News•December 31, 2012

Sandy Tops List of 2012 Extreme Weather & Climate Events

By Andrew Freedman, Michael Lemonick, and Dan Yawitz

From unprecedented heat waves that shattered “Dust Bowl” era records from the 1930s, to Hurricane Sandy, which devastated coastal New Jersey and New York, 2012 was the year Mother Nature had it out for the U.S. No country on Earth rivaled the U.S. in 2012 in terms of extreme weather and climate events, as one rare episode after another rocked the country.

Many served to highlight the growing role that global warming may be playing in tipping the odds in favor of high-impact weather events.

2012's Top 10 Weather & Climate EventsNo. 1: Sandy Alters Climate Conversation
No. 2: Tenacious Drought Punishes U.S.
No. 3: Hottest Year on Record in Lower 48
No. 4: Steamy Arctic Events Alarm Scientists
No. 5: Hot and Dry Conditions Fuel Wildfires
No. 6: Hottest March on Record for U.S.
No. 7: July Is Hottest U.S. Month on Record
No. 8: Hurricane Isaac Creeps Ashore
No. 9: Derecho Blows Into Lexicon
No. 10: 333 Straight Months & Counting(Click on above links to go straight to each story)

The statistics are staggering: The first half of the year was so warm that by early August, the U.S. had already exceeded the number of record-high temperatures set or tied during all of 2011. July 2012 was the hottest month on record in the U.S., as a desiccating drought enveloped the majority of the lower 48 states, stretching its misery from California to Delaware.

The drought has been the most extensive this country has seen since the 1930s. Ranchers were forced to sell off their herds as their fields turned to dust and the price of feed rose steeply; the Mississippi River neared a record-low level, threatening to curtail commerce; and drought-fueled wildfires consumed tens of thousands of acres across the West and threatened a large population center in Colorado Springs.

More than any other event this year, though, Hurricane Sandy brought the subject of climate change to the forefront, with politicians of all stripes expressing new-found interest in taking action after seeing the impacts of the storm.

By the end of 2012 a scientific, and more importantly, a public consensus had emerged that global warming was making its presence felt. This landmark shift in the conversation revolved around the now hard-to-refute recognition that a warming planet means certain types of extreme events are more likely to occur and are more damaging when they do. It also kick-started a discussion about what action is needed to make the country more resilient to extreme weather and climate events, and how to reduce long-term global warming.

Here is our list of the top 10 extreme weather and climate events of 2012, our third annual year-end review. Our ranking system takes into account each event's severity, societal impact, unusualness (was it a relatively routine occurrence or something that stands alone in recorded history?), and the potential ramifications the event suggests for the climate system as a whole.

(Read each of the Top 10 events by following the page links below)

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No. 1 — Hurricane Sandy Devastates East Coast, Alters Climate Conversation

In one night, one of the most powerful storms in U.S. history struck the East Coast, and even more powerfully, seemed to turn the national conversation to climate change.

Hurricane Sandy was the most destructive storm to hit the U.S. since Hurricane Katrina in 2005, and one of the worst storms in recorded history to strike New York and New Jersey. The storm claimed the lives of at least 125 people in the U.S., with 43 of the fatalities occurring in New York City alone.

Coastal flooding in Mantoloking, N.J. as taken from a New Jersey Air National Guard Helicopter.
Credit: NJNG/Scott Anema.

The combination of winds, waves, and heavy precipitation knocked out power to more than 8 million households in the densely populated Mid-Atlantic and Northeast. And it caused an estimated $100 billion in damage, second in the record books behind only Katrina. Homes from coastal Rhode Island to the southern tip of New Jersey were flooded or washed away. New Jersey Governor Chris Christie described the damage done to his state as “unthinkable.”

The storm’s toll put a new spotlight on climate change, including shifting the focus more on near-term adaptation measures that could aid communities in the face of increasingly destructive extreme events. In particular, the task of girding New York City against rising sea levels was given a new sense of urgency.

And it all happened in the time it took a massive storm surge to swamp a metropolis.

At 8 p.m. EDT on October 29th, the center of Sandy, which was at that point an unusual hybrid storm with characteristics of both a tropical hurricane and a cold-season nor’easter, made landfall near Atlantic City, N.J. Sandy’s expansive wind field — measuring nearly 1,000 miles in diameter at one point, which made it the largest hurricane wind field ever recorded — pushed a towering wall of water onto the Mid-Atlantic and Northeastern shoreline.

