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Human-caused global warming is fundamentally altering global atmospheric and oceanic circulation, throwing our weather systems into unprecedented instability. Burning fossil fuels and deforestation release greenhouse gases, trapping heat, supercharging the water cycle, and causing extreme weather events to become the new normal....
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TVTranscript
00:00.
00:09Over the last few years, Britain's weather has become more extreme.
00:17Especially our winters.
00:25Last winter was the wettest on record.
00:28Dad, look behind you!
00:31Deadly storms battered Britain for months.
00:35Causing misery for millions.
00:40Before that, we had a run of cold winters.
00:44Record-breaking temperatures with bitter lows of minus 22.
00:51Now there are big questions everyone wants answered.
00:56Why is our weather getting more extreme?
00:58Can we expect more of it in the future?
01:02And has it got anything to do with climate change?
01:26I'm Helen Cheresky.
01:28I'm John Hammond.
01:30Together we're going to try and make sense of Britain's recent extreme weather.
01:34And find out what's behind these unusual events.
01:37Is there more extreme weather on the way?
01:43Hello, Studio E. Can you hear me? It's John in the Weather Centre.
01:46Yeah, I've got two and a half minute weather for you.
01:48How long till me?
01:5030 seconds. Okay, fine.
01:52I'm a meteorologist.
01:54Hello there, plenty of fine weather to come in the outlook.
01:56But those temperatures, though, aren't exactly high, are they?
01:58Five or six degrees, parts of the Midlands.
02:01And I'm going to find out if there's anything that connects all the different types of recent extreme weather we've
02:06had.
02:07We're regularly on our BBC weather website. I'll be back with more detail on the UK weather in half an
02:12hour's time.
02:12Lower, lower, lower, lower. That's it, that's it.
02:15Okay, go on.
02:15And I'm a physicist.
02:17Off we go.
02:18I'm going to investigate the underlying causes of our extreme weather.
02:23Wow, that's fast.
02:25Together, we want to find out if our recent extreme weather will become our normal weather in the future.
02:36To get to grips with Britain's recent extreme winters, you need to understand what makes our weather so unusual in
02:43the first place.
02:45I've been forecasting the weather now for over 20 years, and for me it's the unpredictability of our weather which
02:51makes it so interesting.
02:54Here in the UK, it can change hour by hour, minutes by minutes sometimes.
03:00It's such a challenge to accurately predict what the weather is going to do next.
03:07We all like to moan about our weather. It changes constantly and it's very hard to predict.
03:14But there's a good reason for that.
03:17It's all down to Britain's unique position on the planet.
03:21Above our heads is a battleground, a constant struggle for supremacy between different types of air.
03:27Now most places in the world aren't like this. They're dominated by one or maybe two air masses.
03:34But here in the UK, we have to cope with four.
03:38During winter, these four major air masses are the Arctic air mass bringing cold snowy weather from the Arctic.
03:45The polar continental air mass dragging bitter winds in from Siberia.
03:51The maritime air mass tracking over the Atlantic bringing mild wet weather.
03:57And the tropical air mass bringing warm air up from the south.
04:04No air mass dominates our weather for long, which is one reason why it's constantly changing.
04:11But which air mass dominates isn't just down to chance.
04:15There is one factor that plays a major role in controlling which air mass sits over Britain.
04:22What determines which air mass dominates and the type of weather we get in the UK
04:28is a phenomenon which lies around 10 kilometres up in the atmosphere.
04:33It's called the Jetstream, a high-speed river of air which circles the globe at speeds of well over 100
04:41miles an hour.
04:44Because the Jetstream dictates the type of weather we get in Britain,
04:49it is the main suspect behind our recent extreme winters.
04:57Last winter was wet and stormy.
05:02Because the Jetstream brought in the maritime air mass and with it wet and windy weather.
05:10What was unusual was the persistence of this weather pattern.
05:16As it dominated for weeks on end.
05:21But the previous winters brought bitterly cold weather.
05:27Between 2008 and 2011, the Jetstream brought air masses in from the north and east.
05:34So Britain shivered under cold Arctic and Siberian winds.
05:42And again, what was unusual was how long this cold air stayed over Britain.
05:53Whether our winters were wet or cold, they all had one thing in common.
05:59One of the four major air masses got stuck over Britain, resulting in extreme weather.
06:10It suggested the Jetstream was doing something strange.
06:14Scientists wanted to understand more about its behaviour.
06:19But it was a challenge because it's an elusive phenomenon.
06:23Just finding it can be a struggle.
06:28There are four different Jetstreams all snaking their way around the planet.
