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Global Warming May Have Fueled March Heat Wave Odds

According to several top scientists, the March heat wave that has shattered records across a wide swath of the U.S. bears some of the hallmarks of global warming.

In email conversations on Wednesday and Thursday, those same scientific researchers who specialize in studying the role climate change plays in influencing individual extreme events — a burgeoning field known as  "extreme event attribution” — said global warming may have made March's soaring temperatures more likely to occur, although they add that natural variability has played a key role as well.

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

Since March 12, more than 7,000 warm temperature records (warm daily highs and warm overnight lows) have been set or tied, including numerous all-time monthly high temperature records.

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.

Gabi Hegerl, Chair of Climate System Science at the University of Edinburgh, said there is evidence that extreme heat events have become more common and more severe, including at the regional level in parts of the U.S. ”This is consistent with observing more and stronger heat waves,” she said.

Hegerl said that in order to draw conclusions about global warming’s role in this particular heat wave, one would need to conduct modeling studies where you compare the odds of this event occurring with and without added greenhouse gases in the atmosphere, such as carbon dioxide, “to see how much the warming has changed the odds.”

The reasoning behind such an approach is because both global-warming trends and shifts in atmospheric circulation may contribute to a certain event, so scientists need to observe the whole climate system at play to investigate how often an event with these extreme characteristics takes place under the two alternative scenarios.

Surface temperature departures from average from march 13-19, showing the heat in the U.S. and Canada (in orange and red). Click on image for a larger version. Credit: NOAA/ESRL.

Studies that have followed this probability-based approach, such as one that examined the deadly European heat wave of 2003, and another that investigated the brutal Russian heat wave of 2010, have found that changes in greenhouse gases can significantly increase the odds that such severe events would occur.

A study published in the Proceedings of the National Academy of Sciences in October 2011 found that the local warming trend in the Moscow area had increased the expected number of records per decade there by five times what it otherwise would have been. The study also found that there is an 80 percent chance that the 2010 July monthly heat record would not have occurred without global warming.

This study would suggest that global warming is an enabler of extreme heat events, making them more likely to occur, but not necessarily causing each specific one, or accounting for each characteristic of a particular event.

In some ways global warming acts as an accomplice in a crime, not necessarily pulling the trigger, but still playing a significant role.

One view that some researchers expressed is that since temperatures are exceeding average values and breaking records by such wide margins, it’s likely that the comparatively modest global warming to date — U.S. temperatures have warmed by an average of about 1 degree Fahrenheit during the past century — doesn’t account for the full magnitude of what’s been going on in so much of the country this month.

As to whether global warming might be contributing to the recent hot wave, Randall M. Dole, a deputy director of research at NOAA’s Earth System Research Laboratory (ESRL) in Boulder, Colo., said, "Absolutely. The planet as a whole is warming, the continents on average are warming faster than the oceans, so there is a great body of scientific evidence that would support such an interpretation. The question is how much.”

“ . . . It's hard to attribute more than a modest fraction of the event magnitude to [manmade climate change] itself. It could well have made a truly extreme event even warmer,” Dole said.

So, according to him, global warming stacked the deck in favor of an unusual heat event, but does not fully account for how intense this event has been. He pointed to the setup of weather systems and a lack of extensive snow cover as being partly responsible for exceeding records by such wide margins.

Satellite picture taken March 21, 2012, showing the blocking high and heat dome in place over the Great Lakes and East, and a slow-moving storm over the South Central states. Click on the image for a larger version. Credit: NOAA.

Unlike other heat waves studied recently, this one occurred during March, which is normally a season of transition when the presence or lack of snow cover can make a huge difference for daily high and low temperatures. Snow reflects incoming sunlight, and tends to keep the surrounding air cooler than it would otherwise be. The lack of an extensive snowpack this year helped allow temperatures to soar to record levels during this heat wave, Dole said.

It’s also worth noting, a few of the researchers said, that this event featured a massive dome of high pressure that blocked the progression of weather systems. Such “blocking patterns” are often associated with temperature and precipitation extremes, and were present during the 2010 Russian heat wave and the 2003 European heat wave, as well. However, there is a lot of uncertainty about what causes blocking events or how global warming influences them.

Martin Hoerling, a research meteorologist at ESRL, agreed with Dole, saying that although global warming is likely playing a role in this event, it probably did not play a major one. “Meteorology, not climate change, is the main ingredient in the current March 2011 U.S. extreme warmth,” he wrote. Of climate change, he said, “. . . its contribution to the magnitude of current conditions (+30°F departures [from average]) is quite small (but not zero) indeed.”

Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research, said he views the situation somewhat differently. “Indeed [greenhouse gas-driven] warming is not dominant, but I suspect when all the evidence is in we will find that the event likely would not have occurred without global warming, the odds will be so low,” he said.

