What's Wrong with Our Weather? Horizon

Over the last few years,

Britain's weather has become more extreme...

..especially our winters.

Last winter was the wettest on record.

Dad, look out behind you!

Deadly storms battered Britain for months,

causing misery for millions.

Before that, we had a run of cold winters,

record-breaking temperatures with bitter lows of minus 22.

Now there are big questions everyone wants answered.

Why is our weather getting more extreme?

Can we expect more of it in the future?

And has it got anything to do with climate change?

-I'm Helen Czerski.

-I'm John Hammond.

Together we're going to try and make sense

of Britain's recent extreme weather.

And find out what's behind these unusual events and

is there more extreme weather on the way?

Hi, Studio E, can you hear me? It's John in the weather centre.

Yeah, got two and a half minute weather for you.

How long until me?

30 seconds. OK, fine.

'I'm a meteorologist.'

Hello there, plenty of fine weather to come in the outlook

but those temperatures, though, aren't high, are they?

Five or six degrees in parts of the Midlands...

'And I'm going to find out if there's anything that connects all

'the different types of recent extreme weather we've had.'

..regularly on our BBC weather website,

I'll be back with more detail on UK weather in half an hour's time.

-Lower, lower, lower, lower, that's it, that's it.

-I can't go any lower.

'And I'm a physicist.'

Off we go.

'I'm going to investigate the underlying causes

'of our extreme weather.'

Wow, that's fast.

Together we want to find out if our recent extreme weather

will become our normal weather in the future.

To get to grips with Britain's recent extreme winters,

you need to understand what makes our weather so unusual

in the first place.

I've been forecasting the weather now for over 20 years

and, for me, it's the unpredictability of our weather

which makes it so interesting.

Here in the UK, it can change hour by hour, minute by minute sometimes.

It's such a challenge to accurately predict

what the weather is going to do next.

We all like to moan about our weather.

It changes constantly and it's very hard to predict.

But there's a good reason for that.

It's all down to Britain's unique position on the planet.

Above our heads is a battleground,

a constant struggle for supremacy between different types of air.

Now most places in the world aren't like this.

They're dominated by one or maybe two air masses

but here in the UK we have to cope with four.

During winter, these four major air masses are the Arctic air mass

bringing cold, snowy weather from the Arctic...

..the polar continental air mass

dragging bitter winds in from Siberia,

the maritime air mass tracking over the Atlantic

bringing mild, wet weather,

and the tropical air mass bringing warm air up from the south.

No air mass dominates our weather for long which is one reason

why it's constantly changing.

But which air mass dominates isn't just down to chance.

There is one factor that plays a major role in controlling

which air mass sits over Britain.

What determines which air mass dominates

and the type of weather we get in the UK is a phenomenon

which lies around 10 kilometres up in the atmosphere.

It's called the jet stream, a high speed river of air

which circles the globe at speeds of well over 100mph.

Because the jet stream dictates the type of weather we get in Britain,

it is the main suspect behind our recent extreme winters.

Last winter was wet and stormy...

..because the jet stream brought in the maritime air mass and with it,

wet and windy weather.

What was unusual was the persistence of this weather pattern...

..as it dominated for weeks on end.

But the previous winters brought bitterly cold weather.

Between 2008 and 2011,

the jet stream brought air masses in from the north and east,

so Britain shivered under cold Arctic and Siberian winds.

And again what was unusual was how long this cold air

stayed over Britain.

Whether our winters were wet or cold,

they all had one thing in common -

one of the four major air masses got stuck over Britain,

resulting in extreme weather.

It suggested the jet stream was doing something strange.

Scientists wanted to understand more about its behaviour...

..but it was a challenge because it's an elusive phenomenon.

Just finding it can be a struggle.

There are four different jet streams

all snaking their way around the planet.

The one that affects us is the polar front jet,

seen here in red and orange lines sweeping over the country.

Its path constantly changes,

getting weaker or stronger from one day to the next

and that makes it hard to predict its behaviour beyond a few days ahead.

