Caption: Volunteers and debris fill the streets of Joplin, Missouri, after an EF-5 tornado touched down on May 22, 2011.
More than a year ago, as the United States rang in the new year at the beginning of 2011, Mother Nature made it abundantly clear that the gloves were off. A series of punishing snowstorms pounded the country, culminating in the massive Groundhog Day's blizzard, which shut down Chicago with more than 20 inches of snow and brought heavy snow to 22 states. The storm cost $1.8 billion, making it only the third billion-dollar U.S. snowstorm since 1980.
But Mother Nature was just getting warmed up. As winter waned and spring severe weather season arrived in April, a series of violent severe storms brought an astonishing eight billion-dollar disasters to the United States in a three-month period. The Plains and Southeast endured an epic onslaught of dangerous tornadoes that killed hundreds and caused tens of billions in damage. Three of the largest tornado outbreaks in U.S. history hit in a six-week period, including the largest and most expensive tornado outbreak in U.S. history—the $10.2 billion dollar Southeast United States Super Outbreak. Even more stunning was the $9 billion late-May tornado outbreak that brought an EF-5 tornado to Joplin, Missouri. The Joplin tornado did $3 billion in damage and killed 159 people—the largest death toll from a U.S. tornado since 1947, seventh deadliest in U.S. history, and the most expensive tornado in world history. Record rains accompanied the storm systems that spawned these two epic tornado outbreaks, triggering billion-dollar floods on the Mississippi and Missouri Rivers.
And after the floods, came the heat. In 2011, if you weren't getting washed away by a flood, you were baking in record heat and drought. According to NOAA's National Climatic Data Center (NCDC), as of November 2011, 56 percent of the contiguous United States had experienced a top-10 percent wettest year or a top-10 percent driest year—the highest such percentage in over a century of record-keeping, and the type of pattern climate models predict we'll see more of as the climate warms. The summer of 2011 was the second hottest in U.S. history, just 0.1 degree below the great Dust Bowl summer of 1936. Oklahoma had the hottest month and Texas the hottest summer of any state in U.S. history in 2011. Texas endured the worst one-year drought in state history, costing at least $10 billion, with many regions 20 inches below average in precipitation. Unprecedented wildfires scorched the tinder-dry grasslands and forests of Texas and surrounding states, burning thousands of buildings and costing over $1 billion.
As summer gave way to fall, hurricane season was relatively merciful to the U.S. and—our coasts experienced less than half of the usual number of strikes from tropical storms and hurricanes. However, the two storms that did hit—Hurricane Irene and Tropical Storm Lee—dumped prodigious rains, creating floods that cost billions. Finally, as cold weather arrived again, one final billion-dollar weather disaster hit—a rare October snowstorm that dumped heavy, wet snow on the Northeast, when the trees had not yet lost their leaves.
In all, 14 billion-dollar weather disasters pummeled the United States in 2011, killing more than 1,000 and costing $55 billion. At least two other weather disasters may end up costing $1 billion by the time the accounting is complete, said NCDC. The previous record for billion-dollar weather disasters was just nine, and an average year has three or four.
During my 30 years as a meteorologist, I've never seen a year that comes close to matching 2011 for the number of astounding U.S. extreme weather events. It boggles my mind that in one year we had weather events that rivaled or exceeded some of America's most iconic extremes—the “Super” tornado outbreak of 1974, the Dust Bowl summer of 1936, and the great Mississippi flood of 1927.
The Director of the NWS, Jack Hayes, made similar comments in December: “In my weather career spanning four decades, I've never seen a year quite like 2011. We experienced record-breaking extremes of nearly every conceivable type of weather.” Was 2011's extreme weather influenced by climate change? If so, what are the mechanisms, and what does the future hold? I argue that though it is difficult to separate the emerging signal of climate change from the noise of natural variability, there is strong scientific evidence that some of the destruction wrought by this year's extreme weather was made more likely by climate change.
Caption: Hurricane Irene strengthened on its path toward the continental United States in late August 2011.
