The Midwest has experienced near record flooding this spring, resulting in four deaths, extensive property damage, and disruptions of agriculture and transportation. Evidence suggests that manmade climate change has increased the frequency of heavy downpours, and will continue to increase flooding risks. But in their ample coverage of Midwestern flooding, major media outlets rarely mentioned climate change.

By Jill Fitzsimmons & Shauna Theel, via Media Matters

Less Than 3 Percent Of Midwest Flood Stories Mention Climate Change

ABC, NBC And CNN Entirely Ignore Climate Connection. ABC, CBS, NBC and CNN devoted 74 full segments to flooding in the Midwest, but only one — on CBS Evening News — alluded to the fact that heavy downpours have increased (one percent of coverage). That segment did not explain that scientists have attributed this to climate change, and did not feature any scientists. MSNBC and Fox News were not included in this analysis because transcripts of their daytime coverage are not available in Nexis. [CBS News, 5/2/13]

USA TODAY Only National Print Outlet To Mention Climate Context Of Floods. USA TODAY, which recently launched a year-long series on the impacts of climate change, was the only national print outlet in our study that mentioned climate change in its reporting on Midwestern floods. The Associated Press, Reuters, Los Angeles Times, New York Times, and Wall Street Journal never mentioned climate change in a total of 35 articles on the floods. The Washington Post did not cover the flooding independently. In total, only 3 percent of national print coverage mentioned climate change. [USA TODAY, 4/22/13] [Media Matters, 3/1/13]

Local Media Largely Ignore Climate Context Of Floods. Less than 4 percent of local newspaper articles on flooding in the Midwest mentioned climate change — only 4 of 107 articles. The Kansas City Star, Des Moines Register, Detroit Free Press, Minneapolis Star-Tribune and Indianapolis Star never made that connection. Flooding in the area has nearly surpassed records, leading to four deaths, delays in planting agriculture, disruptions in transportation, and potential health impacts. [The Atlantic Wire, 4/25/13] [Union of Concerned Scientists, 4/30/13] [Climate Central, 4/26/13]

No Coverage Of Flooding Contribution To The Gulf “Dead Zone.” Aquatic ecologist Don Scavia told Media Matters in an email that “most media coverage is missing an important aspect of such flooding. These massive spring flooding events push an enormous amount of agricultural pollution down the Mississippi system and into the Gulf of Mexico. That flux will most certainly create a large dead zone.” He added that conservation policies for farmers “may no longer be adequate” as flooding risks increase from climate change. Indeed, our study found that aside from one article in the Des Moines Register, the media overlooked that flooding increases fertilizer runoff from Midwestern farms into the Gulf of Mexico, further contributing to the “dead zone” there. [UPI, 4/10/13]

Evidence Suggests Climate Change Worsens Flood Risks In Midwest

Warming Leads To More Overall Precipitation. As the Environmental Protection Agency explained, basic physics indicates that warming leads to more evaporation and thus more precipitation overall:

As average temperatures at the Earth’s surface rise (see the U.S. and Global Temperature indicator), more evaporation occurs, which, in turn, increases overall precipitation. Therefore, a warming climate is expected to increase precipitation in many areas. However, just as precipitation patterns vary across the world, so will the effects of climate change. By shifting the wind patterns and ocean currents that drive the world’s climate system, climate change will also cause some areas to experience decreased precipitation. In addition, higher temperatures lead to more evaporation, so increased precipitation will not necessarily increase the amount of water available for drinking, irrigation, and industry.”

The EPA created this map, based on 2012 data from the National Oceanic and Atmospheric Administration, showing that precipitation in most areas of the U.S. including the Midwest has increased over the last century:

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By Jeff Masters, via Weather Underground

The remarkable storm that brought record-breaking May snows and cold to the Midwest last week continues to spin over the Southeast U.S. The storm is unleashing flooding rains, bringing a case of “Weather Whiplash” to Georgia: flooding where extreme drought had existed just a few months ago. The storm formed when a loop in the jet stream of extreme amplitude got cut off from the main flow of the jet over the weekend, forming a “cutoff low” that is now slowly spinning down as it drifts east over the Southeast U.S. On Sunday, the storm dumped 3.4″ of rain on Atlanta, Georgia–that city’s sixth heaviest May calendar day rain storm since record keeping began in 1878. Remarkably, the rains were also able to bring rivers in Central Georgia above flood stage. This portion of the country was in “exceptional drought”–the worst category of drought–at the beginning of 2013.