But there is a way to track its location and speed.

We live in a world full of sophisticated technology

for monitoring our weather

and keeping an eye on things like the jet stream.

So, for example, we have satellites and radar but there's no substitute

for actually being up there at the place in the sky where

the weather's happening and the piece of kit that gets you there is

crucial for meteorologists

and it's this - a very, very large balloon.

So that just clips on like that.

There you go, let's throw it out.

So attach the balloon to the line.

'Today I'm launching a weather balloon

'with the help of Sam Howett from the Met Office.'

Right, and then just inflate it.

'In Britain, weather balloons are manually launched twice a day,

'one of them from here in Camborne in Cornwall.'

-So, yeah, that's the parachute.

-It does look a tiny bit like

the sort of thing I used to make as a kid

when I was dropping things off the balcony at the top of the stairs.

Absolutely, yeah. I'll just lay that out like that.

The magic bit of string.

'The data collected by these balloons is vital for forecasting

'the daily weather across the whole of Britain.'

In you get.

Should stop laughing at your parachute here.

I know, it's great, isn't it?

The balloon is made of latex

and it's filled with helium which is making it buoyant and that buoyancy

will carry it upwards when it leaves the ground

at five to six metres every second

so it's going to go up really, really quickly.

And as it goes up, it'll expand because there's less air higher up

and somewhere way up there, some little flaw in the latex

will give way and it will pop and the parachute will carry the payload

back down to Earth but by that time we'll already have the data back here

and we'll know whether the jet stream is over the top of us today.

Right, it's all yours.

Don't let go.

-Lower, lower, lower, lower, that's it, that's it.

-I can't go any lower.

OK, go on. Go on, keep going. That's it, OK.

OK. So grip it quite tightly and come out this way.

Keep coming, keep coming. OK.

-OK, let it go.

-OK, ready?

-Yeah, go!

-Off we go.

And off it goes. Wow, that's fast.

It shows how strong the wind is

-because it's basically gone off at 45 degrees.

-Yeah, absolutely.

It hasn't gone straight up at all.

Right now, the balloon's rising through the troposphere

which is the lower level of the atmosphere

where most of our weather happens and up near the top of that layer,

that's where the jet stream runs, about 10 kilometres up.

And as the balloon keeps going up beyond that,

it'll hit the next layer of the atmosphere which is the stratosphere.

The troposphere is around 10 kilometres thick.

It's turbulent and this is where most of our weather happens

but this layer, the one above, the stratosphere,

is much more stable because the air's thin and dry

but what's really critical is the boundary between these two layers

because this is where the jet stream can be found.

But it's hard to predict the exact route that the jet stream

is taking along that boundary on any given day.

'So did the jet stream pass over us today?'

Well, this is the trace so far.

You can just see the balloon is giving us data every two seconds.

-That's these new little green dots coming in?

-That's right,

and it's just about approaching 20 kilometres at the moment.

So, right now, where the balloon is is above where we'd expect

the jet stream to be, so we've gone right through that region

-and not seen anything?

-Unfortunately not.

If we look at about 10 kilometres, we can see there's not that

much of a variation so the jet stream's not above us today.

If this was a day when the weather balloon did go through

the jet stream, we'd expect to see really high wind speeds out here?

That's right. As we can see here, the wind speed,

it's fairly constant between five and 23 metres a second

and with the jet stream you'd expect

a wind speed of between 40 and 50 metres a second.

The weather balloon we launched today didn't go through the jet stream.

It went just to one side of it, but it did go really high up

in the atmosphere, 35 kilometres up into the sky before it popped

and the data that it sent back is already at the Met Office.

It's only three hours since it was launched

but the data is already being incorporated into the models

and it'll be used for the weather forecasts that go out tonight.

The fact that I didn't see the jet stream today

just goes to show how fickle its path is.

But knowing what it's doing is absolutely critical because

this is the key suspect behind last winter's extreme weather.

There were at least 12 major storms last winter.