It will take many months for scientists to conduct the rigorous analyses necessary to determine how global warming may have set the stage for this heat wave. For now, weather forecasters are left scratching their heads over the staggering records that have been set. In some places, such as Rochester, Minn., the overnight low temperature on March 18 was so warm that it broke the daily high temperature record.

In Chicago, where 80°F temperatures are rare even in April, the temperature exceeded that mark for a record eight days this month — including an 87°F high temperature on March 21. Chicago’s average temperature for the first 21 days of March would tie for the seventh-warmest April on record there, the National Weather Service reported.


By Tim Kelley (02066)
on March 23rd, 2012

Extreme Heat, Extreme Cold, Extreme Snow, Extreme Flood, Extreme Drought, Extreme Obesity..
all attributed to Climate Change

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By Andrew
on March 23rd, 2012

Hi Tim,

It may seem that way sometimes, like journalists and advocates are trying to paint every extreme event as being related to climate change. But at least in the case of extreme heat, the scientific evidence is rather robust, since if you shift the mean you get a significant shift in the tails of the distribution.

I’ll have to get around to doing the obesity story you suggest though. It is, after all, an epidemic in this country. wink


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By Jake Gless (Grand Rapids, MI 49509)
on March 23rd, 2012

Andrew, I’m no meteorologist, but I’ve got this theory that the Bakken oil fields are causing this stretch of many months of incredibly warm weather.  Could you please shoot my hypothesis down?

Our mild winter was caused by the jet stream, according to this Scientific American article from January. There were never any of the dips or fluctuations referred to as Alberta Clippers. Prominent excerpt: “...the most extreme configuration of the jet stream ever recorded.”

The phenomenon of Urban Heat Islands often effects the surrounding climate, “including the altering of local wind patterns, the development of clouds and fog, the humidity, and the rates of precipitation”.

For more than 2(?) years now, there’s been a fracking bonfire burning continuously in the NW corner of North Dakota. Why? Because the natural gas they’ve unearthed is so abundant that it’s value has crashed, so they’re burning almost all of it off to get to the oil.

Don’t believe man-made activity could be significant enough to affect the climate? Check out the scope of the drilling through this time-lapse video of 2010 alone:

Don’t trust computer graphics? Watch some film from space. That non-stop bonfire covers more earth than a handful of small states:

Now let’s scroll halfway down temperature anomalies from this January:

All that cold air bottled up by the jet stream has to go somewhere, right? Cold snap, Europe:

Still, I just don’t get it. What’s causing this jet stream-induced heatwave?

So, I surmise that as the jet stream comes west to east over the Rockies, it’s being directed on its path past the continental US. The cause of this is the expansive Bakken oil fields, at the southern edge of the N. Atlantic Oscillation, located where the land had been barren for eons. Note that this is an hypothesis of acute climate change, not global warming.

They say trees are 90% air, but birds can’t fly through them. My uncle says changing the jet stream is akin to peeing in a river. I would really appreciate it if someone could tell me why my theory is a bunch of hooey.

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By chuck greene (ithaca,ny 14853)
on March 24th, 2012

Many factors determine winter weather patterns in US. El Nino-Southern Oscillation has an important influence on winter weather patterns throughout the continental US. In the eastern US, the Polar Jet Stream shifts southward during El Nino years, bringing colder and harsher winter weather, while it shifts northward during La Nina years, bringing warmer and milder winter weather. Negative Arctic Oscillation (AO) conditions can amplify the cold, harsh winters expected during El Nino years, as was observed during the severe winter of 2009/2010. Negative AO conditions can produce severe weather that runs counter to the warm, mild winters expected during La Nina years. This was observed during winter 2010/2011. The unusually mild weather observed in the eastern US this past winter serves as a reminder that no two winters are alike and there will always be some uncertainty in forecasting the weather. As in 2010/2011, this year’s winter weather was forecast to be mild due to a La Nina in the Pacific. When negative AO conditions emerged during mid-January and Europe was pummeled by deadly cold and heavy snowstorms, weather in the eastern US remained unseasonably mild. Unlike the previous year, La Nina continued to steer the Polar Jet Stream further north over North America during mid-winter’s negative AO conditions, allowing the eastern US to experience its fourth warmest winter on record. By early March, a strong blocking pattern developed in the eastern Pacific that further amplified these conditions, resulting in record high temperatures throughout the eastern US and an early onset of tornado season in the Midwest. It is unclear what role greenhouse warming may have played in the development of this blocking pattern and the amplified warming associated with it. Nevertheless, weather extremes have dominated the headlines during recent years, and man-made changes in the climate system will increasingly stack the deck in favor of weather surprises once considered to be low-probability events.                                       

