.S. Fire Administration, 2000). So, even a small change in lightning flash rate has important consequences. Lightning is also a major killer, as an average of 52 people per year were killed by lightning strikes over the 30-year period ending in 2012, accounting for 6% of all U.S. weather-related fatalities.
Summary We currently do not know how tornadoes and severe thunderstorms may be changing due to climate change, nor is there hope that we will be able to do so in the foreseeable future. It does not appear that there has been an increase in U.S. tornadoes stronger than EF-0 in recent decades, but climate change appears to be causing more extreme years--both high and low--of late. Tornado researcher Dr. Harold Brooks of the National Severe Storms Laboratory in Norman, Oklahoma said in a 2013 interview on Andrew Revkin's New York Times dotearth blog: "there’s evidence to suggest that we have seen an increase in the variability of tornado occurrence in the U.S." Preliminary research using climate models suggests that we may see an increase in the number of severe thunderstorms capable of producing tornadoes over the U.S. late this century, but these thunderstorms will be more likely to produce damaging straight-line winds, and less likely to produce tornadoes and large hail. This will not necessarily result in a reduction in deaths and damages, though, since severe thunderstorms can be just as dangerous and deadly as tornadoes--especially when they knock out power to areas suffering high-stress heat waves. Research into climate change impacts on severe weather is just beginning, and much more study is needed.
Video 2. Dr. Harold Brooks of the National Severe Storms Laboratory in Norman, Okla., gave a video interview at a tornado and climate research conference held at Columbia University earlier this year. He discusses why we don't issue seasonal tornado forecasts, but doesn't discuss climate change.
The scientific agreement that climate change is happening, and that it's caused by human activity, is significant and growing, according to a new study published Thursday. The research, which is the most comprehensive analysis of climate research to date, found that 97.1% of the studies published between 1991 to 2011 that expressed a position on manmade climate change agreed that it was happening, and that it was due to human activity.
The study looked at peer reviewed research that mentioned climate change or global warming. Peer review is the way that scientific journals approve research papers that are submitted. In peer review, group of scientists that weren't involved in the study, but who are experts in the field, look at the research being submitted and have approved that it meets scientific process standards, and the standards of that journal.
In 2011, 521 of those peer reviewed papers agreed that climate change is real, and that human activity is the cause. Nine papers in 2011 disagreed.
John Cook, founder of skepticalscience.com and the lead author on the study, said the motivation for the analysis was the importance of scientific consensus in shaping public opinion, and therefore policy. "When people understand that climate scientists agree on human-caused global warming, they're more likely to support climate policy," Cook said. "But when the public are asked how many climate scientists agree that humans are causing global warming, the average answer is around 50%."
This "consensus gap" is what Cook and the research team is trying to close. "Raising awareness of the scientific consensus is a key step towards meaningful climate action," Cook said.
In 2004 Naomi Oreskes, Professor of History and Science Studies at the University of California San Diego, published what many scientists consider the seminal study on climate change consensus. She also co-authored the book Merchants of Doubt, which identifies and examines the similarities between today's climate change conversation and previous controversies over tobacco smoking, acid rain, and the hole in the ozone layer.
Oreskes believes that the public isn't aware of the consensus because of deliberate efforts to cause confusion. "There has been a systematic attempt to create the impression that scientists did not have a consensus, as part of a broader strategy to prevent federal government action," Oreskes said. "The public have been confused because people have been trying to confuse us."
The study published Thursday is the first to take so many papers and authors into account. Doing a search on the popular science article website Web of Science for "climate change" or "global warming" produces over 12,000 results. Of these, 4,014 papers were identified to state a position on climate change. Among those, 3,896, or 97.1% endorsed the consensus that climate change was happening and that it was caused by human activity.
In an interesting result, Cook and his team found that over time, scientists tend to express a position on climate change less and less in their research papers. This is likely a result of consensus -- that if a scientific conclusion has been reached, there's no need to continue to state that conclusion in new research. "Scientists tend to take the consensus for granted," says Cook, "perhaps not realizing that the public still think it's a 50:50 debate."