06:34The one that affects us is the polar front jet, seen here in red and orange lines sweeping over the
06:41country.
06:43Its path constantly changes, getting weaker or stronger from one day to the next.
06:49And that makes it hard to predict its behaviour beyond a few days ahead.
06:56But there is a way to track its location and speed.
07:02We live in a world full of sophisticated technology for monitoring our weather and keeping an eye on things like
07:07the Jetstream.
07:08So, for example, we have satellites and radar.
07:11But there's no substitute for actually being up there at the place in the sky where the weather is happening.
07:16And the piece of kit that gets you there is crucial for meteorologists.
07:21And it's this. A very, very large balloon.
07:27So, it just clips on like that.
07:30There you go, just throw it out.
07:32So, attach the balloon to the line.
07:35Today, I'm launching a weather balloon with the help of Sam Howart from the Met Office.
07:40Right, and then just inflate it.
07:47In Britain, weather balloons are manually launched twice a day.
07:51One of them from here in Cambourne in Cornwall.
07:56So, yeah, that's the parachute.
07:58It does look a tiny bit like the sort of thing I used to make as a kid when I
08:01was dropping things off the balcony at the top of the stairs.
08:03Absolutely, yeah.
08:04I'll just lay that out.
08:06Like that.
08:07The magic bit of string.
08:10The data collected by these balloons is vital for forecasting the daily weather across the whole of Britain.
08:18There you go.
08:19You should've got laughing at your parachute here.
08:21I know, it's great, isn't it?
08:25The balloon is made of latex and it's filled with helium which is making it buoyant.
08:29And that buoyancy will carry it upwards when it leaves the ground at five to six metres every second.
08:35So, it's going to go up really, really quickly.
08:37And as it goes up, it'll expand because there's less air higher up.
08:41And somewhere way up there, some little floor in the latex will give way and it will pop and the
08:48parachute will carry the payload back down to earth.
08:50But by that time, we'll already have the data back here and we'll know whether the jet stream is over
08:56the top of us today.
08:57Right.
08:58It's all yours.
09:00Don't let go.
09:02Lower, lower, lower, lower.
09:03That's it.
09:03That's it.
09:04Okay.
09:05Go on.
09:05Go on.
09:05Keep going.
09:06That's it.
09:07Okay.
09:08Okay.
09:09And so grip it quite tightly and come out this way.
09:13Keep coming.
09:14Keep coming.
09:16Okay.
09:17Okay.
09:17Let it go.
09:18Okay.
09:18Ready?
09:18Yep.
09:19Go.
09:19Off we go.
09:27And off it goes.
09:28Wow.
09:29That's fast.
09:34It shows how strong the wind is because it's basically gone off at four to five degrees.
09:38It hasn't gone straight up at all.
09:47Right now, the balloon's rising through the troposphere, which is the lower level of the atmosphere where most of our
09:52weather happens.
09:53And up near the top of that layer, that's where the jet stream runs, about 10 kilometres up.
09:58And as the balloon keeps going up beyond that, it'll hit the next layer of the atmosphere, which is the
10:03stratosphere.
10:17The troposphere is around 10 kilometres thick.
10:20It's turbulent, and this is where most of our weather happens.
10:23But this layer, the one above, the stratosphere, is much more stable because the air's thin and dry.
10:32But what's really critical is the boundary between these two layers.
10:38Because this is where the jet stream can be found.
10:47But it's hard to predict the exact route that the jet stream is taking along that boundary on any given
10:53day.
10:55So did the jet stream pass over us today?
10:59Well, this is the trace so far.
11:02We can just see the balloon.
11:04It's giving us data every two seconds.
11:07So that's these new little green dots coming in.
11:09That's right.
11:10And it's just about approaching 20 kilometres at the moment.
11:13So right now, where the balloon is, is above where we'd expect the jet stream to be.
11:17So we've gone right through that region and not seen anything.
11:19Unfortunately not.
11:20If we look at about 10 kilometres, we can see there's not that much of a variation.
11:24So the jet stream's not above us today.
11:26If this was a day when the weather balloon did go through the jet stream, we'd expect to see really
11:30high wind speeds out here.
11:32That's right. As we can see here, the wind speed, it's fairly constant between 5 and 23 metres a second.
11:40And with the jet stream, you'd expect a wind speed of between 40 and 50 metres a second.
11:47The weather balloon we launched today didn't go through the jet stream.
11:51It went just to one side of it.
11:52But it did go really high up in the atmosphere, 35 kilometres up into the sky before it popped.
11:59And the data that it sent back is already at the Met Office.
