National Geographic News
Photo of a coal-fired power plant in Georgia.

The pollution emitted by coal-fired power plants may have contributed to cooler temperatures in some regions of the Southeast. This power plant in Cartersville, Georgia, is one of the biggest in the country.

Photograph by Seth Resnick, Science Faction/Corbis

Brian Clark Howard

National Geographic

Published May 9, 2014

Although the Deep South has a reputation for hot, steamy weather, part of the Southeastern United States actually experienced cooler-than-normal temperatures in the years between 1991 and 2012.

That fact has been highlighted in a major report on climate change released by the White House this week. Long-term trends suggest the region, like most of the planet, will continue to warm, but the reasons for the localized cooling may provide insight into how climate systems work, scientists say.

Released Tuesday, the third National Climate Assessment is a peer-reviewed, comprehensive look at the impacts of global warming on the United States. It was written by several hundred scientists and other experts, with input from 13 federal agencies and the public. (See "Federal Climate Change Report Highlights Risks for Americans.")

In the section on the Southeast, the report noted that cooling in part of the region between 1991 and 2012 was "unusual compared to the rest of the U.S. and the globe." The phenomenon has been dubbed the "global warming hole."

Photo of a train loaded with coal.

The area that saw the most cooling during that time, southern Alabama, saw a decrease of 0.5°F to 1°F (0.3°C to 0.5°C) over the previous average.

One of the section's authors, Louisiana-based Virginia Burkett of the U.S. Geological Survey, told National Geographic, "We cannot confidently explain the cooling period that skews the average temperature trend in the Southeast, but there are several theories."

She stressed that the overall region, like the rest of the country, is still experiencing warming on a longer time scale, and that average temperatures are currently rising.

Reasons for the Warming Hole?

Another co-author of the Southeast section of the National Climate Assessment, Jayantha Obeysekera of the South Florida Water Management District, said the exact cause of the warming hole remains a "kind of open question," but it could be related to the cyclical El Niño-La Niña changes in ocean temperatures.

Another report author, climate scientist Leonard Berry of Florida Atlantic University, concurs. "El Niño and La Niña are quite powerful, multiyear oscillations that can have strong impacts on the region," he says.

Eric Leibensperger, a climate scientist at the State University of New York, Plattsburgh, told National Geographic that temperatures in the Southeastern U.S. seem to be particularly sensitive to air flowing from the Gulf of Mexico, which contains a lot of water vapor.

The moist air forms clouds and precipitation, which can lead to cooling. Large-scale cycles like El Niño can accelerate that process.

Obeysekera says changing land-use patterns might have also contributed to the cooling phenomenon. For example, shifts in agricultural practices may have led to more irrigation, which can lead to localized cooling.

Another possible explanation was published by Leibensperger and colleagues in the journal Atmospheric Chemistry and Physics in 2012. "What we've shown is that particulate pollution over the eastern United States has delayed the warming that we would expect to see from increasing greenhouse gases," Leibensperger said at the time.

In particular, sulfate particles emitted by coal-fired power plants can serve as reflectors of the sun's heat, bouncing it back into space, the scientists noted. The Southeast is heavily reliant on coal and has numerous such plants, so localized sulfate pollution could result in regional cooling.

The carbon dioxide emitted from the same coal plants is still a potent greenhouse gas that is driving global warming overall, but Leibensperger's work suggests that the sulfates may have provided some degree of counterbalance on a local level.

Since a number of those plants have been cleaned up in the past few years, there is less sulfate pollution, so there should be less cooling going forward, Leibensperger suggests.

Overall Warming

Berry says that the National Climate Assessment still found that the long-term trend for the Southeast as a region is one of warming.

"The science doesn't blindly say that temperatures have to go up all the time, there are still fluctuations," he says. "But the report says temperatures are going to go up."

Specifically, the report concluded that average temperatures in the Southeast would rise 4°F to 8°F by 2100 (2°C to 4°C), depending on how much greenhouse-gas emissions continue to grow. The states in the interior of the region would see increases greater than the coastal states, by 1°F to 2°F (0.5°C to 1°C).

The report also noted that "major consequences of warming include significant increases in the number of hot days (95°F [35°C] or above) and decreases in freezing events."

Current Impacts

The Southeast has already been feeling the effects of climate change, the report noted, including sea level rise, increased risk of severe storms such as hurricanes, and heat spells. (See "Climate Report Provides Opportunity to Bridge Political Divide.")

