Correlation demonstrated between cosmic rays and temperature of the stratosphere

This offers renewed hope for Svensmark’s theory of cosmic ray modulation of earth’s cloud cover. Here is an interesting correlation published just yesterday in GRL.

Cosmic rays detected deep underground reveal secrets of the upper atmosphere

sh-stratospheric-heating-by-cosmic-rays

Watch the video animation here (MPEG video will play in your media player)

Published in the journal Geophysical Research Letters and led by scientists from the UK’s National Centre for Atmospheric Science (NCAS) and the Science and Technology Facilities Council (STFC), this remarkable study shows how the number of high-energy cosmic-rays reaching a detector deep underground, closely matches temperature measurements in the upper atmosphere (known as the stratosphere). For the first time, scientists have shown how this relationship can be used to identify weather events that occur very suddenly in the stratosphere during the Northern Hemisphere winter. These events can have a significant effect on the severity of winters we experience, and also on the amount of ozone over the poles – being able to identify them and understand their frequency is crucial for informing our current climate and weather-forecasting models to improve predictions.      

Working in collaboration with a major U.S.-led particle physics experiment called MINOS (managed by the U.S. Department of Energy’s Fermi National Accelerator Laboratory), the scientists analysed a four-year record of cosmic-ray data detected in a disused iron-mine in the U.S. state of Minnesota. What they observed was a strikingly close relationship between the cosmic-rays and stratospheric temperature – this they could understand:  the cosmic-rays, known as muons are produced following the decay of other cosmic rays, known as mesons. Increasing the temperature of the atmosphere expands the atmosphere so that fewer mesons are destroyed on impact with air, leaving more to decay naturally to muons. Consequently, if temperature increases so does the number of muons detected.

What did surprise the scientists, however, were the intermittent and sudden increases observed in the levels of muons during the winter months. These jumps in the data occurred over just a few days.  On investigation, they found these changes coincided with very sudden increases in the temperature of the stratosphere (by up to 40 oC in places!).  Looking more closely at supporting meteorological data, they realised they were observing a major weather event, known as a Sudden Stratospheric Warming.  On average, these occur every other year and are notoriously unpredictable. This study has shown, for the first time, that cosmic-ray data can be used effectively to identify these events.

Lead scientist for the National Centre for Atmospheric Science, Dr Scott Osprey said:  “Up until now we have relied on weather balloons and satellite data to provide information about these major weather events.  Now we can potentially use records of cosmic-ray data dating back 50 years to give us a pretty accurate idea of what was happening to the temperature in the stratosphere over this time.  Looking forward, data being collected by other large underground detectors around the world, can also be used to study this phenomenon.”

Dr Giles Barr, co-author of the study from the University of Oxford added: “It’s fun sitting half a mile underground doing particle physics. It’s even better to know that from down there, we can also monitor a part of the atmosphere that is otherwise quite tricky to measure”.

Interestingly, the muon cosmic-ray dataset used in this study was collected as a by-product of the MINOS experiment, which is designed to investigate properties of neutrinos, but which also measures muons originating high up in the atmosphere, as background noise in the detector. Having access to these data has led to the production of a valuable dataset of benefit to climate researchers.

Professor Jenny Thomas, deputy spokesperson for MINOS from University College London said  “The question we set out to answer at MINOS is to do with the basic properties of fundamental particles called neutrinos which is a crucial ingredient in our current model of the Universe, but as is often the way, by keeping an open mind about the data collected, the science team has been able to find another, unanticipated benefit that aids our understanding of weather and climate phenomena.”

Dr Osprey commented: “This study is a great example of what can be done through international partnerships and cross-disciplinary research. One can only guess what other secrets are waiting to be revealed.”

h/t to Ron de Haan

Advertisements

  Subscribe  
newest oldest most voted
Notify of
Ron de Haan

Anthony, I think you are the fastest publisher on earth.

sdk

this is really a fascinating article, and worthy of follow up and more depth. thanks for getting this posted.

Neil Hampshire

Great to read of some real investigative science taking place instead of the recent data manipulation claiming to show Antarctica is warming

Katherine

That animation was an eye-opener! I take it the red symbolized temperature increases of 40°C?