The first on the 5th of December had wind gusts reaching 142mph.

But it wasn't just strong winds we had to battle with.

These rain radar images show the extent

and intensity of the wet weather.

In January, the south of England

received almost three times its normal rainfall

and as the frequency of storms increased,

it led to the wettest winter on record.

'So why were there so many storms with so much rain?

'And how did they get so powerful?'

So the jet's been driving this active cold front

across the country today but then...

'The answers lie with what the jet stream was doing.'

This is the jet stream from last winter.

The most important thing to see is that it's heading

straight across the Atlantic.

Now, normally the wind speeds within the jet stream

are around 100 to 150mph but last winter,

the speeds reached almost 300mph

so that's twice the normal speed and the jet stream

was heading straight towards us and it was this which delivered

storm after storm after storm so quickly, one after the other.

The key to understanding last winter's stormy weather

was figuring out why the jet stream had got so fast.

One of the first clues began to emerge

when scientists realised that something strange had been going on

in the Atlantic before the winter began.

It concerned one of nature's most deadly and powerful weapons...

..hurricanes.

I'm here in Miami and I've come to visit Eric Uhlhorn

who's been studying hurricanes for some 15 years or so

and I want to speak to him because something very unusual

happened with the hurricane season of 2013.

I want to find out more.

'Inside this laboratory, I'm hoping to find the first clue

'behind last winter's fast jet stream.

'It all starts with the unique way that hurricanes interact with

'the sea as they track over its surface.'

As hurricanes come across the Atlantic,

typically they mix up the cold water

and behind the storm, as the storm tracks across the ocean,

you'd typically see what we call a cold wake.

What you're looking at here is a sea surface temperature map

of the North Atlantic.

You can see the United States here, here's Florida,

these are warm ocean waters in orange

and these are the colder ocean waters.

And what you see is a hurricane

tracking across the Atlantic right here

and you can see that it leaves a scar of cool water

behind the storm as it mixes up that cold water below the surface.

And it's typically about 200 to 300 kilometres or so across

and it can last for several weeks after the storm.

So how much cooler does it actually get within that scarring,

that track behind the hurricane?

Typically we see ocean temperatures cool

about three to five degrees Celsius behind the storm.

If just one hurricane can have such a dramatic effect

on the upper ocean, what happens when a whole season of hurricanes

power their way across the Atlantic?

In 2012, we had a very active hurricane season with

ten hurricanes which helped to cool the water across the North Atlantic.

But last year, there weren't many hurricanes.

In 2013, we only saw two hurricanes.

What we see are significantly warmer temperatures

compared to average than we saw in 2012.

So a lack of hurricanes may have resulted in areas of the Atlantic

being warmer than average.

But what's puzzling is how could warmer waters in the Atlantic

produce last winter's super-fast jet?

It turns out that the speed of the jet stream

is driven in part by temperature differences between cold air

over the poles and warm air over the tropics.

When the temperature difference between these two regions

is very big, the jet stream tends to travel very fast.

So last year's warmer than average temperatures in the Atlantic

may have increased this temperature gradient...

..which could have produced a fast jet.

We had warmer temperatures in 2013

so you may see some large temperature gradient

between the North Atlantic and the Arctic region

which may then impact the atmosphere

and therefore develop a large temperature gradient

which then can potentially drive a stronger jet stream.

Well, it makes sense to me as a meteorologist that

something as hugely energetic as a hurricane can have a big influence

on the system, if you like, the atmosphere and the ocean's system.

Of course there are so many other factors involved

and that's the challenge of meteorology.

We can't make a direct link but it's very intriguing.

And it poses the question, could the lack of hurricanes in 2013

have played a role in producing such a strong jet stream?

A few weeks after I met up with Eric,

scientists discovered the answer.

It turned out that the impact of a lack of hurricanes last year

wasn't big enough to have turbo-charged the jet stream.

So the abnormal hurricane season was a red herring.

Although the jet stream still remained the number one suspect,

the hunt to discover why it got so fast would have to start again.