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By Tabuism (Toronto ON. Canada)
on March 24th, 2012

Mountain butterflies, wildflowers hit by climate change

Countless Aspenites who’ve made the wildflower-spotting pilgrimage over West Maroon Pass to Crested Butte have passed through the historic mining settlement at Gothic, and seen the “Rocky Mountain Biological Laboratory” sign affixed to a wooden fence.
Many have likely wondered what goes on in there. A few dozen locals found out Thursday night, as Rocky Mountain Biological Laboratory (RMBL) president Ian Billick outlined recent findings, in a presentation at the Aspen Center for Environmental Studies.
Billick, who first came to Gothic in 1988, reported that the mountain lab’s recent studies shed light on how climate change is affecting mountain ecosystems and the rest of the world. The life cycles of everything from butterflies to wildflowers are changing.

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By Nigel Thornley (Hantsport, Nova Scotia, Canada)
on March 24th, 2012

Too much careful ivory tower science. I think the answer is simple and it points soundly to the extra thickness of the Carbon Dioxide ” blanket ” that is over us. The Weather Network has a neat graph, where they track the present temperatures .... max high and min low… against the 40 year average… and, for the last two years, at least…. the night time temperatures have been, almost without any day exception…. many degrees warmer than that average. Since it is in the night that the blocking of heat loss into the cold of space is most critical… this tells me that the extra insulating layer is having a dramatic affect on our night time weather. In turn, this has set up the conditons for exceptional warmth… in particular this last March… because those elevated night temperatures all winter allowed the snow to melt as fast as it fell, so our ground was basically bare….. and, once the sun got towards a good warming angle, the positive feedback of the heat of the sun hitting dark, bare ground…. made a terrific heat gain, where, in our more normal year… that heat would have been reflected back out into space, by the thick white layer of snow, and it would only have been late April that it would normally melt and allow the heating to begin. I say this has CO2 finger print clearly and strongly all over it. Check the night time temps people….. and they are not affected by any sun output variance… only radiative coolling out into space.

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By Gene Humme (Hartford, Ct)
on March 25th, 2012

Is it possible that there is added volatility to weather patterns do to a change in the tilt of the earth? There does not seem to be any mention of cosmological events that the earth has not been through in a few thousand years. I have to believe that it could factor into some of the dramatic events that are taking place on earth. They are not all weather related, but the weather is most noticeable to us laymen.

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By Steve Bloom (Oakland, CA 94609)
on March 29th, 2012

Hmm, no mention of Francis and Vavrus (2012), Andrew?  Just ask Marty Hoerling, though, since it’s unreasonable to think that the, um, shifting of the entire ocean-atmosphere circulation system would affect anything like meteorology, right?

But a very balanced article overall, in the fine tradition of the press.  Maybe you’ll get an award for it!

What’s especially great about this sort of thing is that it will be covered as a scientific dispute and so will fail to motivate action, and then after it’s been settled won’t be news anymore, since after after all it will have already gotten a bunch of coverage.

Sarcasm aside, overall I would expect better from an article on Climate Central.

Trying to be constructive here, how could you do better?  Well, it would take more time, but off-hand you’d have to look into Marty’s modeling runs (published? credible? what about model results generally as regards blocking event frequency?) and maybe talk to some more people in the field to see if there’s any sort of consensus (a fair number of other folks study large-scale atmosphere dynamics), focusing on blacking event intensity/frequency specifically (we have detailed weather records going back a good number of years, so what are the stats?  how do Stu Ostro’s stats on increasing pressure heights relate?), and consider the extremes in the context of known major circulation changes such as the expansion of the tropics, the shift of the Atlantic front into the Arctic and any changes related to polar amplification.  As Francis says, how could there not be large-scale consequences?

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By Jim (Jacksonville Florida )
on April 9th, 2012

This is not “proof” of global warming.  Remember when it snowed early October 2011 in the northeast US everywhere?  Global Warming scientists kept reminding us it was weather, NOT climate.  But when we have a warm March in the US, its HUGE proof that global warming exists.

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By Jim (Junction City kansas 66441)
on May 16th, 2012

The jet stream has been acting oddly for the past 6 years and has given us both the coldest and wettest winter and the hottest and driest winter, in a long time the cause isn’t global warming when it snowed in Rome and people in Moscow wore shorts it’s the jet stream acting oddly and no one can explain why. Any “reason” given is pure speculation without scientific evidence to prove it. Solar activity has been put forth as well as melting ice caps but no one has any proof on either side of the fence. If you are working in a science lab and yell out eureka before the experiment is over you are a fool and the same applies to complex global climate conditions, we are all in the lab, let’s get our facts together before making any decisions.

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