Figure 1. The record May snowstorm that hit the Midwest U.S. on May 1 – 3, 2013, got cut off from the jet stream and was seen spinning over the Southeast U.S. on Sunday, May 5, in this image from NASA’s MODIS instrument. The 3.4″ of rain that fell on Atlanta, Georgia on May 5 was that city’s sixth heaviest May calendar day rainfall since record keeping began in 1878.

Weather Whiplash

Weather Whiplash — a term originally coined by science writer Andrew Freedman of climatecentral.org to describe extreme shifts between cold and hot weather — is also a excellent phrase we can use to describe some of the rapid transitions between extreme drought and floods seen in recent years. I brought up a remarkable example in mid-April, when a 200-mile stretch of the Mississippi River north of St. Louis reached damaging major flood levels less than four months after near-record low water levels restricted barge traffic, forcing the Army Corp to blast out rocks from the river bottom to enable navigation. As the climate warms, the new normal in coming decades is going to be more and more extreme “Weather Whiplash” drought-flood cycles like we have seen in the Midwest and in Georgia this year. A warmer atmosphere is capable of bringing heavier downpours, since warmer air can hold more water vapor. But you still need a low pressure system to come along and wring that moisture out of the air to get rain. When natural fluctuations in jet stream patterns take storms away from a region, creating a drought, the extra water vapor in the air won’t do you any good. There will be no mechanism to lift the moisture, condense it, and generate drought-busting rains. The drought that ensues will be more intense, since temperatures will be hotter and the soil will dry out more.

Figure 2. Weather Whiplash in Georgia, 2013: the center of the state was in exceptional drought as the beginning of the year, but heavy rains in February, March, and April busted the drought. Heavy May rains have now brought flooding. Image credit: U.S. Drought Monitor.

Weather Whiplash in the Southeast U.S. more likely due to an intensification of the Bermuda High
This year’s “Weather Whiplash” in Georgia is the second time in the past decade the state has gone from exceptional drought to flood. In September 2007, Atlanta, Georgia was in the midst of a 1-in-100 year drought, and was just weeks away from running out of water. Yet just two years later, the drought had been busted, and a phenomenal 1-in-500 year flood ripped through the city, killing ten and causing $500 million in damage. According to a 2011 study by a Duke University-led team of climate scientists, “Changes to the North Atlantic Subtropical High and Its Role in the Intensification of Summer Rainfall Variability in the Southeastern United States”, the frequency of abnormally wet or dry summer weather in the southeastern United States has more than doubled in recent decades, due to an intensification of the Bermuda High. The scientists found that the Bermuda High, which is centered several hundred miles to the east of the Southeast U.S., has grown more intense during summer and has expanded westwards over the past 30 years. Since high pressure systems are areas of sinking air that discourage precipitation, this has made abnormally dry summers more common over the Southeast U.S. However, in summers when the Bermuda High happens to shift to the east, so that high pressure is not over the Southeast U.S., the stronger winds blowing clockwise around the Bermuda High bring an increased flow of very moist subtropical air from the south to the Southeast U.S., increasing the incidence of abnormally wet summers. Thus, the intensification of the Bermuda High has made extreme droughts and extreme floods more likely over the Southeast U.S. Using climate models, the scientists determined that human-caused global warming was likely the main cause of the significant intensification in the Bermuda High. Thus “Weather Whiplash” between drought and flood will probably become increasingly common in the coming decades over the Southeast U.S.