Hurricane Sandy approaches landfall in this enhanced satellite image from NOAA.
Click to enlarge the image.

Record storm surges and waves led to flooding in lower Manhattan that left the city’s famous subway system out of service for days. It took weeks before full service was restored on all subway lines, and the U.S. Army Corps of Engineers had to be called in to assist with the task of “de-watering” the subway system. And in New Jersey, it took workers nearly two months to restore service to the PATH rail line from Manhattan to Hoboken as they pumped more than 10 million gallons of water from the tunnels.

As the ocean poured into New York City, threatening the power grid, power companies cut the electricity off in Lower Manhattan, leading to the unsettling sight of a half-lit “City That Never Sleeps.”

New York City’s major airports were closed for days, and flooding at LaGuardia Airport left flights cancelled for much of the week. Iconic Boardwalks, summer houses, and year-round communities all over the Jersey Shore were left in splinters.

The storm’s unprecedented track — it curved to the west and accelerated before hitting land — maximized the storm surge potential, since an easterly air flow struck the coast at a right angle.

The landscape in the New York City region is shaped so that storm surge is maximized when winds blow from the east or southeast, directing the incoming tide like a funnel aimed at Staten Island, Manhattan, and far northern New Jersey. In contrast, when Tropical Storm Irene struck in 2011, it took a different path, and southerly winds resulted in a far less damaging storm surge.

At The Battery in Lower Manhattan, Sandy’s 9.23-foot storm surge coincided with the time of high astronomical tide, yielding a record-shattering maximum tide level of 13.88 feet. The previous high tide record of 11.20 feet was set during the great hurricane of 1921, according to the National Oceanic and Atmospheric Administration. Other record tide levels were recorded in New Jersey, New York, and Connecticut.

Water pours into the Hoboken PATH station, viewed via a Port Authority surveillance camera and broadcast via social media networks on Oct. 29, 2012.
Credit: kenobryan/Instagram.

Sandy was an unusually powerful storm to strike so far north and so late in the Atlantic Hurricane season.

Hurricane-force winds (74 mph or greater) were recorded as far south as North Carolina and as far north as Massachusetts, and strong winds downed trees and power lines as far west as Ohio, and whipped up the second-highest wave heights ever recorded on Lake Michigan. Parts of West Virginia were even buried under 2-3 feet of snow as cold air was drawn into the massive storm’s circulation.

The impact of Hurricane Sandy accelerated discussions about climate change adaptation efforts.

In Sandy’s wake, many local and state governments have already begun looking at how to prepare for similar storms. Of the $77 billion sought in federal aid from New York and New Jersey, $16.4 billion has been requested for new construction projects intended for adaptation.

Scientists have pointed to two climate change-related factors that may have helped make Sandy more destructive than it otherwise would have been.

First and foremost is sea level rise. During the past century, sea levels have risen by about a foot in some parts of New Jersey and New York, making any storm surge more likely to cause damage. In addition, sea surface temperatures off the East Coast were 3° to 5°F above average during the event, which helped Sandy retain its tropical characteristics as it moved north. Global sea surface temperatures have been increasing as a result of manmade global warming and other factors.

Additionally, some scientists have pointed to the role that an unusual weather pattern — characterized by a sharp dip in the jet stream over the Midwest and East Coast, and a massive dome of high-pressure over northeast Canada and Greenland — played in the event. Such a “blocking pattern” prevented Sandy from moving harmlessly out to sea, and instead forced the storm to take a sharp left — straight into New Jersey.

Some scientists think that the loss of Arctic sea ice may be leading to more frequent blocking events. This year, sea ice melted to the lowest level on record since at least the beginning of the satellite era in 1979.

Related Content
Ongoing Coverage of Historic Hurricane Sandy
How Global Warming Made Hurricane Sandy Worse
Hurricane Sandy Paralyzes New York, New Jersey

(Read each of the Top 10 events by following the page links below)

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No. 2— Tenacious and Punishing Drought Engulfs Much of U.S.

Despite a severe drought that afflicted Texas and several nearby states in 2011, forecasters had no inkling that a far greater disaster was in store for 2012 — the worst drought to strike the U.S. since the 1950s, and one that would show little sign of relenting even as 2012 came to a close.