Alan the Brit

Can’t be true, the BBC Science & Environment blog hasn’t got that story at all. The best they can manage so far is “Climate Shift Killing Trees in Western US”! They’ll let that one slip through the net for a while, then when they feel the need they’ll regurgitate it for general consumption when no one will notice, say when unemployment hits 3M, or a major scandal occurs with some half-wit celebrity somewhere, the usual technique of burying interesting science. Now that’s one for the books, not Climate Change, not Climate Chaos, not Climate Catastrophe, but Climate Shift! Anything to keep us on our toes I guess.
Personally I think it makes fascinating reading & certainly lends ever more credence to Svensmark’s theories on Galactic Cosmic Rays

Chris H

I don’t see how this has anything to do with Svensmark’s theory….

Jonathan

Anthony, I am pretty sure you have got the causal link the wrong way round on this one. The paper is all about how stratospheric warming leads to enhanced detection of muons produced by cosmic rays.
REPLY:I understand where you are coming from, but I said nothing about a “causal link”. You did. The only thing I said is that it “gives new hope for Svenmark’s theory”, since it has been demonstrated that there is a measurable link between cosmic rays and the temperature of the upper atmosphere. This is important, because it has never been shown before. The question is, what drives the change and how is it related to mesons and is the sudden stratospheric warming related to it? Nobody knows the cause of SSW, so this provides a tantalizing clue. – Anthony

DocWat

Heaven forbid, someone should find evidence that the sun affects climate/weather here on earth.

This is a fascinating story, and I will be very interested to know more. Re the CLOUD experiment at CERN, has anyone heard more about when this is scheduled to take place? 2010? I know they’ve had some problems recently at the LHC, just wondering if it has affected the timetable much.

Katlab

Anthony, if you keep publishing articles about SCIENCE, you are going to ruin your reputation, as flat-earther. Maybe after this is investigated we should call ourselves deep-earthers.

jmrSudbury

Your “detector deep underground” phrase in particular caught my attention. Now I have more questions for the scientists that work at SNO (Sudbury Neutrino Observatory) where I have worked for a mere 3 months. We are much deeper underground than the Minos lab; we are at the 6800 foot / 2 km level beneath the surface.
I would like to join the chorus and reiterate Dr Giles Barr’s words in that it is indeed fun to sit far underground doing particle physics. I am not a scientist. I am in the operations group, but I enjoy what I have been learning about various topics while at SNO.
Thanks for posting an article in terms that I can understand.
John M Reynolds

Anthony, there have been a number of interesting press releases in the last few weeks concerning the workings of the Upper Atmosphere. I was only highlighting the two below the other day on another site.
Very interesting indeed. A coherent forcing mechanism seems just around the corner:
Earth Atomsphere “Breathing” due to UV: http://www.colorado.edu/news/r/32abc3f111047d1a5ec153cc27e63d5d.html
Also, Infrared waves due to gaseous molecule radiation from NASAs Langley research:
http://www.livescience.com/space/081216-agu-breathing-atmosphere.html
“The changes in heating that cause the breathing can also impact climate, by triggering the upper atmosphere’s “thermostat,” as study team member Martin Mlynczak of NASA’s Langley Research Center, Hampton, Va., put it. The added UV radiation heats up the atmosphere, in turn causing gaseous molecules to radiate that heat away in the form of infrared radiation.”

Phillip Bratby

It’s good to see real science without any political undertone.

Flanagan

Hi,
i’m not sure I really understand how these (very interesting) results corroborate Svensmar’s theory about cloud formation?
REPLY: I didn’t say it corroborated, you did, I said it provides “new hope for Svensmark’s theory” because it shows a measurable connection between cosmic rays and the upper atmosphere, something that has not been demonstrated before. The question is, what drives the change and how is it related to mesons and is the sudden stratospheric warming related to it?- Anthony

Wait for the alarmists to scream that correlation is not proof and it is not causality either. It is the only way that they can get there 800 year lag in CO2 increases from temperature increases to look like CO2 is driving climate change to our impending doooooooooom.
Sure this needs to be looked at further by other teams and independently verified as befits real and proper science, but it is a very good and exciting start and instinctively (rather than scientifically) makes much more sense to me than CO2 being the driver of climate change.

H.R.