The next clue was hidden away in an event

that happened over 130 years ago.

In 1883, the Krakatoa volcano in Indonesia erupted.

Around 40,000 people died

and ash and dust were hurled 35 kilometres into the air.

But there was an unexpected side effect to this cataclysm,

felt around the world.

These spectacular crayon sketches

were done on the banks of the Thames in Chelsea in London

and I've never seen the sky over London look like this.

These were done on a very specific date in 1883

by the painter William Ashcroft.

They look like the sky's on fire, really bright red,

and the events that caused it were on the other side of the world,

the eruption of the volcano Krakatoa.

While artists were inspired to paint,

scientists wanted to understand how the volcanic dust had spread

so quickly across the globe,

producing these extraordinary sunsets.

So the Royal Society put an advert in newspapers asking the public

to send in any unusual observations in connection with the volcano.

These are just some of the letters that came in and they're so varied.

They're also really hard to read

because the handwriting's almost illegible.

People are describing the sky all of a flare at sunset, fall of ashes,

explosions heard and on the back there's these colour pictures of

a sunset and afterglow, the rings, it says, as they formed in succession.

So imagine sitting in the Royal Society in London and getting

these letters and the huge amount of information that's in them.

And that information let them build up a picture of what had

happened all around the globe in the months following the eruption.

That picture revealed something that had never been observed before.

What these observations show is that the dust and the aerosols

that were carried from the explosion spread westwards around the globe

at the equator, almost as if there was a sort of river of wind

running westwards up in the atmosphere

that was carrying it along.

These winds are similar to the jet stream but they travel much

higher up in the stratosphere

and they're only found near the equator.

What's fascinating about these winds

is that they don't always flow in the same direction.

Up there in the stratosphere, above the equator,

there are winds that either travel to the west or to the east

and it switches direction every 14 months or so.

It's known as the quasi-biennial oscillation,

sometimes called the QBO for short.

Back in the 1970s, scientists discovered that

when these winds flow towards the east, they strengthen the jet stream.

And it's intriguing that last winter these same winds were flowing

towards the east.

Finally, scientists had the first concrete piece of evidence

that something was helping to speed up the jet stream

but there was a problem.

These winds have travelled towards the east many times in the past

without producing a record-breaking winter like last year.

So whilst they probably played a small role

in strengthening the jet stream, on their own, they weren't enough.

As the winter ended,

scientists began to develop a third explanation.

They knew the perfect conditions for a fast jet stream involve

a big temperature difference between the poles and the tropics

so they looked for any signs that showed that this temperature gradient

increased last winter.

Throughout the next few days,

temperatures will fall to the low, around minus 13 -

everyday activities may not be feasible.

It led them to investigate the unusual weather conditions

in North America.

These bone-chilling temperatures normally stay locked up over

the Arctic but last winter this freezing air was dragged

southwards over North America.

Professor Dame Julia Slingo, the Met Office chief scientist,

has been looking at what might have caused this cold air

to be dragged south.

Surprisingly her search began with a deadly flood

that happened on the other side of the world in Indonesia.

Last December, unusually intense rain persisted for weeks.

The fatal floods that followed displaced 60,000 people

and left areas under more than two metres of water.

The Indonesian region has been a large part of what's been happening.

You might say well that's an awfully long way from the UK and it is,

but what happens in Indonesia affects profoundly

the weather patterns around the world.

This extraordinary amount of rain triggered off a sequence of events

that would ultimately contribute to a super-fast jet stream.

First, the intense rainfall in Indonesia helped to dramatically

alter the normal path taken by a different jet stream,

the Pacific jet.

That usually follows its path across and well north of California.

In this year, it's gone a very long way north

and then made a very deep curve down over the US and Canada,

what we call a great "buckle" in the jet stream.

This buckle in the Pacific jet stream helped drag

the freezing cold air from the Arctic down over Canada and America.

This cold air then shoved up against the warm air over the Atlantic

which produced a big temperature gradient,

the perfect conditions for a fast jet stream.