Climate-related disasters usually have multiple causes, and the 2012 drought was no exception. The natural, periodic La Niña climate oscillation played a role, as did the pattern of sea surface temperatures in the Atlantic Ocean, but so did manmade global warming, and plain old bad luck. As the century progresses, however, the atmosphere’s growing concentration of greenhouse gases will very likely make severe droughts more common, more intense and longer lasting.

That’s bad news considering how the 2012 drought devastated America’s agricultural sector, with withering effects on corn, soybeans, winter wheat and cattle. Formal numbers won’t be available from the U.S. Department of Agriculture until February, 2013, but estimates of damage from crop losses alone could total more than $35 billion, according to the reinsurance company Aon Benfield. In fact, it’s quite possible that damage from the drought will eclipse the total bill from Hurricane Sandy, which some estimates place at more than $100 billion. Overall, the drought could end up robbing the limping U.S. economy’s GDP of a full percentage point, said Deutsche Bank Securities.

Back in the spring, the U.S. Department of Agriculture was predicting a record corn yield for the upcoming season. Then, an unprecedented March heat wave was followed by a record warm spring, and the combination of low precipitation and record high temperatures spiked the drought footprint of the lower 48 states from a worrisome 38 percent to a devastating 64 percent — an expansion so blazingly fast that experts called it a “flash drought.”

Credit: USDAgov/flickr

This was no mere garden-variety dry spell: parched conditions would linger in many parts of the country for months, at times covering more than 70 percent of the land area of the continental U.S, and only rarely and all-too-briefly dipping below 60 percent, even after the cooler temperatures of autumn took hold. The drenching downpours brought on by Hurricane Isaac, in August and Superstorm Sandy in late October never made it to the heart of the drought area in the High Plains and West, so they didn’t do much to alleviate the dryness, either.

The crisis was most acute in Kansas and Nebraska, where significant parts of both states were still suffering in early December from what scientists who produce the U.S. Drought Monitor call “exceptional drought” — the very worst category. Parts of Alabama and Georgia were also hard hit.

Of course, Earth’s climate system doesn’t take notice of the calendar year, so while the 2012 drought technically ends on December 31, forecasters expect the dryness to continue at least through the coming winter.

Related Content
Ongoing Coverage of Historic Drought in U.S.
Drought Has Ties to La Niña, with Global Warming Assist
Lack of Warning on Drought Reflects Forecasting Flaws

(Read each of the Top 10 events by following the page links below)

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No. 3— Turning Up the Heat: Hottest Year on Record in the Lower 48 States

2012 will go down in history as the hottest year on record in the continental U.S., pushing 1998 into second place. In line with the global warming trend spurred by steadily rising carbon emissions, seven of the top 10 warmest years in the 48 states have occurred in the past 15 years.

Like so much recent record-breaking weather, 2012 isn’t just going to top the previous record, 2012 is looking to smash it, by more than 1°F. In mid-December, Climate Central projected that 2012 average temperature for the continental U.S. at 55.34°F compared to the previous record set in 1998 of 54.32°F. For perspective, 1°F is one-quarter of the difference between the coldest and warmest years ever recorded in the U.S.

Average annual temperature in contiguous U.S. from 1895 to 2012.
Click on the image for a larger version.

The country endured many unusually long-lasting and severe heat waves as temperatures spiked in March, and never fell below average. March was the warmest such month on record in the lower 48 states, exceeding the average monthly temperature by 7°F. After the March heat wave, the contiguous U.S. recorded its warmest Spring, largest seasonal departure from average, third-warmest summer, and warmest 12-month period, all new marks since records began in 1895.

The average springtime temperature in the lower 48 was so far above the 1901-2000 average — 5.2°F, to be exact — that the country set a record for the largest temperature departure for any season on record.

The dominance of mild conditions is starkly apparent when looking at the balance between daily record highs set or tied this year and daily record lows. During 2012, there were 33,753 daily record-high temperatures set or tied, compared to just 6,303 daily record-low temperature records. That means that for every five daily record highs, there was just one daily record low set or tied.

When taking into account record-warm overnight low temperatures as well as record-cold overnight low temperatures, the ratio was closer to 4-to-1. No matter how you slice it, warm temperatures absolutely dominated the U.S. weather in 2012.

