Is U.S. climate getting more extreme?

By: Dr. Jeff Masters , 1:54 PM GMT on March 13, 2009

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Is the climate in the U.S. getting more extreme? The answer to this question depends upon how one defines "extreme". For example, the number of extreme tornadoes (violent EF-4 and EF-5 twisters) has not increased in recent years. We lack the data to judge whether there has been an increase in severe thunderstorms and hail. There has been a marked increase in Atlantic hurricane activity since 1995 (though the possible contribution of human-caused global warming to this increase is not something hurricane scientists agree upon). Since it is difficult to quantify how severe storms like tornadoes and hurricanes are changing, a better measure of how climate extremes are changing is to look at temperature and precipitation, which are well-measured. NOAA's National Climatic Data Center (NCDC) has developed a Climate Extremes Index to attempt to quantify whether or not the U.S. climate is getting more extreme. The Climate Extremes Index (CEI) is based upon three parameters:

1) Monthly maximum and minimum temperature
2) Daily precipitation
3) Monthly Palmer Drought Severity Index (PDSI)

The temperature data is taken from 1100 stations in the U.S. Historical Climatology Network (USHCN), a network of stations that have a long period of record, with little missing data. The temperature data is corrected for the Urban Heat Island effect, as well as for station and instrument changes. The precipitation data is taken from 1300 National Weather Service Cooperative stations. The Climate Extremes Index defines "much above normal" as the highest 10% of data, "much below normal" as the lowest 10%, and is the average of these five quantities:

1) The sum of (a) percentage of the United States with maximum temperatures much below normal and (b) percentage of the United States with maximum temperatures much above normal.

2) The sum of (a) percentage of the United States with minimum temperatures much below normal and (b) percentage of the United States with minimum temperatures much above normal.

3) The sum of (a) percentage of the United States in severe drought (equivalent to the lowest tenth percentile) based on the Palmer Drought Severity Index (PDSI) and (b) percentage of the United States with severe moisture surplus (equivalent to the highest tenth percentile) based on the PDSI.

4) Twice the value of the percentage of the United States with a much greater than normal proportion of precipitation derived from extreme (equivalent to the highest tenth percentile) 1-day precipitation events.

5) The sum of (a) percentage of the United States with a much greater than normal number of days with precipitation and (b) percentage of the United States with a much greater than normal number of days without precipitation.


Figure 1. The Annual Climate Extremes Index (CEI), updated through 2008, shows that U.S. climate has been getting more extreme since the early 1970s. Image credit: National Climatic Data Center. On average since 1910, 20% of the U.S. has seen extreme conditions in a given year (thick black line).

As summarized by Gleason et al. (2008), the National Climatic Data Center concludes that based on the Climate Extremes Index, the percentage of the U.S. seeing extreme temperatures and precipitation generally increased since the early 1970s. These increases were most pronounced in the summer. No trend in extremes were noted for winter. The annual CEI index plot averaged for all five temperature and precipitation indices (Figure 1) showed that five of the fifteen most extreme years on record occurred since 1997. Shorter-lived periods with high CEI values occurred in the 1930s and 1950s, in association with widespread extreme drought and above-average temperatures. The most extreme year in U.S. history was 1998, with 1934 a close second. The year 1998 was the hottest year in U.S. history, with a record 78% of the U.S. experiencing minimum temperatures much above normal. That year also had a record 23% of the U.S. with much greater than normal precipitation from extreme 1-day precipitation events. The 1934 extreme in CEI was due in large part because of the most widespread drought of the century--a full 52% of the U.S. was affected by severe or extreme drought conditions. That year also saw a record 64% of the U.S. with much above normal maximum temperatures.

The impact of maximum and minimum temperatures on the Climate Extreme Index
It is very interesting to look at the five separate indices that go into the Climate Extremes Index. Today I'll look at temperature, and next week, I'll focus on drought and precipitation. The portion of the U.S. experiencing month-long maximum temperatures either much above normal or much below normal has been about 10% over the past century (black lines in Figure 2). However, over the past decade, about 20-25% of the U.S. has been experiencing monthly maximum temperatures much above normal, and the portion of the U.S. experiencing much colder than normal high temperatures has been near zero. Minimum temperatures show a similar behavior, but have increased more than the maximums (Figure 3). Over the past decade, minimum temperatures much above normal have affected 25-35% of the U.S. This means that the daily range of temperature (difference between minimum and maximum) has decreased over the past decade, which is what global warming says should be happening if greenhouse gases are primarily to blame for the rise in temperatures.