From the article:
“What they observed was a strikingly close relationship between the cosmic-rays and stratospheric temperature – this they could understand: the cosmic-rays, known as muons are produced following the decay of other cosmic rays, known as mesons. Increasing the temperature of the atmosphere expands the atmosphere so that fewer mesons are destroyed on impact with air, leaving more to decay naturally to muons. Consequently, if temperature increases so does the number of muons detected.”
Hmmmm… another proxy for temperature. What I make of it is that it will be a more accurate proxy than tree rings (I know, I know. One’s a proxy for troposphere and the other for stratosphere temps – apples and oranges).
Unfortunately, unless I misunderstood, we have data of this sort that only goes as far back as when people started digging holes in the ground and started looking for neutrinos – that’s 50 years – but it seems to be good data.

wow, what a beautiful piece of work and what beautiful timing. It starts to look as if cosmic rays might affect not only low-cloud formation but also the temperature direct as well as the ozone concentration. Great that this has come through the mainstream where I hope they will look further at the solar wind correlation.

realitycheck

Very interesting article – thanks Anthony. So this would basically imply that Sudden Stratospheric Warmings (SSW) should be more frequent during solar minimums (since low solar flux allows an increase in the cosmic radiation recieved at Earth)
It is generally accepted that the Artic Oscillation and/or North Atlantic Oscillation tend to shift negative up to 10 to 20 days following these SSW events – i.e. in meteorological terms SSWs tend to lead to high-latitude “blocking” – patterns which tend to generate intense cold surface high pressure systems near the poles and displace them southwards into the mid-latitudes. i.e. they typically lead to strong cold outbreaks into the mid-latitudes.
This is interesting since we appear to be undergoing a significant SSW as we write…
http://www.cpc.ncep.noaa.gov/products/stratosphere/temperature/10mb9065.gif

Ozzie John

Interesting Article….!
Are there any ideas thrown forward which might explain the sudden increase in the cosmic rays? I noticed an article published by NASA recently mentioning holes in the earths magnetic field being caused by CME’s. Not sure if these events could cause cosmic rays to suddenly stream into our atmosphere ?
Will look forward to further updates on this topic !

Anthony – thanks for this – its really useful to have you as a monitor of breaking science. I would love to know how this stratospheric heating ties in with UV heating and the shift of the jetstream studied by Drew Shindell at NASA – he had a paper in 2001 I think that found a correlation with UV heating, southerly shift of the jetstream and the Maunder Minimum – maybe this effect also contributed.

Alan Chappell

Dr. Scott Osprey commented.
“one can only guess what other secrets are waiting to be revealed”
Unlike Nero (fiddling) or Gore/Hansen this is not a ‘done’ scientist, well done Scott.

lgl

How is this a “renewed hope for Svensmark’s theory of cosmic rays”?
Have they found a correlation between stratospheric temp and weather in lower troposphere?

Now there’s some data I’d like to get my hands on… yummy.

The explanation of the video featuring sudden stratospheric warming occurring in the southern hemisphere states that ‘This is the only such event recorded in the southern hemisphere (they normally occur in the northern hemisphere).’
Well, that’s pretty interesting. Wonder if that has anything to do with Antarctica’s relative ice stability as compared to the Arctic.

mark

based on this: ““What they observed was a strikingly close relationship between the cosmic-rays and stratospheric temperature – this they could understand: the cosmic-rays, known as muons are produced following the decay of other cosmic rays, known as mesons. Increasing the temperature of the atmosphere expands the atmosphere so that fewer mesons are destroyed on impact with air, leaving more to decay naturally to muons. Consequently, if temperature increases so does the number of muons detected.”
it sounds to me like jonathan was correct. what am i missing? it sounds like this is saying that WHEN the stratosphere warms, more mesons get through…..wouldn’t this be almost the opposite of what is being suggested on here? not being argumentative….i agree with most of what i read on here, but i am having difficulty understanding this one….

Richard Heg

This article from sciencedaily which says that seasons have changed in a way not predicted by the IPCC(therefore AGW), suggests that there have been changes in atmospheric circulation that have changed the seasons. Could the change in the stratosphere cause these changes in atmospheric circulation? then in turn cause the change in global temperature? Just a taught
http://www.sciencedaily.com/releases/2009/01/090121144053.htm
“Although the cause of this seasonal shift – which has occurred over land, but not the ocean – is unclear, the researchers say the shift appears to be related, in part, to a particular pattern of winds that also has been changing over the same time period. This pattern of atmospheric circulation, known as the Northern Annular Mode, is the most important wind pattern for controlling why one winter in the Northern Hemisphere is different from another. The researchers found that the mode also is important in controlling the arrival of the seasons each year.
Whatever the cause, Stine said, current Intergovernmental Panel on Climate Change (IPCC) models do not predict this phase shift in the annual temperature cycle.”