As the climate has warmed during the past several decades, there has been a growing imbalance between record daily high temperatures in the contiguous U.S. and record daily lows. A study published in 2009 found that rather than a 1-to-1 ratio, as would be expected if the climate were not warming, the ratio has been closer to 2-to-1 in favor of warm temperature records during the past decade (2000-2009). This finding cannot be explained by natural climate variability alone, the study found, and is instead consistent with global warming.

The study used computer models to project how the records ratios might shift in future decades as the amount of greenhouse gases in the air continues to increase. The results showed that the ratio of daily record highs to daily record lows in the lower 48 states could soar to 20-to-1 by mid-century, and 50-to-1 by 2100.

(Read each of the Top 10 events by following the page links below)

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No. 4— Steamy Arctic Events Alarm Climate Scientists

The continental U.S. had its warmest year on record in 2012, but things were unprecedentedly steamy at the top of the globe as well. And what transpired there—from a falling snow cover, an alarming rate of melting of Greenland’s ice cap, and a melt back of Arctic sea ice—was worrisome to climate scientists, who have long predicted that climate change would strike the Arctic faster and harder than the rest of the globe.

During the spring, snow cover across North America and Eurasia dropped to the lowest level ever seen. A few months later in July, fully 97 percent of Greenland’s ice cap—which covers most of that enormous island—experienced at least some surface melting. That hasn’t happened since at least the 1800s. During that same month, a glacier in northern Greenland shed a slab of ice twice as big as Manhattan Island. And finally in August, the ice that covers the Arctic Ocean melted back more than it has since satellite observations began in the 1970s, and kept on dropping right through mid-September.

But the loss of sea ice was perhaps the most startling news.

Late-summer sea ice in the Arctic Ocean has been on a downward spiral since the late 1970s, but the downward trend isn’t perfectly smooth. Some years actually see more ice than the year before, as wind patterns and ocean currents in a given summer might enhance or hold back that season’s melting. However, the long-term trend has been toward diminished summer sea ice cover. Studies have shown that this trend is due to a combination of manmade climate change and natural climate variability.

In 2012, ice coverage reached a new low — and not by a small amount, either. As of September 16, according to the National Snow and Ice Data Center (NSIDC), just 1.32 million square miles (2.41 million square kilometers) of ice were left floating in the Arctic Ocean, a whopping 18 percent less than the previous record of 1.61 million square miles (4.17 million square kilometers), set in 2007. The amount of sea ice that melted between March and September, the NSIDC said, would be enough to cover Canada and Texas, combined. The melt rate was so fast, in fact, that the old record was shattered weeks before the new low was reached.

Graphic showing how thick multiyear ice is being replaced by thin, first-year ice.
Click on the image for a larger version. Credit: NSIDC.

NSIDC scientist Julienne Stroeve told Climate Central that’s especially significant because “the 2007 minimum was, in large part, driven by [unusual weather patterns], and this summer we shattered the 2007 minimum under more normal summer circulation patterns.”

It’s all consistent, however, with scientists’ understanding of the role sea ice plays in the Arctic Ocean and the global climate. When it’s there, the ice’s bright, white surface reflects sunlight back into space. When it melts, the darker sea water it exposes absorbs more of that light, heating the ocean and the air above. When the ice refreezes in the fall and winter, it’s thinner, and prone to melting even more easily in future seasons, exposing even more water and setting up a feedback loop that could leave the Arctic Ocean seasonally ice-free by later this century. Computer models predict this may occur by 2050, and some scientists think it will happen even earlier.

The side effects of declining Arctic sea ice are already being seen, and are occurring faster than scientists expected just a few years ago. To start with, some scientists think that warmer Arctic sea and air temperatures are now influencing the jet stream in the northern hemisphere, leading to more extreme weather events.

A wavier jet stream that features more frequent “blocking” events, or stuck weather patterns, could be leading, paradoxically, to stormier winters in the U.S. and Europe. This is an area of active scientific research, and many climate scientists remain skeptical that Arctic sea ice loss has already started to change the weather in lower latitudes, although it’s widely recognized that melting sea ice is warming the Arctic climate.

Arctic sea ice cover off Ellsmere Island in Canada.
Click on the image for a larger version. Credit: Michael D. Lemonick.