12:02It's only three hours since it was launched, but the data is already being incorporated into the models
12:08and it'll be used for the weather forecasts that go out tonight.
12:12The fact that I didn't see the jet stream today just goes to show how fickle its path is.
12:20But knowing what it's doing is absolutely critical, because this is the key suspect behind last winter's extreme weather.
12:34There were at least 12 major storms last winter.
12:38The first, on the 5th of December, had wind gusts reaching 142 miles per hour.
12:46But it wasn't just strong winds we had to battle with.
12:51These rain radar images show the extent and intensity of the wet weather.
13:02In January, the south of England received almost three times its normal rainfall.
13:09And as the frequency of storms increased, it led to the wettest winter on record.
13:21So why were there so many storms with so much rain?
13:27And how did they get so powerful?
13:30So the jet's been driving this active cold front across the country today.
13:34The answers lie with what the jet stream was doing.
13:38This is the jet stream from last winter.
13:41The most important thing to see is that it's heading straight across the Atlantic.
13:45Now, normally the wind speeds within the jet stream are around 100 to 150 miles an hour.
13:50But last winter, the speeds reached almost 300 miles an hour.
13:55So that's twice the normal speed.
13:57And the jet stream was heading straight towards us.
14:00And it was this which delivered storm after storm after storm so quickly, one after the other.
14:07The key to understanding last winter's stormy weather was figuring out why the jet stream had got so fast.
14:29One of the first clues began to emerge when scientists realised that something strange
14:35had been going on in the Atlantic before the winter began.
14:47It concerned one of nature's most deadly and powerful weapons.
14:53Hurricanes.
15:01I'm here in Miami and I've come to visit Eric Yulhorn, who's been studying hurricanes for some 15 years or
15:08so.
15:08And I want to speak to him because something very unusual happened with the hurricane season of 2013.
15:14I want to find out more.
15:22Inside this laboratory, I'm hoping to find the first clue behind last winter's fast jet stream.
15:30It all starts with the unique way that hurricanes interact with the sea as they track over its surface.
15:39As hurricanes come across the Atlantic, typically they mix up the cold water and behind the storm, as the storm
15:45tracks across the ocean,
15:47you can typically see what we call a cold wake.
15:50What we're looking at here is a sea surface temperature map of the North Atlantic.
15:54You can see the United States here.
15:56Here's Florida.
15:58These are warm ocean waters in orange and these are the colder ocean waters.
16:02And what you see is a hurricane tracking across the Atlantic right here.
16:05And you can see that it leaves a scar of cool water behind the storm as it mixes up that
16:10cold water below the surface.
16:14And it's typically about 200 to 300 kilometres or so across, and it can last for several weeks after the
16:20storm.
16:22So how much cooler does it actually get within that scarring, that track behind the hurricane?
16:28Typically, we see ocean temperatures cool about 3 to 5 degrees Celsius behind the storm.
16:34If just one hurricane can have such a dramatic effect on the upper ocean, what happens when a whole season
16:41of hurricanes power their way across the Atlantic?
16:44In 2012, we had a very active hurricane season with 10 hurricanes, which helped to cool the water across the
16:50North Atlantic.
16:53But last year, there weren't many hurricanes.
16:56In 2013, we only saw two hurricanes.
17:00What we see are significantly warmer temperatures compared to average than we saw in 2012.
17:12So a lack of hurricanes may have resulted in areas of the Atlantic being warmer than average.
17:23But what's puzzling is how could warmer waters in the Atlantic produce last winter's super-fast jet?
17:35It turns out that the speed of the jet stream is driven in part by temperature differences between cold air
17:42over the poles and warm air over the tropics.
17:49When the temperature difference between these two regions is very big, the jet stream tends to travel very fast.
18:01So last year's warmer-than-average temperatures in the Atlantic may have increased this temperature gradient.
18:11Which could have produced a fast jet.
18:16We had warmer temperatures in 2013, so you may see some large temperature gradient between the North Atlantic and the
18:25Arctic region,
18:26which may then impact the atmosphere and therefore develop a large temperature gradient,
18:31which then can potentially drive a stronger jet stream.
18:42What makes sense to me as a meteorologist that something as hugely energetic as a hurricane can have a big
18:50influence on the system, if you like,
18:52the atmosphere and the ocean system.
18:54Of course, there are so many other factors involved, and that's the challenge of meteorology.
18:58We can't make a direct link, but it's very intriguing.
19:01It poses the question, could the lack of hurricanes in 2013 have played a role in producing such a strong
19:09jet stream?
19:12A few weeks after I met up with Eric, scientists discovered the answer.