Pearland Aggie

I guess I missed the part about how this discovery is linked to low level nucleation and cloud formation?

DR

Is there a link between Earth’s magnetic field and low-latitude precipitation?
http://geology.gsapubs.org/cgi/reprint/37/1/71.pdf
“In addition to supporting the notion that variations in the geomagnetic field may have influenced Earth’s climate in the past, our study also provides some
degree of support for the controversial link between GCR particles,
cloud formation, and climate.”

Alan the Brit

IgI:-(
Perhaps not yet, but who knows what’s around the corner. It’s like so many things in life around the world, if you can’t find the right answers, perhaps you’re not asking the right questions! Modern Climate Science is in its infancy, only 35-45 years or so old, in contrast to the paleoclimate records we have uncovered, & even then it’s only a learned assumption. I believe we know an awful lot less than we have yet to discover! If computer models know all about it, why are the perpetrators sending teams out to the Pacific to study cloud formation & water vapour effects?;-)

anna v

“What they observed was a strikingly close relationship between the cosmic-rays and stratospheric temperature – this they could understand: the cosmic-rays, known as muons are produced following the decay of other cosmic rays, known as mesons. Increasing the temperature of the atmosphere expands the atmosphere so that fewer mesons are destroyed on impact with air, leaving more to decay naturally to muons. Consequently, if temperature increases so does the number of muons detected.”
To a particle physicist, as I am, this is gobbledygook. I need to read the paper. Does anyone have a link?

Mike Bryant

Prediction… Within thirty days, a new study will assert that this recent exciting find has nothing whatsoever to do with global warming, or that this newly discovered relationship has been increasing in intensity because of increased CO2 in the atmosphere. The new study will come out of England.

J.Hansford.

Well lgl, I suppose it shows evidence of Cosmic rays having an effect on the atmosphere causing a change in the stratosphere…. It gives credibility for further investigation as to other interactions, namely cloud formation….. at a guess anyway.
Whereas the AGW proponents had been completely dismissive of any effect from Cosmic rays…. they are now forced to revisit and rethink….
That would be my thoughts on it anyway.

Bill Marsh

Not sure I would agree with the beginning of your post that this ‘offers renewed hope’ for Svensmark’s theories. I personally don’t think his theories are in need of ‘renewed hope’ as this implies that they have been falsified and this may offer some validation to restore them. I don’t think his theory has been falsified, at least not from the work I’ve seen.
Anyway, this is interesting and I plan on reading it more thoroughly after work.
Thanks for posting it.
REPLY: I can see where you are coming from there. But every theory, including Svensmark’s is just a theory until observation and experiment can validate it. This is one step closer to validation for Svensmark becuase whil it is not exactly the same as his theory, it does demonstrate that GCR’s do modulate the Earth’s atmosphere. I hope it will spur renewed searching for cloud modulation links or some other link related to GCR’s – Anthony

Chris H (and many others)

I don’t see how this has anything to do with Svensmark’s theory….

If the elevated muon flux seen 1/2 mile underground also shows up in clean low level air over oceans, then there should be an increase in low level stratus clouds. If observed, that would support Svensmark’s hypothesis.
However, this can’t be a global event, because the whole atmosphere would expand and the same amount of air would would be in the atomospheric column.
From the mine’s location, the referenced video, and my lack of knowledge of stratospheric warming events, I don’t know if these occur at lower latitudes. It may be that an event at higher latitudes pushes enough air toward the equator that there is decrease in muon flux there and hence fewer muon-induced clouds.
Clearly, there is work to do….
A couple photos of the lab are at http://www.dnr.state.mn.us/state_parks/soudan_underground_mine/physicslab.html

Anthony,
Daily like so many reading you I am fascinated by the information you provide and especially, like others who comment, in a way I can understand.
The story also seems ground breaking (excuse the Pun) as it tells of a district possibility for more accurate weather predictions using accessible stable underground cosmic ray modulation data directly to understand waether activity that now has to be sent back by weather balloons.
You meanton Dr Giles Barr, Professor Thomas and the comment by Dr Scott Osprey. I can imagine they are ecstatic as are any colleagues who worked with them to help research and publish these findings and the great video to back it up. Congratulations must go to them for this fact based intelligent research and scientific contribution.
Tracking some of the comments too I think testifies the value to this work.
I look forward to the Anthony day when you educate me enough to be able to add more than thank you notes. In the meantime like 7 million others I am staying tuned.