The end result was a whole series of storms

so, in a sense, you've had a double whammy if you like.

You've had the cold air coming down and setting up things

on the north side of the jet, but you've had disturbances

also coming into the south side of the jet.

It is a bit like a row of dominoes.

I mean, you know it takes about a week for something

that happens in Indonesia to have its domino effect, if you like,

and we see it in our weather over the UK.

But the connections are so clear this year.

Last winter, there were many different factors at play.

These all worked together to produce a fast jet stream.

The QBO was powering along towards the east...

..and intense rain in Indonesia

knocked the Pacific jet stream off its normal path.

Dad, look out behind you!

This helped increase the temperature gradient...

..which led to our jet stream thundering its way towards Britain.

In many ways, last winter was the perfect storm.

Everything that could have come together

to change the jet stream did.

And it's incredible to think that

so many different factors could have affected our weather back here

in the UK but that's exactly what happened and it just goes to show

how many pieces there are in this giant jigsaw puzzle.

But there is one more piece to this puzzle.

It is perhaps the most controversial and complex piece of all...

..climate change.

How much of an impact did climate change have on

last winter's stormy weather?

It's one of the hardest questions to answer because our climate is

so complex and because there are so many competing factors

that influence our weather.

But we have one very effective tool for understanding

the role of climate change - computer models.

They incorporate the best of our current understanding.

They represent the collective work of thousands of scientists.

They're an amazing achievement

and when they get as good as they are now,

it's possible to use them like a sort of flight simulator for a planet.

It's an amazing tool to have.

Today, super-computers like this one at the Met Office

can do more than 100 trillion calculations every second

and can look at the impact climate change may have

on our future weather.

I think today the incredible complexity and power and skill

of these models that we use, they are one of the great achievements

of modern science and you realise that we're entering,

I think, a golden age for climate science and it's good that

we are because we have some really, really big questions to answer

for the world in terms of what climate change will mean for us all.

Models have predicted that, in the future,

climate change will lead to an increase in extreme weather.

So was last year's extreme winter an early sign of this becoming true?

There can't be a definitive answer on that just yet

because there's quite a lot of research that needs to be done.

That being said,

I think there are various factors that we understand

from the science of climate change

that again would suggest that it's been an additional factor.

To fully understand the impact that climate change had last winter,

more research needs to be done.

But perhaps that's missing the point

because we may never be able to say that one particular weather event

or one unusual season is because of climate change.

But it seems likely that one consequence of climate change

will be more intense rain.

I think it's important to remember on top of all this discussion

of global weather patterns that there is this basic bit of physics

that says that in a warmer world rainfall will be more intense.

No-one's produced any evidence to counter that idea

and it's widely accepted

and so it's reasonable to expect that in the future,

as the world warms, we will get more intense rainfall.

And more intense rain will increase the potential for flooding.

So regardless of whether last winter was made worse by climate change,

flooding is something we may have to get used to.

But our extreme winters haven't just been about rain and storms...

..because previous winters have sent Britain into a deep freeze.

It started in 2008 when temperatures dropped to minus 12...

..as cold Siberian air from the east brought snow across the country.

A year later, Northern Scotland had the coldest winter on record

as once again Britain shivered under cold Siberian winds

for weeks on end.

The following winter,

we had the coldest December in 100 years,

as bitterly cold air from the Arctic

brought a blanket of snow across Britain.

And there is one thing all these recent cold winters had in common.

Once again, the main suspect was the behaviour of the jet stream.

The track of the jet stream varies a lot but, during a typical winter,

it takes this sort of path, straight across the UK.

But in recent cold winters it's done something rather peculiar.

It's taken a meander and, instead, it's moved its way northwards

and then dived southwards which has meant that the UK

has been very much on the northern side of the jet and

that's exposed us to particularly cold air in recent winters.

These big meanders dragged in

either the Arctic air mass from the north

or the polar continental air mass from the east.

Both brought bitterly cold winds and snow.

So what caused these big meanders in the jet stream?