19:18It turned out that the impact of a lack of hurricanes last year wasn't big enough to have turbocharged the
19:25jet stream.
19:27So the abnormal hurricane season was a red herring.
19:33Although the jet stream still remained the number one suspect,
19:36the hunt to discover why it got so fast would have to start again.
19:51The next clue was hidden away in an event that happened over 130 years ago.
20:01In 1883, the Krakatoa volcano in Indonesia erupted.
20:08Around 40,000 people died.
20:12And ash and dust were hurled 35 kilometres into the air.
20:23But there was an unexpected side effect of this cataclysm felt around the world.
20:43These spectacular crayon sketches were done on the banks of the Thames in Chelsea in London.
20:49And I've never seen the sky over London look like this.
20:52These were done on a very specific date in 1883 by the painter William Ascroft.
20:59They looked like the sky is on fire, really bright red.
21:03And the events that caused it were on the other side of the world,
21:07the eruption of the volcano Krakatoa.
21:15While artists were inspired to paint,
21:18scientists wanted to understand how the volcanic dust had spread so quickly across the globe.
21:33So the Royal Society put an advert in newspapers, asking the public to send in any unusual observations in connection
21:43with the volcano.
21:47These are just some of the letters that came in.
21:50And they're so varied.
21:52They're also really hard to read because the handwriting is almost illegible.
21:56People are describing the sky all of a flare at sunset, fall of ashes, explosions heard.
22:02And on the back there's these colour pictures of a sunset and afterglow, the rings it says as they formed
22:10in succession.
22:12So imagine sitting in the Royal Society in London and getting these letters and the huge amount of information that's
22:19in them.
22:20And that information let them build up a picture of what had happened all around the globe in the months
22:26following the eruption.
22:32That picture revealed something that had never been observed before.
22:37What these observations show is that the dust and the aerosols that were carried from the explosion spread westwards around
22:45the globe at the equator,
22:47almost as if there was a sort of river of wind running westwards up in the atmosphere that was carrying
22:53it along.
23:00These winds are similar to the jet stream but they travel much higher up in the stratosphere.
23:07And they're only found near the equator.
23:12What's fascinating about these winds is that they don't always flow in the same direction.
23:19Up there in the stratosphere above the equator, there are winds that either travel to the west or to the
23:25east and it switches direction every 14 months or so.
23:30It's known as the Quasi-Biennial Oscillation, sometimes called the QBO for short.
23:41Back in the 1970s, scientists discovered that when these winds flow towards the east, they strengthen the jet stream.
23:53And it's intriguing that last winter these same winds were flowing towards the east.
24:00Finally, scientists had the first concrete piece of evidence that something was helping to speed up the jet stream.
24:07But there was a problem.
24:10These winds have travelled towards the east many times in the past without producing a record-breaking winter like last
24:17year.
24:18So whilst they probably played a small role in strengthening the jet stream, on their own, they weren't enough.
24:33As the winter ended, scientists began to develop a third explanation.
24:43They knew the perfect conditions for a fast jet stream involve a big temperature difference between the poles and the
24:50tropics.
24:51So they looked for any signs that showed that this temperature gradient increased last winter.
25:01Throughout the next few days, temperatures will fall to the low around minus 13.
25:07Everyday activities may not be feasible.
25:10It led them to investigate the unusual weather conditions in North America.
25:18These bone-chilling temperatures normally stay locked up over the Arctic.
25:24But last winter, this freezing air was dragged southwards over North America.
25:35Professor Dame Julia Slingo, the Met Office Chief Scientist, has been looking at what might have caused this cold air
25:42to be dragged south.
25:46Surprisingly, her search began with a deadly flood that happened on the other side of the world, in Indonesia.
25:57Last December, unusually intense rain persisted for weeks.
26:07The fatal floods that followed displaced 60,000 people and left areas under more than two metres of water.
26:17The Indonesian region has been a large part of what's been happening.
26:22You might say, well that's an awfully long way from the UK, and it is.
26:27But what happens in Indonesia affects profoundly the weather patterns around the world.
26:39This extraordinary amount of rain triggered off a sequence of events that would ultimately contribute to a super fast jet
26:47stream.
26:50First, the intense rainfall in Indonesia helped to dramatically alter the normal path taken by a different jet stream, the
26:58Pacific jet.
27:01That usually follows its path across and well north of California.
27:06In this year, it's gone a very long way north and then made a very deep curve down over the
27:13US and Canada.
27:15What we call a great buckle in the jet stream.
27:19This buckle in the Pacific jet stream helped drag the freezing cold air from the Arctic down over Canada and
27:26America.