Figure 3. Observed June-July-August departure of precipitation from average over the SE United States for a 60-yr period (mm day−1). Horizontal dashed lines represent 1 standard deviation of the summer rainfall. Note that summer precipitation extremes exceeding one standard deviation have more than doubled during the most recent 30-year period compared to the previous 30-year period. Image credit: Li et al., 2011, Journal of Climate.

– Jeff Masters is the co-founder of the Weather Underground. This piece was originally published at the Wunderblog and was excerpted with permission.

New Orleans post-Katrina. Credit: NOAA.

By Dana Nuccitelli via Skeptical Science

The link between human-caused global warming and extreme weather is often difficult to pin down, particularly with regards to hurricanes.  As Kevin Trenberth has discussed, all weather now occurs in a climate that humans have altered:

“It is important to recognize that we have a ‘new normal’, whereby the environment in which all storms form is simply different than it was just a few decades ago.  Global climate change has contributed to the higher sea surface and sub-surface ocean temperatures, a warmer and moisteratmosphere above the ocean, higher water levels around the globe, and perhaps more precipitation in storms.”

Two new papers have recently been published examining the link between global warming and hurricane intensity.  In both cases, the scientists have found evidence that the most intense hurricanes are already occurring more often as a result of human-caused global warming.  However, their predictions about future hurricane changes differ somewhat.

Grinsted on Hurricane Storm Surges

Last year, Tamino examined Grinsted et al. (2012), which demonstrated that the most extreme storm surge events can mainly be attributed to large landfalling hurricanes, and that those events are strongly linked to hurricane damage.  The study also found that there have been twice as many Katrina-magnitude storm surge events in globally warm years as compared to cold years.

In a new paper, Grinsted et al. (2013) constructed a storm surge index beginning in 1923 from six long tide gauge records in the southeastern USA.  The idea is that surges in sea level recorded at tide gauge stations can tell us about strong hurricane events.  Consistent with their 2012 results, the authors found:

“The strong winds and intense low pressure associated with tropical cyclones generate storm surges. These storm surges are the most harmful aspect of tropical cyclones in the current climate, and wherever tropical cyclones prevail they are the primary cause of storm surges.”

They compared their storm surge index to changes in global surface temperature, to temperatures in the Main Development Region (MDR; a part of the Atlantic Ocean where most hurricanes form), and to MDR warming relative to the tropical mean temperatures (rMDR).  They found that averaged sea surface temperatures over the MDR are the best predictor of Atlantic cyclone activity, followed by global average surface temperature, with MDR warming relative to the tropics being the worst predictor of hurricane activity (Figure 1).

Figure 1: (A) Average surge index over the cyclone season. (B) Observed frequency of surge events with surge index greater than 10 units per year (surge index > 10 units). (C) Global average temperature, MDR temperature, and rMDR temperature anomaly. Inset shows locations along the US coast of the six tide gauges used in the surge index.

Grinsted et al. then used the relationships between hurricane storm surges and global and MDR temperatures to predict how storm surges will change in the future.  They used the Representative Concentrations Pathway (RCP) 4.5 scenario, which represents a future in which we slowly reduce human greenhouse gas emissions such that they peak around the year 2040.  In this scenario, there is approximately 2.4°C global surface warming over the 21st century.  The results are shown in Figure 2.

“The response to a 1°C warming is consistently an increase [in Katrina-levelstorm surges] by a factor of 2–7…. This increase does not include the additional increasing surge threat from sea level rise.”

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A new public opinion survey finds nearly two thirds of Americans say the weather has gotten worse in recent years.

The survey by Yale and George Mason Universities, “Extreme Weather and Climate Change in the American Mind April 2013” also found that nearly six in ten understand that global warming is affecting our weather.

Last month, Gallup’s polling confirmed that the public’s understanding and concern about global warming is on the rise. In February, a poll released by the Brookings Institution showed a 7 percent increase in the number of Americans who say that the planet is warming — with that increase influenced by extreme weather events.