Bill Marsh

Ozzie,
I don’t think the increase in GCR has been sudden and it most likely is related to the decrease in solar activity. At least according to Svensmark, the more energetic cosmic rays coming from beyond the solar system are not affected by the earth’s magnetic field overmuch, they contain too much energy. They are affected by the Sun’s magnetic field and by the intensity of the solar wind, both of which have decreased. This is what has allowed more of the existing GCR to penetrate the earth’s atmosphere and create those interesting showers of secondary particles.
According to some work I’ve read the level of GCR changes over geologic timescales based on the solar systems orbit of the Galaxy and its location in the plane of the Galaxy, but those changes occur over millions of years, not a few decades or a few years. I’ve also seen some speculation that the solar system entered a sort of ‘magnetic bubble’ in near space that is increasing the number of GCR, but that again is over the course of a few million years.
Bottom line is the number of GCR available seems to be relatively constant in the short term (barring a supernova event locally or some other sudden event) but the intensity of the sun’s magnetic field and solar wind regulate the number that approach earth. This can change over the short term.

Ninderthana

Anthony,
The article suggests that an increase in the level of muons detected
in deep mines is dependent on the stratospheric temperature. This would seem to indicate that muon level is being driven by the stratospheric temperature, assuming that the meson cosmic ray flux at the top of the atmosphere remains unchanged.
Is there any evidence that a significant increase in the meson cosmic ray flux at the top of the atmosphere can actually lead to significant heating in the stratosphere?

MartinGAtkins

Oceans are cooling according to NASA.
http://tinyurl.com/afnvub

DaveE

lgl (04:05:28) :
“How is this a “renewed hope for Svensmark’s theory of cosmic rays”?
Have they found a correlation between stratospheric temp and weather in lower troposphere?”
From the article.
“These events can have a significant effect on the severity of winters we experience, and also on the amount of ozone over the poles – being able to identify them and understand their frequency is crucial for informing our current climate and weather-forecasting models to improve predictions.”
DaveE.

Tom in getting warmer Florida

So which comes first, the stratospheric warming by other causes leading to greater detection of muons or lower solar flux allowing more mesons to hit the stratosphere causing the warming which then shows up as a detection of more muons?

Luis Dias

This is becoming utterly ridiculous. Correlation IS NOT causation, and as someone pointed out earlier, you got the causation exactly the other way around.
More Temperature => More Muons.
NOT:
More Muons => More Temperature.
This means that this paper doesn’t say absolutely nothing about Svenmarks’ Theory.
It just defends that Muon counters are very good proxies of stratospheric temperatures.
The level of absurdity is reaching idiocy. The amount of people that “read” this post and comment on how “warmists are gonna have hard time denying the EVIDENCE!” is too large for me to ignore.
Or perhaps it’s only the Sturgeon’s Law…

Stephen Wilde

There is possibly a link between events such as this and my supposition that when the air is deprived of energy from the oceans during a negative oceanic cycle then the response of the air is to adjust the circulation patterns in order to restore the energy balance of the air.
In that case there would be the observed sudden upper air warming followed by larger northern high pressure systems sending out more polar air over more oceans to pick up more energy and thereby restore the balance between energy going into space and energy being collected from the oceans.
On that basis I would speculate that the initial response of the planet to a developing energy deficit would first be a contraction of the equatorial high pressure systems which then allows an expansion of the high latitude high latitude systems.
A movement of the jet streams equatorward would follow with the climate shifts we observe on the ground.
Lots to be investigated here and highly relevant to the validity or otherwise of AGW theory because the same process of the air adjusting it’s own energy balance in response to variations in oceanic energy input could apply when the air’s own energy input is changed by, say, human CO2.

Mark

Whoa, whoa, whoa! Someone needs to read this article again! I believe this DOES NOT suggest that cosmic rays lead to temperature changes, and therefore DOES NOT support the concept of sunspot minima leading to global cooling. On the contrary, it says that if the Earth’s temperature (in the stratosphere) goes up, THAT causes cosmic rays to be more easily detected in their underground station. The causality they’re talking about here goes from Earth’s temperature to cosmic rays, not cosmic rays to Earth’s temperature. If I’m reading the article wrong, can someone explain?
I’m certainly interested in anything that would explain why solar sunspot minima seem to correlate with cooling on Earth, but this particular study doesn’t seem to provide any light on that issue.

foinavon

Ozzie John (03:43:04) :

Interesting Article….!
Are there any ideas thrown forward which might explain the sudden increase in the cosmic rays?