27:28This cold air then shoved up against the warm air over the Atlantic, which produced a big temperature gradient.
27:36The perfect conditions for a fast jet stream.
27:40The end result was a whole series of storms.
27:44So, in a sense, you've had a double whammy, if you like.
27:47You've had the cold air coming down and setting up things on the north side of the jet.
27:52But you've had disturbances also coming into the south side of the jet.
27:57It is a bit like a row of dominoes.
27:59You know, it takes about a week for something that happens in Indonesia to have its domino effect, if you
28:06like.
28:06And we see it in our weather over the UK.
28:10But the connections are so clear this year.
28:23Last winter, there were many different factors at play.
28:29These all worked together to produce a fast jet stream.
28:36The QBO was powering along towards the east.
28:42And intense rain in Indonesia knocked the Pacific jet stream off its normal path.
28:52This helped increase the temperature gradient.
28:58Which led to our jet stream thundering its way towards Britain.
29:14In many ways, last winter was the perfect storm.
29:18Everything that could have come together to change the jet stream did.
29:25And it's incredible to think that so many different factors could have affected our weather back here in the UK.
29:31But that's exactly what happened.
29:33And it just goes to show how many pieces there are in this giant jigsaw puzzle.
29:42But there is one more piece to this puzzle.
29:46It is perhaps the most controversial and complex piece of all.
29:51Climate change.
30:03How much of an impact did climate change have on last winter's stormy weather?
30:13It's one of the hardest questions to answer.
30:16Because our climate is so complex.
30:19And because there are so many competing factors that influence our weather.
30:25But we have one very effective tool for understanding the role of climate change.
30:32Computer models.
30:34They incorporate the best of our current understanding.
30:38They represent the collective work of thousands of scientists.
30:42They're an amazing achievement.
30:43And when they get as good as they are now, it's possible to use them like a sort of flight
30:48simulator for a planet.
30:49It's an amazing tool to have.
31:00Today, supercomputers like this one at the Met Office can do more than a hundred trillion calculations every second.
31:08And can look at the impacts climate change may have on our future weather.
31:17I think today, the incredible complexity and power and skill of these models that we use, they are one of
31:27the great achievements of modern science.
31:29And you realise that we're entering, I think, a golden age for climate science.
31:34And it's good that we are because we have some really, really big questions to answer for the world in
31:41terms of what climate change will mean for us all.
31:46Models have predicted that in the future, climate change will lead to an increase in extreme weather.
31:54So was last year's extreme winter an early sign of this becoming true?
32:00There can't be a definitive answer on that just yet, because there's quite a lot of research that needs to
32:06be done.
32:07That being said, I think there are various factors that we understand from the science of climate change that again
32:18would suggest that it's been an additional factor.
32:38To fully understand the impact that climate change had last winter, more research needs to be done.
32:49But perhaps that's missing the point.
32:53Because we may never be able to say that one particular weather event or one unusual season is because of
33:01climate change.
33:06But it seems likely that one consequence of climate change will be more intense rain.
33:15I think it's important to remember, on top of all this discussion of global weather patterns, that there is this
33:21basic bit of physics that says that in a warmer world, rainfall will be more intense.
33:28No one's produced any evidence to counter that idea, and it's widely accepted.
33:34And so it's reasonable to expect that in the future, as the world warms, we will get more intense rainfall.
33:48And more intense rain will increase the potential for flooding.
33:53So regardless of whether last winter was made worse by climate change, flooding is something we may have to get
34:00used to.
34:10But our extreme winters haven't just been about rain and storms.
34:17Because previous winters have sent Britain into a deep freeze.
34:25It started in 2008, when temperatures dropped to minus 12.
34:31As cold Siberian air from the east brought snow across the country.
34:39A year later, northern Scotland had the coldest winter on record.
34:43As once again Britain shivered under cold Siberian winds for weeks on end.
34:51The following winter, we had the coldest December in 100 years.
34:57As bitterly cold air from the Arctic brought a blanket of snow across Britain.
35:08And there is one thing all these recent cold winters had in common.
35:15Once again, the main suspect was the behaviour of the jet stream.
35:21The track of the jet stream varies a lot.
35:23But during a typical winter, it takes this sort of path straight across the UK.
35:29But in recent cold winters, it's done something rather peculiar.
35:33It's taken a meander.
35:35And instead it's moved its way northwards and then dived southwards.
35:39Which has meant that the UK has been very much on the northern side of the jet.
35:42And that's exposed us to particularly cold air in recent winters.
35:49These big meanders dragged in either the Arctic air mass from the north.