This isn’t really so surprising given that the last two years have brought a stunning series of extreme weather events: two record heat waves, an historic drought, above-average destructive wildfires, and two powerful hurricanes that slammed into the East Coast. In 2012, the U.S. experienced the most extreme year for weather ever recorded, according to NOAA’s Climate Extremes Index.

The world’s largest reinsurance firm, Munich Re, released a report in October concluding that the growing number of weather extremes are a “strong indication of climate change”:

“Climate­-driven changes are already evident over the last few decades for severe thunderstorms, for heavy precipitation and flash flood­ing, for hurricane activity, and for heatwave, drought and wild­-fire dynamics in parts of North America.”

“In all likelihood, we have to regard this finding as an initial climate-change footprint in our US loss data from the last four decades. Previously, there had not been such a strong chain of evidence. If the first effects of climate change are already perceptible, all alerts and measures against it have become even more pressing,” said Peter Höppe, the head of Munich Re’s Geo Risks Research unit.

At the same time, non-climatic events (earthquakes, volcanos, tsunamis) have hardly changed, as the figure shows.

So our weather is getting more extreme, thanks in large part to climate change and, NOAA’s latest research confirms, to warming-driven Arctic ice loss as well. And the public has noticed. Now we just need our political leadership to notice.

Bowling Green Subway Station in New York, October 30, 2012, closed after Superstorm Sandy hit. (Credit: AP)

Scroll to the end of this post for an excellent infographic on this topic.

Superstorm Sandy devastated New Jersey, New York, and other areas along the eastern seaboard six months ago on October 29, 2012. It took at least 72 lives in the United States and caused nearly $50 billion in damages. Congress eventually provided $60 billion in disaster relief and recovery aid after weeks of deliberating and partisan bickering. These recovery efforts continue to this day.

Sandy was the worst natural disaster in the United States in terms of destruction and deaths since Hurricanes Katrina and Rita in 2005, but it wasn’t the only one. In 2011 and 2012 alone, the United States experienced 25 floods, storms, droughts, heat waves, and wildfires that each caused at least $1 billion in damages. Combined, these extreme weather events were responsible for 1,107 fatalities and up to $188 billion in economic damages.

The Center for American Progress conducted an analysis and found that the federal government—which means taxpayers—spent $136 billion total from fiscal year 2011 to fiscal year 2013 on disaster relief. This adds up to an average of nearly $400 per household per year.

Nearly all of this disaster spending was for relief and recovery from these and other smaller natural disasters. Most of these disasters are symptomatic of the man-made climate change resulting from massive amounts of carbon emissions and other pollutants in the atmosphere, which warm the oceans and the Earth. As climate change accelerates, so will federal spending on disaster relief and recovery, which will ultimately be paid for by taxpayers.

The nearly $400 per household spent annually over the past three years could be the beginning of a very costly future as climate-related extreme weather multiplies. This issue brief explores federal spending on disaster relief and offers up recommendations for how we can respond to the potential growth in these expenditures.

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Daniel J. Weiss is a Senior Fellow and Director of Climate Strategy at the Center for American Progress. Jackie Weidman is a Special Assistant on the Energy team at the Center. The authors would like to thank Cathleen Kelly, Senior Fellow, Mari Hernandez, Research Associate, and Mayhah Suri, intern, all at the Center, for their contributions to this analysis. This piece was reposted from CAP.

A 2010 study found “global warming is the main cause of a significant intensification in the North Atlantic Subtropical High that in recent decades has more than doubled the frequency of abnormally wet or dry summer weather in the southeastern United States.” Below, meteorologist Dr. Jeff Masters relates how wild climate swings are becoming the norm in the Midwest, too.–JR

Runoff from heavy rains flooded parts of a cemetery in St. Louis County on April 18, 2013. (Photo credit: Robert Cohen)

By Jeff Masters, via Weather Underground

It seems like just a few months ago barges were scraping bottom on the Mississippi River, and the Army Corps of Engineers was blowing up rocks on the bottom of the river to allow shipping to continue. Wait, it was just a few months ago–less than four months ago! Water levels on the Mississippi River at St. Louis bottomed out at -4.57′ on January 1 of 2013, the 9th lowest water level since record keeping began in 1861, and just 1.6′ above the all-time low-water record set in 1940 (after the great Dust Bowl drought of the 1930s.)