While there have been a few years (1921, 1934) when the portion of the U.S. experiencing much above normal maximum temperatures was greater than anything observed in the past decade, the sustained lack of maximum temperatures much below normal over the past decade is unique. The behavior of minimum temperatures over the past decade is clearly unprecedented--both in the lack of minimum temperatures much below normal, and in the abnormal portion of the U.S. with much above normal minimum temperatures. Remember that these data ARE corrected for the Urban Heat Island effect, so we cannot blame increased urbanization on the increase in temperatures. Recall that the all-time record maximum and minimum temperature data, which I presented in a post in February, are not corrected for the Urban Heat Island Effect, but look very similar to the CEI maximum and minimum temperature trends presented here.

A lot of people have told me that they believe we are experiencing more wild swings of temperature from hot to cold from day to day in recent years, but the CEI data does not answer this question. To my knowledge, a study of this kind has not been done.


Figure 2. The Annual Climate Extremes Index (CEI) for maximum temperature, updated through 2008, shows that 20-25% of U.S. has had maximum temperatures much above normal over the past decade. Image credit: National Climatic Data Center.


Figure 3. The Annual Climate Extremes Index (CEI) for minimum temperature, updated through 2008, shows that 25-35% of U.S. has had minimum temperatures much above normal over the past decade. Image credit: National Climatic Data Center.

References
Gleason, K.L., J.H. Lawrimore, D.H. Levinson, T.R. Karl, and D.J. Karoly, 2008: "A Revised U.S. Climate Extremes Index", J. Climate, 21, 2124-2137.

Annual WeatherDance contest ready for registration!
Armchair forecasters, now's your chance to shine! WeatherDance, based on teams in the men's and women's NCAA basketball tournaments, allows players to predict which team's city will be hotter or colder on game day in each round of the Big Dance. Beginning today, players can make their forecasts at the Weather Dance Web site at: www.weatherdance.org. The site will be updated with cities promptly after NCAA seeding announcements. First round Weather Dance selections must be entered by 11:59 p.m. EDT Wednesday, March 18.

"Officially, Weather Dance began as a class project to get students involved in weather forecasting, but we kept it around because it got popular. People think they can do better forecasting than the meteorologists. Well, here's their shot!" said Perry Samson, WeatherDance creator, co-founder of the The Weather Underground, Inc., and Professor in the Department of Atmospheric, Oceanic and Space Sciences at the University of Michigan.

This is the fifth year for the game. Last year more than 2,000 people played. Most play merely for the thrill, but many K-12 science teachers involve their classes as part of meteorology units. The winning teacher will receive an expense-paid trip to join the Texas Tech/University of Michigan Storm Chasing team this spring for a day of tornado chasing in Tornado Alley. Other winners will receive a Weather Underground umbrella, "Extreme Weather" mugs, or a copy of the book "Extreme Weather," by Christopher C. Burt.

I'll talk about drought and precipitation trends in my next post, Monday or Tuesday.

Jeff Masters

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I've been off most of the winter
over at Sully's blog to watch the northeast.

Won't be long before we see some tropical events again though, eh?

I'll be around.
Thanks for keeping this blog going
Doc Masters...and all of you...!
Member Since: Posts: Comments:
Just want to say thanks for the
"Take a long walk off a short pier" remarks.

That was a favourite saying of my Mom's
and today would have been her 81st birthday.


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Quoting KoritheMan:


Isn't that what a lot of us (myself included) said last year, and well... >_>

'Course, I'm only joking, and you are entitled to your opinion about the season. I just thought I'd point it out that we all thought the same thing about 2008 and nature proved us wrong.


The water is cooler than it was this time last year.
Member Since: September 7, 2008 Posts: 17 Comments: 1604
Quoting KoritheMan:


If you're talking about how SSTs are well below average in the Caribbean, this will obviously make a big difference during the heart of the season, where quite a few storms typically traverse the Caribbean and enter the Gulf of Mexico. But I highly doubt March SSTs have any bearing on the season as a whole. After all, the Gulf is starting to warm up, with a few patches of 80F water -- this is because the cold is finally receding, albeit gradually. Once the heat kicks in, it doesn't take long for things to begin to heat up. The Gulf also happens to be above average, particularly the central and north-central parts of it, which could lead to trouble for storms that develop within the Gulf.


Have you seen the water south of Panama? There is a 90F spot surrounded by 60F water...lol
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Quoting BahaHurican:
I wonder if we are going to have the numbers without the ACE, like '06. It doesn't seem the heat is going to accumulate enough to give us large numbers of long-lived powerful storms.