Yes. As the authors of the article in Geophys. Res. Letts. under discussion state:
There are two mechanisms by which the condition of the atmosphere affects the muon rate. Firstly, an increase in temperature causes the atmosphere to expand so muons are produced higher up and therefore have a larger probability to decay before being detected. Secondly, the mesons may interact (and thereby be lost) as well as decay. As the temperature increases, the probability of interaction becomes smaller because the local atmospheric density decreases, so more mesons decay, causing an increase in the muon rate. In deep underground detectors where muons with a high surface energy are measured, the second effect dominates and this causes a positive correlation between temperature and muon rate.
In other words, contrary to the impression in the top article of this thread and many comments, the article is about the ability to detect muons on Earth (or underground!), as a function of the temperature of the stratosphere. There’s no evidence that the cosmic rays are influencing the atmosphere at any level. The atmosphere (or stratosphere) is influencing the ability to detect the muons.

Dear Anthony, thanks for being the fastest blogger to report on this cool work – that includes an ex-colleague of mine at Harvard, Gary Feldman.
You must be careful before using this work as a proof of cosmoclimatology because the muon flux (production from pions) is partly influenced by the temperature, so the causation in the opposite direction explains at least a part of this observed correlation.
More comments:
http://motls.blogspot.com/2009/01/us-global-warming-is-least-concern.html

Pamela Gray

I am curious as to what causes the stratosphere to suddenly heat up? An electronic charge or magnetic event? I have a magnetic heater. It helps to unfreeze galvanized pipes. You plug it in, stick it to a metal pipe, and it heats the thing up, unfreezing the line. If a heated stratosphere then lets in more cosmic rays, would that lead to more cloud seeding? Or is it a way for us to understand how the Earth breaths out heat? If it is more porous coming in during these events, is that an indirect measure of porosity going out? Is it a way for heat to escape, suddenly?

John McDonald

I remember doing an experiment back in High School with alcohol, dry ice, and a jar to make a cloud chamber. When we stuck a radioactive “glow in the dark” old wrist watch in the cloud chamber little streaks of vapor “clouds” condensed in semi-circles that you could see quite nicely.
Is this the same basic idea that Svensmark has about radiation causing clouds.

Paul Schnurr

I am not a scientist but this seems to indicate the existence of another negative forcing at work (as marked by the cosmic ray records even if not caused). I agree with those that believe that the earth is a negative feedback system and the threat of “runaway” global warming is unlikely.
The threat of a small amount of heat increase (caused by CO2 increase) tripping “runaway” global warming seems to be the most plausible (and scariest) scenario on the AGW side. We shall see, but if all it took was heat there certainly appear to have been times in the past when the temperature was rising (heat was being applied by some means) and no “runaway” occurred.

davidgmills

Perhaps time to re-post another article I posted several weeks ago:
http://journals.royalsociety.org/content/77543w3q4mq86417/
Unfortunately, this is not a link to the actual paper, just an abstract.
But these authors looked at 50 years of cosmic ray data collected in Colorado, and compared them to 50 years of cloud records in the UK.
As I understand it, increased cosmic radiation caused about a 20% increase in likelihood for a cloudy day and cloud density increased 2 or 3% on average with high cosmic radiation.
I have the actual paper bookmarked on another computer and if there is interest, I will repost the link.
This paper seemed to more closely verify Svensmark’s theories.

Ron de Haan (23:57:09) :
Anthony, I think you are the fastest publisher on earth.

And the worst name speller too. 😉
I posted an excerpt from wikipedia in response to your post on the Ross Hays letter thread.
One reason for major stratospheric warmings to occur in the Northern hemisphere is because orography and land-sea temperature contrasts are responsible for the generation of long (wavenumber 1 or 2) Rossby waves in the troposphere. These waves travel upward to the stratosphere and are dissipated there, producing the warming by decelerating the mean flow. This is the reason that major warmings are only observed in the northern-hemisphere, with one exception. In 2002 a southern-hemisphere major warming was observed. This event to date is not fully understood.
So maybe the conventional wisdom is that the sudden stratospheric warming is caused by rossby waves and the QBO, which is caused by gravity waves.