35:54Or the polar continental air mass from the east.
35:58Both brought bitterly cold winds and snow.
36:03So what caused these big meanders in the jet stream?
36:07The search for answers soon became an international one.
36:11Because big meanders in the jet stream also played a role in one of America's most deadly storms.
36:29The ferocious power of Hurricane Sandy was the most destructive hurricane of 2012.
36:38On the 29th of October, it collided head on with the coast of New Jersey in America.
36:47Over 70 died.
36:49Half a million buildings were ripped apart.
36:52And the clean-up bill cost over $50 billion.
37:10Dr Jennifer Francis has been investigating this hurricane.
37:16She's taking me to a part of the New Jersey coastline that was badly damaged.
37:22So just how unusual was Hurricane Sandy?
37:25So Sandy was a very unusual storm.
37:28This part of the coast of New Jersey was one of the worst hit.
37:32In fact, the ocean, which is on our left here, came right across the seawall.
37:37So this whole area was under, what, five foot of water?
37:40Something like that.
37:41And, you know, of course, with waves on top.
37:43And the roadway was covered with one or two feet of sand after the storm.
37:47I can't even imagine what it looked like.
37:57What made Hurricane Sandy so devastating was the unusual path it took.
38:06Well, normally, hurricanes tend to steer right out into the Atlantic, out to the east.
38:11But Hurricane Sandy did something completely different.
38:17It encountered the jet stream.
38:20And that created the winds that blew her onto her very unusual path,
38:25taking a sharp left turn right into New Jersey.
38:29And the sheep of the jet stream was really critical for steering Sandy into the coast.
38:41The jet stream had taken a big meander, which helped push Hurricane Sandy off its normal path.
38:52But why had the jet stream developed this large meander over such a vast area?
39:03To find out, you need to understand what causes the jet stream to change shape in the first place.
39:12And you can look in the most unlikely of places for the answer.
39:18The jet stream is a bit like a river in the sky, but it's 10 kilometers up and invisible, so
39:23we can't see it.
39:24And it can be hard to understand its behavior.
39:27But something that can help us understand what it's doing is to think about rivers down here on Earth,
39:32like this one, the River Cookmere in Sussex.
39:40The reason that the water in rivers moves is that it's flowing down a gradient.
39:45In places where the ground is steep, the gradient is steep, and water flows quickly,
39:49and it usually flows in a straight line.
39:54But down here on the floodplain, it's a little bit different.
40:01Here I'm down near the end of the river, where the land is almost flat.
40:06There's only a really shallow gradient in height.
40:08But that gradient is enough to keep the river flowing,
40:11and I can measure how fast it's flowing using this, and this is a flow meter.
40:21And it's come out at 26 centimeters a second.
40:26So that's relatively slow.
40:28There's only a shallow height gradient here, and the river's running slowly.
40:33And to see the effect of that, I need to go up there on the hillside.
40:43From up here, we can see what we couldn't see down below.
40:52This river isn't running in a straight line.
40:55It's got these big loops in it called meanders.
40:58And they develop when rivers run more slowly.
41:01The reason it's useful to look at this is that the same thing happens up in the sky with the
41:06jet stream.
41:07When it slows down, it changes shape and develops meanders just like this.
41:24The jet stream's speed is also linked to a gradient.
41:29But that gradient is different to that of a river.
41:38The river here is running because of a gradient in height.
41:42It's running from the ground, the higher ground, inland, out to the ocean.
41:47But the jet stream is running because of a gradient in temperature.
41:50And just like the river, when that temperature gradient becomes shallower, the jet stream slows down and starts to meander.
42:00So weak, lazy jet streams develop big meanders which can get stuck, resulting in one air mass sitting over Britain
42:08for weeks on end.
42:11Which is exactly what happened during our recent cold winters.
42:18So to understand what caused them, scientists needed to find out why the jet stream had slowed down and produced
42:25these big meanders.
42:30It's a search which has led scientists to some of the most contentious areas of climate research.
42:36Because Jennifer thinks the answer might be found with dramatic changes that are going on in the Arctic.
42:42I've been studying the Arctic my whole life and we started realising in the late 1990s that things were changing
42:51really fast out there.
43:03The Arctic is warming almost twice as fast as the rest of the world.
43:16It's a phenomenon known as Arctic amplification.
43:25Three quarters of the volume of summer sea ice has disappeared in just 30 years.
43:34The scale of the ice loss is just truly breathtaking.
43:37In 2007, we had a new record for the least amount of ice in the Arctic Ocean at the end
43:44of the summer.
43:45And since then, it's just been every year has been very low.