In addition, the extra heat building in the Arctic is accelerating the pace of global warming overall. Another problem is that warm air is already starting to melt the permafrost that pervades the soil in northern Alaska, Canada and Siberia. That could release carbon from organic matter that has been frozen for many thousands of years — yet another feedback that could accelerate global climate change.

This year also saw a record low spring snow cover in North America and Eurasia, and the rate of snow cover loss in springtime has been even faster than the rate of sea ice loss. The loss of spring snow cover affects the length of the growing season, the timing and dynamics of spring river runoff, permafrost thawing, and wildlife populations. This was the third time in five years that North America saw a record-low spring snow cover, and the fifth year in a row that Eurasia has. Warmer air temperatures have played a large role in driving this trend, since during the past decade no part of the Arctic has been cooler than the 30-year average.

Warmer ocean water, meanwhile, is destabilizing glaciers that drain Greenland’s massive ice cap, which holds enough fresh water to raise sea level by more than 20 feet. Warm air over the Arctic in 2012 triggered the most widespread surface melt on the Greenland ice cap on record.

All the melting has also encouraged an increase in both shipping and oil exploration in Arctic waters, threatening to bring pollution to these formerly pristine waters — and also posing new risks to the men and women living and working there.

(Read each of the Top 10 events by following the page links below)

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No. 5 — Hot and Dry Conditions Fuel Western Wildfires

Thanks to the warmest temperatures on record for the continental U.S. and the worst drought since the 1950s, wildfires spread like, well, wildfire in 2012, scorching nearly 9.2 million acres of forest, brushland and grassland across the nation through November, the most recent month for which numbers are available. That’s the second-largest area burned since the National Interagency Fire Center began keeping records in 2000 (the worst was 2006), and nearly 50 percent more than the 10-year average over the first decade of this century.

According to the fire center, 11 people died in wildfires nationwide in 2012.

It also represents the least number of individual fires, at 55,505. That’s because the average fire burned 165 acres of land in 2012, nearly twice the average area burned per fire from 2000-2010 — a number very much in keeping with research showing that climate change has been boosting the number of large wildfires in the American West since the 1970s.

And as climate change continues to force temperatures upward over the rest of this century, that trend, and the total acreage torched, are likely to rise as well.

One of the hardest-hit states was Colorado, where the Waldo Canyon fire near Colorado Springs consumed more than 18,000 acres in June and early July, damaged some 347 homes and killed two people, making it the most destructive wildfire in state history in terms of houses lost. The previous record was set just weeks earlier, when the High Park fire, near Fort Collins, burned a whopping 87,000-plus acres, destroyed 259 homes and killed one person.

In Oregon, meanwhile, the Long Draw fire was the largest in more than a century, scorching more than 557,000 acres, while the Holloway fire burned 461,000 acres, although the human and economic toll was less because both fires happened in sparsely populated areas.

Nationwide, 11 people died in wildfires nationwide, according to the fire center.

Climate studies show that warmer and drier conditions in coming decades will likely cause a continued increase in the burned area from wildfires. A recent NASA study found that the burned area from wildfires may double in size by 2050, and that high wildfire years, such as 2012, would likely occur more frequently.

(Read each of the Top 10 events by following the page links below)

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No. 6— Hottest March on Record in Continental U.S.

Of all the heat waves that roasted the U.S. in 2012, it was the March heat event that was most unusual, and most consequential. In the northern Plains and Upper Midwest, March is typically a wintry month. But during March 2012, an already thin snow and lake-ice cover was essentially vaporized by unrelenting warm weather with temperatures that soared as high as 40 degrees above average in some locations. In many cases, overnight low temperatures were so warm that they broke the daytime-high temperature record.

By melting the snow and drying out soils, the March heat wave set the stage for the dramatic and rapid expansion of the drought in subsequent months, catching many farmers and ranchers—who had been expecting a bumper corn crop—off guard.

Land surface temperature departures from average from March 8-15, 2012.
Click on the image for a larger version. Credit: NASA.

March was the hottest such month since U.S. temperature records began in 1895. For the month, more than 7,000 daily record-high temperatures were set or tied, and a nearly equal amount of warm overnight low temperatures were reached.

The ratio of daily record highs to daily record lows was staggeringly out of balance—a mind-boggling 27-to-1—according to data from the National Climatic Data Center. In many ways, the heat in March exceeded the hot weather that followed in the summer months. For example, not only were there twice as many warm temperature records broken in March compared to July, but on average they were broken by 3.22°F more than the average July record, according to a Climate Central analysis.