If you're talking about how SSTs are well below average in the Caribbean, this will obviously make a big difference during the heart of the season, where quite a few storms typically traverse the Caribbean and enter the Gulf of Mexico. But I highly doubt March SSTs have any bearing on the season as a whole. After all, the Gulf is starting to warm up, with a few patches of 80F water -- this is because the cold is finally receding, albeit gradually. Once the heat kicks in, it doesn't take long for things to begin to heat up. The Gulf also happens to be above average, particularly the central and north-central parts of it, which could lead to trouble for storms that develop within the Gulf.
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Quoting SevereHurricane:


All indications point to an active season.
But I do think we will see a slow start and when August rolls around KA BOOM!


Isn't that what a lot of us (myself included) said last year, and well... >_>

'Course, I'm only joking, and you are entitled to your opinion about the season. I just thought I'd point it out that we all thought the same thing about 2008 and nature proved us wrong.
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Quoting SevereHurricane:


All indications point to an active season.
But I do think we will see a slow start and when August rolls around KA BOOM!
I wonder if we are going to have the numbers without the ACE, like '06. It doesn't seem the heat is going to accumulate enough to give us large numbers of long-lived powerful storms.
Member Since: October 25, 2005 Posts: 19 Comments: 22357
Quoting BahaHurican:
Agreed. I'm not convinced - yet - by the tentative conversations about a new el Nino, mainly because the ongoing PDO seems to be heavily reinforcing this current ENSO- cycle. Even if we do get to el Nino this year, which I doubt, it seems likely to be a weak one. Again, ideal conditions for a potentially active season.


All indications point to an active season.
But I do think we will see a slow start and when August rolls around KA BOOM!
Member Since: September 7, 2008 Posts: 17 Comments: 1604
Quoting Orcasystems:


No, 1935 would be a warming period


Oops... looking at the wrong thing.
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Quoting SevereHurricane:
I think we are going to see La Nina hang in there a little while longer than expected then go to Neutral for the Climax for Hurricane Season, hence a Highly Active Season looks to be in store for us this year...

Agreed. I'm not convinced - yet - by the tentative conversations about a new el Nino, mainly because the ongoing PDO seems to be heavily reinforcing this current ENSO- cycle. Even if we do get to el Nino this year, which I doubt, it seems likely to be a weak one. Again, ideal conditions for a potentially active season.
Member Since: October 25, 2005 Posts: 19 Comments: 22357
Quoting beell:


It's that "we all have to sleep sometime" thing that would scare the crap outta me.
Hot grits.

It's not just for breakfast anymore.. . .

Member Since: October 25, 2005 Posts: 19 Comments: 22357
Quoting hurristat:


You mean 1935?


No, 1935 would be a warming period
Member Since: October 1, 2007 Posts: 81 Comments: 26511
Quoting Orcasystems:


Figure 3. The Annual Climate Extremes Index (CEI) for minimum temperature, updated through 2008, shows that 25-35% of U.S. has had minimum temperatures much above normal over the past decade.

I don't want to call this cherry picking data to make a point... but I will do some cherry picking to make the exact opposite point.

The exact opposite statement could have been made in 1922, 1955, 1970, 1980.


You mean 1935?
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Figure 3. The Annual Climate Extremes Index (CEI) for minimum temperature, updated through 2008, shows that 25-35% of U.S. has had minimum temperatures much above normal over the past decade.

I don't want to call this cherry picking data to make a point... but I will do some cherry picking to make the exact opposite point.

The exact opposite statement could have been made in 1922, 1955, 1970, 1980.
Member Since: October 1, 2007 Posts: 81 Comments: 26511
Quoting hurristat:


I'm sorry... but spelling is atrocious...

MADNESS!!!


Even though the spelling was bad I liked his site, it was pretty cool and it was relativly organized.
Member Since: September 7, 2008 Posts: 17 Comments: 1604
Quoting TampaSpin:


Its not the spelling its the typing....if i typed what i was thinking now who knows what would be on print.......LOL


sorry... : !
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Quoting SevereHurricane:


Man your not lying,
Before my groth spurts started I was scared to death of my little sister... lol


Ya it's something i say that because i'm 6'2" lol
Member Since: August 12, 2007 Posts: 2 Comments: 2838
Anyways you'al betta get ye picks in. Ya hear......LMAO
Member Since: September 2, 2007 Posts: 178 Comments: 20443
Quoting hahaguy:


I hear ya my g/f is 5' 1" and 105lbs and she can fight and i've learned don't make her mad lol


Man your not lying,
Before my growth spurts started I was scared to death of my little sister... lol
Member Since: September 7, 2008 Posts: 17 Comments: 1604
Quoting hurristat:


I'm sorry... but spelling is atrocious...

MADNESS!!!


Its not the spelling its the typing....if i typed what i was thinking now who knows what would be on print.......LOL
Member Since: September 2, 2007 Posts: 178 Comments: 20443
Here is a graph showing both Dynamical and Statistical Models and you tell me what you think is gonna happen again...