Sea level has already risen five to nine inches (13 to 23 centimeters) in the region over the past 50 years or so, says Berry. It has been most noticeable at high tides, and it has already caused damage along the coast.

"We're looking at seven inches [18 centimeters] by 2030 and up to two feet [61 centimeters] by 2060," says Berry. (See "Rising Seas" in National Geographic magazine.)

Already, local officials are starting to plan for those changes.

In Miami, reversible pumps have been installed to push rising water out. In Ft. Lauderdale, a coastal road that was washed out by Hurricane Sandy is being narrowed from four lanes to two and is getting reinforced with a stronger barrier to withstand the pressures of rising water.

"Things are happening now, and local people are in planning mode," says Berry. "But at the state and national level, planning is still slow."

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Brady Leights
Brady Leights

Wow that's probably the worst place for a "hole", considering the people of the southeast are well known for believing global warming is a hoax.  

Karl Backhaus
Karl Backhaus

The need for more natural gas burning is the next largest step toward a cleaner, not greener earth. Our society is based around hydrocarbons and would not be the same without it. All the people stating that "we must stop using all hydrocarbons" do not think past their own thoughts. Every basic necessity is somehow, even in the smallest bit can be linked back to a hydrocarbon. The electric car is not a step forward since the energy that is being burned to create the electricity is once again....a hydrocarbon. I live in Upstate New York where acid rain run effects every forest around. Most lakes within the Adirondack mountains are sterile since the rain falling from the sky's pH level ranges from 2-4. This is caused by sulfates being released by coal burning electric plants in the Midwest. A transition to a more natural gas run energy industry would greatly reduce the sulfate emissions and cleaner air in most places around the world. As a geologist, the Marcellus and Utica Shale beds would produce enough natural gas to run this country for over 500-1000 years at the minimum. This is our next big step toward a cleaner earth, and reducing carbon emissions.

John Monroe
John Monroe

A high school teacher once said in class that humans will survive because we adapt to changes so well. I believe global warming will be humans undoing and make that statement false. Coal, gas and oil will continue to be used adding green house emission until we soon reach the tipping point where  reversing the coming disaster  is no longer possible. And the children will say HOW COULD YOU LET THIS HAPPEN!

Gary Conn
Gary Conn

'Climate Change' has become a platitude for 'the weather'.

Human nature is a factor of environment change, but not the only one.

Global warming is one scenario; cooling and more precipitation is another.

Government, industry, and science institutions must work together to improve our environment.

Doug Hornby
Doug Hornby

Wow, 1/2 of one degree, derived from another "dream" model.

I just cannot find the words to express my contempt for this kind of article. Let me find a few ways.

1. 1/2 of a degree, you pick the scale. What are the error limits on this?

2. The causation, is a "Theory" dressed up in the article as a fact. It isn't. IT IS A WAG! "Wild a%$ed guess"

3. They are already "feeling the effects, heat spells. BUT YOU ARE SAYING IT'S COOLER.

4. More effects, more storms. The latest data from NASA says few storms in the last few decades. weaker too. WHAT EFFECT?

5. Sea rise. I am smelling a blivet here, another model, and no hard data to back it up.

That should be enough questions regarding this article. Fear mongering is a tried and true method of obtaining readership. The sad fact is that other than people like myself, the media is loosing their following, thanks in part to "fear mongering".

Gerard Van der Leun
Gerard Van der Leun

"NEW YORK—Warning that such occurrences pose a grave threat to the global economy and millions of human lives, a report presented Thursday at a United Nations summit on magical realism highlights an alarming increase in incidences in which the whole world is completely flooded by the tears of a grieving woman.

The U.N. paper states that these fantastic and potentially catastrophic events, in which a woman unleashes an endless torrent of tears over her deceased husband or the sting of unrequited love, may raise global sea levels by as much as 30 centimeters by the year 2050, which according to projections could lead to widespread coastal erosion and leave many of the world’s largest cities—including New York, Mumbai, and Jakarta—at least partially underwater." -- America's Finest News source

Jimmy Gilliland
Jimmy Gilliland

The children will drown in a Cacophony of "gurgle...text text gurgle.... My parents are dead no matter I have a water proof I device... Gurgle gurgle...

Mary Waterton
Mary Waterton

@John Monroe  

I assume then that you ride a bike to work in lieu of a car, right? Or are you another of those hypocritical "environmentalists" who talks the talk but doesn't walk the walk?


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