But according to National Weather Service, the exceptional April rains and snows over the Upper Mississippi River watershed will drive the river by Tuesday to a height 45 feet higher than on January 1. The latest forecast calls for the river to hit 39.4′ on Tuesday, which would be the 8th greatest flood in history at St. Louis, where flood records date back to 1861. Damaging major flooding is expected along a 250-mile stretch of the Mississippi from Quincy, Illinois to Thebes, Illinois next week.

At the Alton, Illinois gauge, upstream from St.Louis, a flood height of 34′ is expected on Tuesday. This would be the 6th highest flood in Alton since 1844, and damages to commercial property in the town of Alton occur at this water level. In addition, record flooding is expected on at least five rivers in Illinois and Michigan over the next few days. A crest 1.5′ above the all-time record has already occurred on the Des Plaines River in Chicago. This river has invasive Asian Carp that could make their way into Lake Michigan if a 13-mile barrier along the river fails during an extreme flood. Fortunately, NPR in Michigan is reporting today that U.S. Army Corps of Engineers crews stationed along the 13-mile Asian carp barrier have seen no evidence of the fish breaching the structure, and it would have taken a flood much larger than today’s record flood to breach the structure. A crest on the Grand River in Grand Rapids, Michigan nearly 4′ above the previous record (period of record: at least 113 years) is expected this weekend. At this flood level, major flooding of residential areas is expected, though the flood wall protecting downtown Grand Rapids will keep the commercial center of the city from flooding.

Figure 1. The rains that fell in a 24-hour period ending at 7 am EDT Thursday, April 18, 2013 over Northern Illinois were the type of rains one would expect see fall only once every 40 years (yellow colors), according to METSTAT, Inc. (http://www.metstat.com.) METSTAT computed the recurrence interval statistics based on gauge-adjusted radar precipitation and frequency estimates from NOAA Atlas 14 Volume 2, published in 2004 (http://dipper.nws.noaa.gov/hdsc/pfds/.) METSTAT does not supply their precipitation recurrence interval forecasts or premium analysis products for free, but anyone can monitor the real-time analysis (observed) at: http://metstat.com/solutions/extreme-precipitation-index-analysis/

Damages from the April 2013 Midwest U.S. flood in Illinois, Michigan, and Missouri are likely to run into the hundreds of millions of dollars. Some of the impacts at the flood levels predicted include:

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Source: Environment America

The vast majority of Americans have experienced a weather-related disaster in the past six years, according to a report released today by the Environment America Research and Policy Center.

The report, which is based on disaster declaration data from the Federal Emergency Management Agency, found that 243 million Americans — nearly four out of five — have lived in a county that has been hit by at least one federally declared weather-related disaster since 2007. The breadth and impact of these disasters, which include drought, tropical cyclones, flooding, tornadoes, wildfires and snow and ice storms, has been significant. According to the report:

• Every U.S. state besides South Carolina has experienced a weather-related disaster since 2007, and in 18 states and the District of Columbia, weather-related disasters have affected every county.
• More than 19 million Americans live in counties that have averaged one or more weather disaster per year since 2007.
• In 2012 alone, there were at least 11 disasters that caused more than $1 billion in damage, including Hurricane Sandy, which, with a price tag as high as $70 billion, was the costliest weather-related disaster in the U.S. since Hurricane Katrina.
• Eight U.S. counties in South Dakota, Oklahoma and Nebraska have been hit by ten or more weather disasters since the beginning of 2007.