Photobucket

I think we are going to see La Nina hang in there a little while longer than expected then go to Neutral for the Climax for Hurricane Season, hence a Highly Active Season looks to be in store for us this year...

Member Since: September 7, 2008 Posts: 17 Comments: 1604
Quoting TampaSpin:
Get your Portlight March Maddenss Picks in....

http://tampaspinsweather.webs.com/index.htm


I'm sorry... but spelling is atrocious...

MADNESS!!!
Member Since: Posts: Comments:
350. beell
10" of rain in one day would cause problems most anywhere. Most S. Floridians would be up for it I bet.

An immediate end to the fire danger in the short-term if not the drought as a first benefit
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Get your Portlight March Maddenss Picks in....

http://tampaspinsweather.webs.com/index.htm
Member Since: September 2, 2007 Posts: 178 Comments: 20443
Quoting TampaSpin:


My 5'ft 100lb'er would kill my butt also.....LOL..i have came to the conclusion the smaller the meaner......LOL


I hear ya my g/f is 5' 1" and 105lbs and she can fight and i've learned don't make her mad lol
Member Since: August 12, 2007 Posts: 2 Comments: 2838
Quoting Vortex95:
Just wondering lets say 10 inches of rain falls tommorow in so fla would it flood more in a drought or if it has been raining a few days in a row?


Is the question 10 in in one day or 10 over a few days or raining for a few days then 10 in in one day?

The order of flash floods would be
3
2
1

I am not sure about the last two however.
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Quoting presslord:
One thing I know for sure about my wife....if I ever hit her, I'd better make sure I kill her with the first punch....otherwise, I'm a goner...


My 5'ft 100lb'er would kill my butt also.....LOL..i have came to the conclusion the smaller the meaner......LOL
Member Since: September 2, 2007 Posts: 178 Comments: 20443
343. beell
Quoting presslord:
One thing I know for sure about my wife....if I ever hit her, I'd better make sure I kill her with the first punch....otherwise, I'm a goner...


It's that "we all have to sleep sometime" thing that would scare the crap outta me.
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Quoting Vortex95:
Just wondering lets say 10 inches of rain falls tommorow in so fla would it flood more in a drought or if it has been raining a few days in a row?


Vortex 10 inches of rain anywhere is alot of rain....that would flood under any conditions but, with florida primarily an all sand base landscape water soaks up very fast unlike the northern states. Theres alot of places for it to run off in Florida also....
Member Since: September 2, 2007 Posts: 178 Comments: 20443
One thing I know for sure about my wife....if I ever hit her, I'd better make sure I kill her with the first punch....otherwise, I'm a goner...
Member Since: August 13, 2007 Posts: 0 Comments: 10492
You guys still following Redoubt?
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Redoudt...

Member Since: September 2, 2007 Posts: 178 Comments: 20443
Quoting TampaSpin:


Did he ask for a life vest....ROFLMAO


Or how bout sending fish to "go live with the humans"??
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I think i will ask to sit on the Board of Directors of Portlight Stratagies and become the HR Director in charge to Short Piers!!!!...LMAO
Member Since: September 2, 2007 Posts: 178 Comments: 20443
Quoting presslord:
I just told my son to go take a long walk off a short pier...I just love that...


Did he ask for a life vest....ROFLMAO
Member Since: September 2, 2007 Posts: 178 Comments: 20443
329. beell
Be careful there, press...
"Hello? Police? My Dad just punched my Mom..."

(See Post 310. for the Truth)
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Quoting aquak9:
Portlight is supported by many WU members, DDR.

So is Surfmom.

You? long walk, short pier, please.

'nuff said.


Heck, Portlight is supported by WU itself and run through WU almost.
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Quoting presslord:
I just told my son to go take a long walk off a short pier...I just love that...

LMAO i bet he was scratching his head after that one.
Member Since: August 12, 2007 Posts: 2 Comments: 2838
I just told my son to go take a long walk off a short pier...I just love that...
Member Since: August 13, 2007 Posts: 0 Comments: 10492
Good talk tonight!
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Quoting presslord:
It's a date KEH...and Tampa...thanks for the Portlight effort....


Gonna push it hard....hope everyone can email everyone on their email list outside of WU and help the Cause. It will be needed!
Member Since: September 2, 2007 Posts: 178 Comments: 20443
Press how do you like the Portlight March Madness Event. Hope it goes well!
Member Since: September 2, 2007 Posts: 178 Comments: 20443
It's a date KEH...and Tampa...thanks for the Portlight effort....
Member Since: August 13, 2007 Posts: 0 Comments: 10492

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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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