Climate Change and Disaster Losses
Economic losses from natural disasters have been steadily increasing worldwide and in the United States since 1980. However, since the U.S. and world population are steadily increasing, a portion of this extra damage is due to more people living in harm's way. In addition, wealth has been increasing, so damages have increased due to more people having more “stuff” that gets destroyed. The number of damaging weather-related natural disasters being reported has also increased significantly since 1980, but this could be due to better communications, an increase in population, and a higher proportion of people now living in areas more vulnerable to natural disasters.
A 2010 paper in the Bulletin of the American Meteorological Society by Netherlands researcher Laurens Bouwer titled, “Have Disaster Losses Increased Due to Anthropogenic Climate Change?”, looked at 22 disaster-loss studies worldwide, published between 2001 and 2010 All of the studies showed an increase in damages from weather-related disasters in recent decades. Fourteen of the 22 studies concluded that there were no trends in damage after correcting for increases in wealth and population, while eight of the studies did find upward trends even after such corrections, implying that climate change could be responsible for the increased disaster losses. In all 22 studies, increases in wealth and population were the “most important drivers for growing disaster losses.” Thus, it is still controversial whether or not climate change is increasing disaster losses. There is too much year-to-year variability in the weather and disasters to tell. Extreme events, by their nature, are rare, making it tough to study them.
The Tornadoes of 2011
The tornado and severe thunderstorm season of 2011 was, to me, the most remarkable part of 2011's weather. Preliminary damage estimates from Munich Re insurance company put 2011's insured losses due to thunderstorms and tornadoes at $25 billion, more than double the previous record set in 2010. There is a clear trend in the data showing increased losses from thunderstorms and tornadoes since 1980 (Figure 1). But how much of this is due to a change in the climate, and how much might be due to other factors? The number of tornadoes being reported has increased in recent decades, but this increase may be due entirely to factors, such as the following, that are unrelated to climate change:
Population growth has resulted in more tornadoes being reported.
Advances in weather radar, particularly the deployment of about 100 Doppler radars across the United States in the mid-1990s, have resulted in a much higher tornado detection rate.
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.
If we look at changes in the strongest tornadoes—EF-1, EF-2, EF-3, EF-4, and EF-5 twisters, the ones most likely to have a reliable long-term detection rate due to their destructive power—we see no sign of an increasing trend in recent decades (Figure 2), even if we include 2011. However, it is difficult to make solid conclusions on how tornadoes may be changing, since the quality of the historical tornado data set is so poor. This is largely due to the fact that we never directly measure a tornado's winds—a tornado has to run over a building before we can make an EF-scale strength estimate, based on the damage. As tornado researcher Chuck Doswell said in a 2007 paper, “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.”
A better way to assess how climate change may be affecting tornadoes is to look at how the large-scale environmental conditions favorable for tornado formation have changed through time. The most important ingredients for tornado formation are usually high atmospheric instability (as measured by the convective available potential energy, or CAPE) and high amounts of wind shear between the surface and 6 km altitude. High instability—created when warm, moist air near the surface is paired with cold, dense air aloft—creates strong updrafts, as air moving upwards finds itself more buoyant than its surroundings. If these strong updrafts are coupled with strong upper-level winds that dramatically change in speed and direction with height—high wind shear—this shearing action creates spin in the atmosphere that can lead to rotating supercell thunderstorms capable of spawning tornadoes. A 2009 study led by German scientist Katrin Riemann-Campe found that globally, CAPE increased significantly between 1958 and 2001, though little change in CAPE was found over the eastern two-thirds of the United States during spring and summer between 1979 and 2001. 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. However, preliminary work by J. Sander of Munich Re insurance company, presented at the December 2011 American Geophysical Union meeting in San Francisco, found that the number of days with very high CAPE values over the eastern two-thirds of the United States between 1970 and 2009 did increase significantly.