43:49And then in 2012, five years later, we hit another new record low, much lower than even 2007.
43:56So it's just been a steady decline in the amount of ice.
44:07Arctic amplification has not only caused sea ice to retreat, but to reduce in thickness too.
44:16Today, it's around 50% thinner compared to previous decades.
44:25And this loss of sea ice has a feedback on the climate system.
44:30Less ice means less sunlight is reflected back into space.
44:37Instead, the ocean surface absorbs more heat from the sun, so the Arctic warms faster, resulting in yet more sea
44:45ice melting.
44:53The scale of the ice loss gave me chills because it is such a huge change to such a fundamental
45:00part of the Earth's climate system to see that change happening so rapidly.
45:04So it just got me thinking, how is this rapid warming in the Arctic affecting areas farther south?
45:14To find out, Jennifer looked back over the last 30 years, the period of major ice loss in the Arctic.
45:22She was looking for any changes in the size of the jet stream's waves, how loopy they got.
45:30It was a simple measure, but her findings were dramatic.
45:34We found that in the last couple of decades, the waves actually do appear to be getting larger.
45:42They appear to be extending northward more often.
45:46And particularly in the North Atlantic, which is important for the UK, these very large swings in the jet stream
45:53are happening more often.
45:55It appears to be the case.
45:58Jennifer believes warming in the Arctic is reducing the temperature difference between the poles and the tropics.
46:05This is slowing down the flow of the jet stream, making it more prone to big meanders.
46:10As that difference in temperature between those two bands of the Earth gets smaller because the Arctic is warming so
46:18much faster, the jet stream is weakening.
46:20And because those waves are what create the weather that we experience down here on the surface, if those waves
46:27are moving more slowly, then the weather patterns should change more slowly in any given place.
46:32So it feels like the weather that you're experiencing is stuck.
46:37Stuck in a rut.
46:37Stuck in a rut.
46:39And that's, you know, we've seen that happen over and over again in the last decade or so.
46:44It just seems to be happening more often now.
46:47And when one of those big dips happens just south of the UK, then all that cold air from the
46:53Arctic can come down over that area and create a very cold winter.
46:59So Arctic amplification produces a smaller temperature gradient between the poles and the tropics, which Jennifer believes produces more meanders
47:10in the jet stream.
47:14And when these meanders happen over Britain, they can drag in the Arctic air mass from the north, bringing cold
47:20Arctic winds, which produce bitter winters.
47:41It's really interesting what Jennifer had to say to me.
47:43I mean, there's no doubt that the Arctic is losing ice at an alarming rate.
47:49You don't really have to be a meteorologist, a climatologist, to conclude that such a fundamental and rapid change to
47:56the system is going to have knock-on effects to the atmosphere and to the weather which we experience.
48:03Also quite compelling on the face of it is the fact that we have gone through a run of prolonged
48:08spells of unusually severe weather.
48:14However, that could all be a red herring because it's a challenge to pick out what is actually natural variability,
48:22just fluff, just noise from what is a genuine signal.
48:29And the challenge is made even harder by a lack of data for scientists to study.
48:36Because dramatic changes in the Arctic have only been seen in the last 30 or so years.
48:44Which isn't long enough to know if the connection between a warming Arctic and a meandering jet stream is real.
48:56And it's particularly wise to be cautious about the role of Arctic amplification, because there are other theories behind our
49:04recent cold winters.
49:06One of which has its origins in a very different part of the climate system.
49:12For decades it was thought that all our weather happened in just one layer of the atmosphere, the troposphere.
49:20But scientists have discovered that the layer above, the stratosphere, is also fundamental to our weather.
49:33Professor Adam Scaife is investigating this important part of the atmosphere.
49:40It's only recently that the computer models that we use to make weather forecasts and climate predictions have properly started
49:47to take into account the full depth of the atmosphere and to properly include the stratosphere.
49:54So at first sight the stratosphere seems very remote from the surface weather.
49:58We're talking about very thin, tenuous air, 50 or so kilometres above the surface.
50:04But there is an important connection there.
50:08Adam believes something going on in the stratosphere could provide another explanation behind some of our cold winters.
50:18And he's brought me to Chesil Beach in Dorset to show me what it is.
50:23So the reason I've brought us here is because although what you see behind us, these breaking waves, might seem
50:30remote and completely irrelevant for the cold winters that we've had.
50:34There is actually a deep underlying similarity between the breaking waves here and breaking waves really high in the atmosphere
50:42during these cold winter events.
50:51Scientists have discovered that the thin air of the stratosphere is home to giant atmospheric waves, which behave in a
50:59similar way to crashing waves in the sea.