The report notes that the frequency and intensity of several weather-related disasters has increased over the last several years and is predicted to continue increasing as the climate warms. Intense rainfall, for instance, has become more frequent in the U.S., with “the rainiest 1 percent of all storms delivering 20 percent more rain on average at the end of the 20th century than at the beginning,” and this increase is predicted to continue. And it’s likely the record-breaking heat waves, drought and wildfires that have gripped the country in the last few years will also become more common as the planet warms. The link between tornadoes and climate change is more tenuous and complicated (see here).

The report is in line with other studies of its kind — in February, a report from the Center for American Progress found that in 2011 and 2012, 43 states experienced extreme weather events that caused at least $1 billion in damage, and these events caused 1,107 fatalities and up to $188 billion in damage in total. To slow the increase of many kinds of extreme weather events, the Environment America report calls on federal and state governments to implement caps on greenhouse gases — especially on high-emitting sources, such as power plants and the transportation sector — and adopt goals of reducing emissions by at least 35 percent below 2005 levels by 2020 and at least 85 percent by 2050.

A new report confirms that the extreme heat waves, floods, droughts and wildfires that have wracked Australia over the past decade have been exacerbated by climate change. The report, commissioned by the Australian government’s Climate Commission, makes clear that these weather events will only get worse in the coming years, and warns that health and emergency professionals as well as citizens must prepare for their impacts now.

The study’s chief commissioner Tim Flannery said the study’s results mean Australia needs to take action to slow climate change.

FLANNERY: Records are broken from time to time, but record-breaking weather is becoming more common as the climate shifts. Only strong preventative action, with deep and swift cuts in emissions this decade, can stabilize the climate and halt the trend towards more intense extreme weather.

The study examines five impacts of climate change that have already begun to affect Australia:

• Heat: Australia broke 123 weather records in 90 days this summer. In January, Sydney hit a record 114 degrees, and the south Australian town of Moomba hit 121.3 degrees. The report notes that in Australia, there has been more than three times the number of record hot days than record cold days in the past 10 years. The heat has majorly impacted the country: in 2009, a heatwave led to 980 heat-related deaths, which is three times the average mortality rate for heatwaves, and heat has contributed to bleaching of the Great Barrier Reef.
• Rainfall: Extreme rainfall in Queensland in 2010 and 2011 led to record-breaking and damaging floods, which broke river height records at more than 100 observation stations. As temperature increases, the report found that the likelihood of torrential rainfall events will increase as well in some areas — a finding consistent with climate change predictions.
• Drought: Australia emerged from a decade-long drought in 2009, which was said to be the worst in the country’s history. The report states the drought was estimated to have caused an 80 percent reduction in grain production and a 40 percent reduction in livestock production, and climate models predict that rainfall in southern and eastern Australia will continue to decrease as the century progresses.
• Bushfires: Australia has seen an increase in fire weather (hot, dry, windy days) over the last 30 years, and the fire season in southeast Australia has extended into November and March. The Black Saturday fires of 2009 killed 173 people and cost about A$4.4 billion. As the duration of hot, dry days is likely to increase in much of Australia, wildfire risk is also predicted to go up.
• Sea level rise: Climate change has already contributed to a 21 centimeter rise in global sea levels, the report states, and  major flooding in 2005, 2006, 2009 and 2010 in Australia’s Torres Strait Islands were likely made more damaging by the increases in sea level.

The study confirms what climate scientists have warned for years — that climate change will likely lead to an increase in extreme weather events. It comes at a time when the effects of climate change are being felt not just in Australia, but around the world: Drastic melting of Arctic sea ice has been linked to a bitterly cold Spring in parts of Europe and North America that has devastated sheep farmers, and the record-breaking drought that affected more than half of the continental U.S. in 2012 is expected to continue into this spring and summer.

Waves hit the sea wall in Wimereux, France. (Photo: Reuters)

According to new climate modeling carried out at the Royal Netherlands Meteorological Institute, global warming likely means Europe will see more Sandy-style superstorms by the end of this century.