Predictions of how climate change might affect tornadoes in the future are varied. In 2007, a team of climate modelers from NASA Goddard led by Anthony Del Genio used a climate model with doubled carbon dioxide to show that a warming climate would make the atmosphere more unstable (higher CAPE) and so prone to more severe weather. However, their model found that decreases in wind shear offset this effect, resulting in little change to the number of severe thunderstorms in the Central and Eastern United States by late this century. A separate model study led by Purdue's Robert Trapp found that the decrease in 0 to 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 almost the entire United States, by the end of the 21st century. These increases were particularly high for many locations in the Eastern and Southern United States, including Atlanta, Georgia; New York City; and Dallas, Texas.
In summary, there is not enough evidence to judge whether or not climate change is affecting severe thunderstorms and tornadoes in the United States. Recent research indicates the possibility that the future climate may see an increase in severe thunderstorms, but much more work needs to be done. Until we have better evidence to the contrary, the extraordinary tornado season of 2011 will have to be regarded as a natural atmospheric freak occurrence.
Caption: Figure 1. Insured losses in 2011 dollars due to thunderstorms and tornadoes in the U.S. between 1980 and 2011. Data rom Property Claims Service MR NatCat SERVICE.
Caption: Figure 2. Number of EF-1, EF-2, EF-3, EF-4 and EF-5 tornadoes from 1950 to 2011. The total shown for 2011 is preliminary and uses unofficial numbers through November 17, but 2011 now ranks in 2nd place behind 1973. There is not a decades-long increasing trend in the numbers of tornadoes stronger than EF-0, implying that climate change, as yet, is not having a noticeable impact on U.S. tornadoes. However, statistics of tornado frequency and intensity are highly uncertain. Major changes in the rating process occurred in the mid-1970s (when all tornadoes occurring prior to about 1975 were retrospectively rated) and again in 2011, when scientists began rating tornadoes lower because of engineering concerns and unintended consequences of National Weather Service policy changes. Also, beginning in 2007, NOAA switched from the F-scale to the EF-scale for rating tornado damage, causing additional problems with attempting to assess if tornadoes are changing over time. Data provided by Harold Brooks, NOAA/National Severe Storms Laboratory.
The Floods of 2011
Almost as remarkable as the tornadoes of 2011 were the floods of 2011. Can we draw a climate change connection to these floods? The best way to answer this question is to do a detailed attribution study where we use a computer model to investigate the particular event in question. By running the model both with and without the changes to the Earth and atmosphere wrought by climate change, one can come up with a “fraction of attributable risk” to see how much more likely the event was with climate change. Such studies are time-consuming and hard to do, though, and are typically not available until years after the event has occurred. So, it is best to consider some atmospheric theory instead.
There is a well-established relationship in atmospheric physics called the Clausius-Clapeyron equation, which says that atmospheric moisture will increase by six percent to seven percent for every degree Centigrade increase in Earth's temperature. Observations show that atmospheric moisture has increased by four percent since 1970 and five percent since 1900. However, the amount of rainfall a hurricane or storm system can now drop is more than four percent to five percent. Extra moisture in the atmosphere helps intensify storms by releasing “latent heat” energy when it condenses into rain. Latent heat is the extra energy that is required to convert liquid water to gaseous water vapor, which the vapor retains until it condenses back into water. The released latent heat energy invigorates the updrafts in a storm, allowing them to draw in moisture from an area greater than usual. This effect is thought to be the main reason why heavy precipitation events—those most likely to cause floods—have been increasing over the past 50 years, in general agreement with the predictions of climate models (see Figure 6).
A 2007 study led by Dr. Kevin Trenberth of the National Center for Atmospheric Research, called “Water and Energy Budgets of Hurricanes: Case Studies of Ivan and Katrina,” found that global warming likely increased the amount of rain dropped by these hurricanes by six to eight percent. The authors wrote:
We conclude that the environmental changes related to human influences on climate have very likely changed the odds in favor of heavier rainfalls and here we suggest that this can be quantified to date to be of order 6 to 8% since 1970. It probably also results in more intense storms. The key point is that the value is not negligible, and nor is it large enough to dominate over the natural processes already in place. Such changes can cause thresholds to be exceeded (the straw that breaks the camel's back).