The search for answers soon became an international one

because big meanders in the jet stream also played a role

in one of America's most deadly storms.

The ferocious power of Hurricane Sandy

was the most destructive hurricane of 2012.

On the 29th of October,

it collided head on with the coast of New Jersey in America.

Over 70 died, half a million buildings were ripped apart

and the clean-up bill cost over 50 billion.

Dr Jennifer Francis has been investigating this hurricane.

She's taking me to a part of the New Jersey coastline

that was badly damaged.

So just how unusual was Hurricane Sandy?

Sandy was a very unusual storm.

This part of the coast of New Jersey was one of the worst hit.

In fact the ocean, which is on our left here,

came right across the sea wall.

So this whole area was under, what, five foot of water?

Something like that and you know, of course, with waves on top

and the roadway was covered with one or two feet of sand

after the storm, I can't even imagine what it looked like.

What made Hurricane Sandy so devastating

was the unusual path it took.

Well, normally hurricanes tend to steer right out into the Atlantic,

out to the east, but Hurricane Sandy did something completely different.

It encountered the jet stream

and that created the winds that blew her onto her very unusual path,

taking a sharp left turn right into New Jersey.

And the shape of the jet stream was really critical for steering

Sandy into the coast.

The jet stream had taken a big meander which helped push

Hurricane Sandy off its normal path.

But why had the jet stream developed this large meander

over such a vast area?

To find out, you need to understand what causes the jet stream

to change shape in the first place.

And you can look in the most unlikely of places for the answer.

The jet stream's a bit like a river in the sky

but it's 10 kilometres up and invisible so we can't see it

and it can be hard to understand its behaviour but something that

can help us understand what it's doing is to think about rivers

down here on Earth, like this one, the River Cuckmere in Sussex.

The reason that the water in rivers moves

is that it's flowing down a gradient.

In places where the ground is steep, the gradient is steep

and water flows quickly and it usually flows in a straight line.

But down here on the flood plain, it's a little bit different.

Here I'm down near the end of the river where the land is almost flat.

There's only a really shallow gradient in height

but that gradient is enough to keep the river flowing and I can

measure how fast it's flowing using this and this is a flow meter.

And it's come out at 26 centimetres a second so that's relatively slow.

There's only a shallow height gradient here

and the river's running slowly

and to see the effect of that, I need to go up there on the hillside.

From up here, we can see what we couldn't see down below.

This river isn't running in a straight line.

It's got these big loops in it called meanders

and they develop when rivers run more slowly.

The reason it's useful to look at this is that the same thing

happens up in the sky with the jet stream.

When it slows down, it changes shape

and develops meanders just like this.

The jet stream speed is also linked to a gradient

but that gradient is different to that of a river.

The river here is running because of a gradient in height.

It's running from the ground, the higher ground, inland,

out to the ocean but the jet stream is running because of a gradient

in temperature and just like the river,

when that temperature gradient becomes shallower,

the jet stream slows down and starts to meander.

So weak, lazy jet streams develop big meanders which can get stuck,

resulting in one air mass sitting over Britain for weeks on end.

Which is exactly what happened during our recent cold winters.

So to understand what caused them, scientists needed to find out

why the jet stream had slowed down and produced these big meanders.

It's a search which has led scientists to some of the most

contentious areas of climate research

because Jennifer thinks the answer might be found with dramatic changes

that are going on in the Arctic.

I've been studying the Arctic my whole life and we started realising

in the late 1990s that things were changing really fast up there.

The Arctic is warming almost twice as fast as the rest of the world.

It's a phenomenon known as Arctic amplification.

Three-quarters of the volume of summer sea ice

has disappeared in just 30 years.

The scale of the ice loss is just truly breathtaking.

In 2007, we had a new record for the least amount of ice

in the Arctic ocean at the end of the summer

and since then it's just been every year has been very low and then

in 2012, five years later, we hit another new record low, much lower

than even 2007, so it's just been a steady decline in the amount of ice.