Although studies have not yet been conducted on the main factors that triggered this heat wave and whether global warming may have tilted the odds in favor of the event, scientific studies of previous heat events clearly show that global warming increases the odds of heat extremes, in much the same way as using steroids boosts the chances that a baseball player will hit more home runs in a given year.

Temperature records from normally cold locations help tell the story of an event that left many longtime meteorologists scratching their heads, searching in vain to find a historical precedent.

In Minneapolis, for example, March was by far the warmest such month on record, with an average temperature that was 15.5°F above average, beating the previous record set in 1910. March saw the earliest occurrence of an 80°F temperature on record in the Twin Cities, on March 17, and the greatest number of 70°F days during the month as well.

Surface temperature departures from average during March 2012.
Click on the image for a larger version. Credit: NOAA/NCDC.

In Detroit, the mid-March heat wave was enough to vault the month to the top of the list of warmest Marches on record. Not only that, but the records for the most consecutive March days with highs above 60°F, 70°F, and 80°F were also set.

Chicago easily set a record for the warmest March, after breaking or tying daily high temperature records on nine consecutive days from March 14-22 — the second-longest stretch of daily temperature records of any type for the Windy City dating back to 1871. The previous warmest March on record there occurred in 1910 and again in 1945, when average monthly temperatures were 48.6°F. March 2012 far exceeded that, averaging 53.5°F. If this had been April, that average monthly temperature would have been high enough to rank as the seventh-warmest April on record.

In Chicago, there were eight days that reached or exceeded 80°F. Prior to this March, the Weather Service reported there had been just 10 days in March with highs in the 80s. In other words, 44 percent of Chicago’s 80°F March days since 1871 occurred in March 2012.

International Falls, Minn., located at the Canadian border and known as the “icebox of the nation,” failed to live up to its name by tying or breaking its record high on every day except one from March 10-22.

Milwaukee and Madison, Wis., also set warm temperature records. Both cities had their warmest March on record, and in addition to setting numerous daytime high temperature records, Milwaukee tied or broke record high minimum temperatures for six days in a row during the heat wave.

The warm weather caused water temperatures in the Great Lakes to climb to levels never before witnessed so early in the year. The water temperature on Lake Erie at Buffalo reached 40°F in March, which was the first time that has occurred since records began in 1927.

(Read each of the Top 10 events by following the page links below)

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No. 7—July Became the Hottest Month in U.S. History

A persistent area of High Pressure, popularly known as a “Heat Dome,” parked itself in the South Central U.S. during July, yielding an unrelenting heat wave that smashed thousands of records from the High Plains to the Northeast. July 2012 topped the Dust Bowl-era month of July 1936 for the title of the warmest month since U.S. weather records began in 1895.

In total, there were 4,420 daily record-high temperatures set or tied in the U.S. during July, along with 3,673 record-warm overnight low temperatures set or tied. The ratio of daily record highs compared with daily record lows was nearly as lopsided as it was during March, as record highs outpaced record lows by a ratio of about 17:1, according to a Climate Central analysis of NCDC data.

Triple-digit heat affected all of Oklahoma on July 31, 2012.
Click to enlarge the image. Credit: Oklahoma Mesonet.

The heat both accelerated the drought and was partly fueled by it, since drier soils allow air temperatures to soar higher than they would in wetter conditions. When the soil is wet, much of the incoming solar energy goes into evaporating moisture, whereas when it’s dry, the same energy goes into raising the thermometer. Nebraska, Iowa, Illinois, and Missouri had precipitation totals among the 10 lowest on record for the month.

The hot and dry weather propelled the U.S. Climate Extremes Index—which keeps track of the highest and lowest extremes in temperatures, precipitation, and other events—to a record 46 percent for the period January-July, 2012, which is twice the average for the period. That means that nearly half the country was affected by extreme weather conditions. The record (42 percent) was last set in 1934 — again, during the Dust Bowl.

Much of the explanation for the high index was due to exceedingly warm daytime temperatures and warm overnight temperatures across a record-large area of the nation. The overnight warmth is what distinguishes July, 2012 from July, 1936. Warm nights, however, don't have much to do with soil moisture, so they're a more robust signal that the planet is warming overall in response to increasing concentrations of greenhouse gases. They're also more dangerous than daytime heat, since they prevent people from recovering after a very hot day.