Such a back-breaking rainfall event might have occurred on September 8, 2011, when the remains of Tropical Storm Lee deluged the watershed of the Susquehanna River. Lee's torrential rains brought Binghamton, New York, its heaviest one-day rainfall on record. The soils, already saturated from the heavy rains of Hurricane Irene just 10 days before, could hold little water, and the runoff caused the Susquehanna River to rise 20 feet in 24 hours. The flood walls in Binghamton were topped by 8.5 inches, flooding large sections of the city, and causing tens of millions in damage. Just a six percent reduction in the flood height would have led to no overtopping of the flood walls and a huge decrease in damage. Extra moisture in the air due to global warming could have easily contributed this six percent of extra flood height.
Caption: A tornado in Nebraska on June 20, 2011.
Caption: Figure 3. Percent increase in the amount falling in heavy precipitation events (defined as the heaviest 1% of all daily events) from 1958 to 2007, for each region of the U.S. There are clear trends toward more very heavy precipitation events for the nation as a whole, and particularly in the Northeast and Midwest. Climate models predict that precipitation will increasingly fall in very heavy events in coming decades. Image credit: United States Global Change Research Program. Figure updated from Groisman, P.Ya., R.W. Knight, T.R. Karl, D.R. Easterling, B. Sun, and J.H. Lawrimore, 2004: Contemporary changes of the hydro-logical cycle over the contiguous United States, trends derived from in situ observations.
Caption: Fields flooded by nearby Wallkill River during Hurricane Irene and again after Tropical Storm Lee, near Campbell Hall, New York, in late August 2011.
Is Global Warming to Blame?
Weather has natural extremes, and 2011 was unquestionably a naturally extreme year in the United States. But when we add heat-trapping gases like carbon dioxide to the air, we are changing the environment in which natural extremes occur. A warmer atmosphere has more energy to power stronger storms, and more energy to evaporate water from the oceans and generate heavier rains and bigger floods. While the influence of our changing atmosphere on severe thunderstorms and tornadoes is uncertain, what is certain is that we've loaded the dice to make dangerous extreme weather events like the floods, heat waves, and droughts of 2011 more likely. Since our flood control systems are designed for the climate of the 20th century and not the climate of the future, we will likely see an increasing number of “straw that broke the camel's back” damaging flooding events in the coming decades if our climate continues to warm as expected.
I've argued in my blog on wunderground.com that Earth's weather in 2010 may have been the most extreme since the infamous “Year Without a Summer” in 1816, caused by the massive Tambora volcanic eruption. The wild weather in the United States in 2011 and worldwide in 2010–2011 was so unusual that it was unlikely to have occurred without some powerful climate-altering force at work, boosting the natural weather extremes. The best science we have maintains that human-caused emissions of heat-trapping gases are the most likely source of such a climate-altering force. One possibility that concerns me is that our changing climate may be doing more than just boosting natural extremes--it may be showing early signs of instability. We are pushing the climate system very hard by adding huge amounts of heat-trapping gases to the atmosphere. The laws of physics demand that the atmosphere must respond in a significant way, and at some point the climate could become unstable as it transitions to a new equilibrium. In a 2004 paper called “Assessing Climate Stability” published in the Bulletin of the American Meteorological Society, Harvard University's Paul Epstein and James McCarthy argue, “There is no assurance that the rate of greenhouse gas buildup will not force the system to oscillate erratically and yield significant and punishing surprises.” Significant and punishing surprises are exactly what we had in the United States during 2011, and over many other regions of the globe in 2010. If the wild weather events of 2010–2011 indeed mark the early stages of an unstable transition period to a new climate state, we can expect many more significant and punishing surprises in the years ahead.
JEFF MASTERS is the founder and director of meteorology at Weather Underground, www.wunderground.com.