51:06When one of these waves breaks, it generates something called sudden stratospheric warming.
51:16Adam's discovered that these events have occurred during some of our recent cold winters.
51:26So two out of three of the recent very cold winters that we've had have occurred in conjunction with sudden
51:32stratospheric warming.
51:33This initially occurs really high in the atmosphere, 50 kilometres or 30 miles above the surface.
51:39And it happens when a wave in the atmosphere breaks at really high altitude.
51:44That breaking wave actually pushes the wind opposite to its normal direction.
51:55Normally the winds in the stratosphere blow in the same direction as the jet stream from west to east.
52:02But as this enormous stratospheric wave breaks, it pushes these winds in the opposite direction towards the west.
52:12These winds then burrow their way down through the stratosphere until they hit the jet stream.
52:20And because the jet stream flows in the opposite direction, these winds act like a brake, slowing it down.
52:31The whole process of this burrowing down through the atmosphere can occur on a timescale of a few days, maybe
52:36a week or two, until it reaches the jet stream.
52:39And at this point, it kind of switches off the jet stream and blows cold air from Siberia in towards
52:45the UK
52:45and leads to those dramatic cold snaps that we've experienced in recent winters.
52:57So a sudden stratospheric warming slows the jet stream, which can produce a big meander.
53:05As a result, the polar continental air mass is dragged in from the east across Britain.
53:11And with it comes bitterly cold air from Siberia.
53:17Since these events can influence our winters so dramatically, scientists want to know if there's a pattern to their occurrence.
53:26But it isn't so simple.
53:29Now there is no regular pattern to when these events occur.
53:33On average, they're every two years.
53:36But just like tossing a coin, you could get three heads in a row.
53:39Sudden stratospheric warmings can occur in runs of winters, or you can have long periods like the 1990s,
53:45when there were no sudden stratospheric warmings for several years on end.
53:53The thing that's a bit frustrating about these sudden stratospheric warming events is that there's no clear pattern to them.
53:59But as humans, we're always looking for patterns.
54:02If a coin falls heads lots of times in a row, we start to ask why.
54:07But sometimes that's just the look of the draw.
54:09And so it may well be that we've had some cold winters and we're looking for a pattern.
54:14But really, there isn't one.
54:27So it looks as if at least two factors could have caused some of our recent cold winters by weakening
54:33the jet stream.
54:36Sudden stratospheric warming.
54:42An Arctic amplification.
54:53We still don't know which one will dominate in the future.
55:01Or indeed whether other factors, like the behaviour of the oceans or changes in solar activity,
55:07could play a role too in influencing our winter weather.
55:13All this makes it a challenge to know whether we face cold or wet stormy winters in the future.
55:29But despite this uncertainty, there may be something we can say about our future winters.
55:37Once again, it all comes back to the jet stream.
55:43For me, the strongest signal to emerge as we struggle to understand the recent extreme weather
55:48is the idea that the jet stream can become stuck in certain configurations.
55:54At one end of the spectrum, a very straight, fast jet stream which brought the storms of last winter.
56:00At the other end of the spectrum, a much slower, meandering jet stream
56:05which has brought the recent run of particularly cold winters.
56:09But either end of the spectrum is capable of bringing prolonged and extreme weather.
56:15And perhaps this is something we should expect more of in the near future.
56:40Scientists are continuing to improve their understanding of the jet stream.
56:44But even now, there's a lot we can do to prepare for our future.
56:56We don't control the weather, but we're not helpless.
57:00And being uncertain about the future isn't the same as knowing nothing.
57:05I think the science here is in a really good state.
57:07There's lots of debate, lots of different ideas, lots of evidence that's available for everyone to see.
57:14And I feel optimistic.
57:15I think that we're really getting to grips with the science of these extreme weather events.
57:27And that means we can also begin to get to grips with how to deal with our changing climate.
57:39When I was a boy, I lived on the side of a hill and I'd sit overlooking the valley and
57:44I'd watch the weather coming my way.
57:46And that was my world really.
57:48And as far as I was concerned, the weather was contained within the valley.
57:57We all have a tendency to think locally, but we have to look beyond the parochial confines of our valley,
58:04our country, our ocean even.
58:07We have absolutely no control over the weather.
58:11But what we can do is understand it and adapt our society for the changes which lie ahead.
58:32More wild and wet weather on BBC 4 Monday night at 8, then understanding our atmosphere, how we discover the
58:38composition of the air we breathe at 9.
58:41Next tonight on BBC 2, a discovery that could destroy her family in Hugo Blick's The Honourable Woman.
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