The team ran a simulation from 2094 to 2098, building in future greenhouse gas emissions, and found that the yearly arrival of hurricanes in the Bay of Biscay — which sits on the east coast between Spain and France — would increase from one to six. Researcher Reindert Haarsma put the number as high as thirteen by the end of the century. Many of the storms will likely also feature the same hybrid make-up of Hurricane Sandy: a core of warm, moist tropical air forming a hurricane-force core, with drier and colder air wrapping around the exterior and driving gale-force winds hundreds of miles further out.

Currently, hurricane formation in the tropics usually occurs far enough west that most storms hit North America. A rare number are caught by the Atlantic jet stream and pulled into Europe. But global warming will increase those instances by expanding the area of tropical ocean over which hurricanes form:

The rise in Atlantic tropical [sea surface temperatures] extends eastwards the breeding ground of tropical cyclones, yielding more frequent and intense hurricanes following pathways directed towards Europe. En route they transform into extra-tropical depressions and re-intensify after merging with the mid-latitude baroclinic unstable flow. Our model simulations clearly show that future tropical cyclones are more prone to hit Western Europe, and do so earlier in the season, thereby increasing the frequency and impact of hurricane force winds.

The study focused on winds, but Kevin Trenberth, the head of the Climate Analysis Section at the National Center for Atmospheric Research, told Summit County Citizens Voice that precipitation could be equally as bad. Global warming not only drives up the power of storms by adding heat energy to the oceans, it also leads to higher levels of precipitation because warmer air holds more moisture. “The resulting flooding episodes will likely dwarf the wind damage,” Trenberth said. “Although [in] some places like the Netherlands, both together could be disastrous.”

For instance, Sandy’s arrival in the northeastern United States killed 74 people and caused at least $20 billion in damage. That alone would make it one of the costliest American hurricanes ever, and some estimates put the damage as high as $45 billion.

Pakistani flood victims move to higher grounds in 2011. (Photo credit: AAP)

By Vladimir Petoukhov and Stefan Rahmstorf, via The Conversation

The northern hemisphere has experienced a spate of extreme weather in recent times. In 2012 there were destructive heat waves in the U.S. and southern Europe, accompanied by floods in China. This followed a heat wave in the U.S. in 2011 and one in Russia in 2010, coinciding with the unprecedented Pakistan flood — and the list doesn’t stop there.

Now we believe we have detected a common physical cause hidden behind all these individual events: Each time one of these extremes struck, a strong wave train had developed in the atmosphere, circling the globe in mid-latitudes. These so-called planetary waves are well-known and a normal part of atmospheric flow. What is not normal is that the usually moving waves ground to a halt and were greatly amplified during the extreme events.

Looking into the physics behind this, we found it is due to a resonance phenomenon. Under special conditions, the atmosphere can start to resonate like a bell. The wind patterns form a regular wave train, with six, seven or eight peaks and troughs going once around the globe (see graph). This is what we propose in a study published this week together with our colleagues of the Potsdam Institute for Climate Impact Research (PIK).

Planetary waves

Normally, an important part of the global air motion in the mid-latitudes of the Earth takes the form of waves wandering around the planet, oscillating irregularly between the tropical and polar regions. So when they swing northward, these waves suck warm air from the tropics to Europe, Russia, or the US; and when they swing southward, they do the same thing with cold air from the Arctic. This is a well-known feature of our planet’s atmospheric circulation system.

However, during several recent extreme weather events these planetary waves almost froze in their tracks for weeks. So instead of bringing cool air after having brought warm air before, the heat just stays. And stays. And stays. In fact, we detected a strong amplification of the usually weak, slowly moving component of these waves.

Time is critical here: two or three days of 30°C are no problem, but 20 or more days lead to extreme heat stress. Since many ecosystems and cities are not adapted to this, prolonged hot periods can result in a high death toll, forest fires, and devastating harvest losses.

The northward wind speed (negative values, blue on the map, indicate southward flow) in the mid-latitudes of the northern hemisphere. During the extreme event (a record-breaking heat wave in the US), the normally weak and irregular waves were replaced by a strong and regular wave pattern. (Credit: Vladimir Petoukhov)

What does climate change have to to with it?

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