Many cities across the U.S. experienced record-setting temperatures during July, as the weather pattern kept cooling rains at bay.

People tried to beat the heat any way they could during a scorching July.
Credit: AP

For example St. Louis, Mo., exceeded its all-time record for the greatest number of days with temperatures of 105°F or above, beating the 10 such days that occurred in 1934. In Wichita, Kan., July was the fourth-warmest month on record, tied with 1934. Wichita recorded 21 100-degree days during the month, the second largest tally ever recorded there during the month of July.

July was also Denver’s warmest month on record, with an average temperature of 78.9°F, beating the previous record that was set in 1934 by more than a full degree. The “Mile High City” had seven 100-degree days, and there were 27 days with a temperature of 90°F or higher.

Of all the areas that were hammered by the heat, the Mississippi Ozarks and southeast Kansas may have had it worst, particularly during mid-to-late July, when a brutal combination of high heat and a pronounced lack of rainfall took hold. In Joplin, Mo., the average high temperature in July was 99.7°F, and the city only received a trace of rainfall. Joplin’s average temperature during July was 6.4°F above average, and the month ranked as the city’s fourth-warmest on record.

The heat also extended northward to Chicago, where the Windy City experienced its third-warmest July, with an average temperature of 81.1°F—7.1°F above normal. In Rockford, Ill., July was the warmest such month on record, with an average temperature that was 7°F above average. On the East Coast, Washington D.C. had its second-hottest July, exceeded only by July 2011, which was 0.5°F warmer. This July, however, set new standards for the greatest number of 100-degree days, with seven, and tied 1930 for the longest string of consecutive 100-degree days, with four.

Studies show that manmade global warming is shifting the odds in favor of more extreme heat events, and projections also show that drought conditions may become more common and severe in parts of the world as greenhouse gases continue to build up in the atmosphere.

For the U.S., the warm July followed a warmer-than-average June, which came on the heels of the warmest spring on record, which in turn was the culmination of the warmest March, third-warmest April, and second-warmest May. This year marked the first time that all three spring months ranked among the 10 warmest.

There was one major benefit to the hot and dry July, though, as the weather pattern that led to the heat also squelched tornado activity. The month had the fewest tornadoes of any July since reliable tornado statistics began in 1954. With just 24 tornado reports in July, the U.S. saw fewer tornadoes month than Canada did, which is unusual.

(Read each of the Top 10 events by following the page links below)

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No. 8— Hurricane Isaac Creeps Along, Dumping on Louisiana

To many Americans, Hurricane Isaac began as “The Storm that Might Cancel the Republican National Convention,” but will be remembered as one of the slowest-moving and wettest hurricanes to traverse the state of Louisiana, taking more than 36 hours to crawl from the coastline to the Arkansas state border. In its wake, flooding from Isaac led to seven deaths in the U.S., and at least 29 more in Haiti and the Dominican Republic. In total, the storm caused more than $2 billion in damage.

Satellite view of Hurricane Isaac approaching Louisiana on August 28, 2012.
Click to enlarge the image. Credit: NASA.

In late August, the storm looked like it would threaten Tampa, where the RNC convention was taking place. However, Tropical Storm Isaac merely brushed the west coast of Florida, after dumping significant rainfall over Haiti and the Dominican Republic. The storm then strengthened into a hurricane and moved slowly toward Southeast Louisiana. Hurricane Isaac made landfall near Port Fourchon, La., on the night of August 28th.

Isaac moved slowly after it came ashore, and continued to bring heavy rain and strong winds to the region as it crept along the Mississippi River at a glacial pace. Sustained 80 mph winds pummeled coastal Louisiana for hours, and parts of Mississippi and Alabama received up to a foot of rain. The storm’s greatest threat was its 11-foot storm surge, which forced hasty evacuations of low-lying communities in southeast Louisiana after levees were weakened and deemed to be in danger of failing.

While New Orleans, with its $14.5 billion in post-Hurricane Katrina flood protection improvements, survived the brunt of the storm, high winds and flooding caused hundreds of millions of dollars in damage from downed trees and disrupted power services from Miami to central Louisiana.

After its slow-motion journey across Louisiana, Isaac brought welcome rain to Arkansas, Missouri and Illinois, before petering out over southern Indiana and Kentucky. However, rains from Isaac brought little relief to the heart of the drought-stricken region, from the Plains to the Upper Midwest, which was suffering from historically low rainfall since at least early spring.

Hurricane Isaac may help change the way the National Hurricane Center in Miami warns the public about the storm surge risk from an approaching hurricane. While Isaac was technically a Category 1 storm when it made landfall, its low barometric pressure and broad wind field made its impacts more characteristic of a Category 2 storm in several ways. Furthermore, the storm’s slow pace ensured that the storm surge would batter the coast during multiple high tides. Both those factors made Isaac more dangerous than its Category 1 classification indicated, yet it seemed that many coastal residents did not get or heed that message before the storm hit.

Currently, the Saffir-Simpson scale only takes a storm’s maximum sustained winds into account, leaving out the crucial factor of storm surge flooding. Recognizing the need to better inform the public about storm surge, the NHC is developing specially-tailored storm surge warnings that forecasters hope to roll out within the next three years.

(Read each of the Top 10 events by following the page links below)

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No. 9— You Say Derecho, We Say Massive Power Outage

This was the year that the word “derecho” (pronounced de-Ray-cho) went viral, after an unusually powerful storm with an uncommon Spanish name name ripped through 12 states in a single day.

Early on a scorchingly hot afternoon on Friday, June 29th, an exceptionally strong thunderstorm complex took shape over eastern Iowa and Illinois. The storms roared eastward at speeds of up to 60 mph, picking up strength and feeding off the warm, moist air and strong jet stream winds. By evening, the storm system, known as a derecho (derived from the Spanish word meaning “straight ahead”) had traveled more than 700 miles, producing straight-line winds that knocked out power to more than 4 million customers from southern Indiana to Maryland.

The derecho, which produced winds of greater than 90 mph, was one of the most widespread and damaging straight-line wind events to strike the Mid-Atlantic region in many years. It led to at least 24 deaths in seven states, and left millions without power for several days or more in the midst of one of the hottest stretches of the summer.

Tree damage in Bethesda, Md., as a result of the derecho event.
Credit: Wikimedia Commons.

The power outage was the largest non-hurricane related outage in Virginia’s history. Nearly two-thirds of Ohio residents lost power during the storm. The DC-metro area received a direct blow that disrupted Internet and electrical services, taking down servers for popular websites such as Amazon’s website hosting services and Pinterest, which disrupted commerce far beyond the nation’s capital.

The complex storm system formed at the outer fringes of an area of high pressure, referred to as a “heat dome,” over the South Central states. This high was responsible for pumping warm and humid air from the Central U.S. to the Midwest and Mid-Atlantic states, and the extreme heat led to an unusually unstable air mass that allowed the thunderstorms to intensify as they barreled southeastward. In Washington, both Reagan National Airport and Dulles Airport had their warmest June days on record before the storms struck. At Reagan National Airport the high temperature was 104°F, beating the old June record of 102°F.

Since climate change can boost the odds of major heat waves, and the extreme heat contributed to the severe weather, it's plausible that climate change played some role in the derecho event. However, it will require rigorous scientific analysis to determine that.

Derechos are relatively frequent events in the Mid-Atlantic states, occuring once every few years. However, this one was unusually ferocious, and it will long be remembered for the regional blackout it caused, as well as the fatalities and widespread tree damage.

(Read each of the Top 10 events by following the page links below)

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No. 10— 333 Straight Months of Warmer Temperatures, and Counting

While the heat in the U.S. was especially notable in 2012, that doesn’t tell the whole story of what is happening to the global climate system. Globally, the year was much warmer than average as well, despite the presence of a cooling La Nina event during the first half of the year.

Click to enlarge the image. Credit: NOAA

The month of November continued an unbroken streak of warmer-than-average temperatures that dates back to 1985 — the year the hit film “Back to the Future” first hit theaters — with global average surface temperatures ranking as the fifth-warmest such month on record, according to figures recently released by the National Oceanic and Atmospheric Administration (NOAA).

November was the 333rd month in a row with a global average surface temperature that was above the 20th century average, a clear sign of the warming trend that scientific evidence shows is due at in large part to man-made emissions of greenhouse gases, such as carbon dioxide. And with mild temperatures in early December, that string of record warm months seems likely to continue.