Arctic amplification has not only caused sea ice to retreat

but to reduce in thickness too.

Today, it's around 50% thinner compared to previous decades.

And this loss of sea ice has a feedback on the climate system.

Less ice means less sunlight is reflected back into space.

Instead, the ocean surface absorbs more heat from the sun

so the Arctic warms faster,

resulting in yet more sea ice melting.

The scale of the ice loss gave me the chills

because it is such a huge change to such a fundamental part of

the Earth's climate system, to see that change happening so rapidly.

So it just got me thinking, how is this rapid warming

in the Arctic affecting areas farther south?

To find out, Jennifer looked back over the last 30 years,

the period of major ice loss in the Arctic.

She was looking for any changes in the size of the jet stream's waves,

how loopy they got.

It was a simple measure but her findings were dramatic.

We found that in the last couple of decades,

the waves actually do appear to be getting larger.

They appear to be extending northward more often

and particularly in the North Atlantic

which is important for the UK,

these very large swings in the jet stream are happening more often.

It appears to be the case.

Jennifer believes warming in the Arctic is reducing

the temperature difference between the poles and the tropics.

This is slowing down the flow of the jet stream,

making it more prone to big meanders.

As that difference in temperature

between those two bands of the Earth gets smaller

because the Arctic is warming so much faster,

the jet stream is weakening and

because those waves are what create the weather that we experience

down here on the surface, if those waves are moving more slowly,

then the weather patterns should change more slowly

in any given place

so it feels like the weather that you're experiencing is stuck.

Stuck in a rut.

Stuck in a rut and that's, you know,

we've seen that happen over and over again in the last decade or so.

It just seems to be happening more often now and when one of

those big dips happens just south of the UK, then all that cold air from

the Arctic can come down over that area and create a very cold winter.

So Arctic amplification produces a smaller temperature gradient

between the poles and the tropics

which Jennifer believes produces more meanders in the jet stream.

And when these meanders happen over Britain,

they can drag in the Arctic air mass from the north

bringing cold Arctic winds which produce bitter winters.

It's really interesting what Jennifer had to say to me.

I mean, there's no doubt that the Arctic is losing ice

at an alarming rate.

You don't really have to be a meteorologist,

a climatologist, to conclude that such a fundamental

and rapid change to the system is going to have knock-on effects

to the atmosphere and to the weather which we experience.

Also quite compelling on the face of it is the fact that we have

gone through a run of prolonged spells of unusually severe weather.

However, that could all be a red herring

because it's a challenge to pick out

what is actually natural variability,

just fluff, just noise, from what is a genuine signal.

And the challenge is made even harder by a lack of data

for scientists to study

because dramatic changes in the Arctic have only been seen

in the last 30 or so years,

which isn't long enough to know if the connection

between a warming Arctic and a meandering jet stream is real.

And it's particularly wise to be cautious about the role

of Arctic amplification because there are other theories

behind our recent cold winters...

..one of which has its origins in a very different part

of the climate system.

For decades it was thought that all our weather happened in just

one layer of the atmosphere, the troposphere,

but scientists have discovered that the layer above,

the stratosphere, is also fundamental to our weather.

Professor Adam Scaife is investigating

this important part of the atmosphere.

It's only recently that

the computer models that we use to make

weather forecasts and climate predictions have properly

started to take into account the full depth of the atmosphere

and to properly include the stratosphere.

So at first sight,

the stratosphere seems very remote from the surface weather.

We're talking about very thin tenuous air, 50 or so kilometres

above the surface, but there is an important connection there.

Adam believes something going on in the stratosphere

could provide another explanation behind some of our cold winters.

And he's brought me to Chesil Beach in Dorset to show me what it is.

So the reason I've brought us here is because although what you

see behind us, these breaking waves, might seem remote and completely

irrelevant for the cold winters that we've had, there is actually

a deep underlying similarity between the breaking waves here and

breaking waves really high in the atmosphere

during these cold winter events.

Scientists have discovered that the thin air of the stratosphere

is home to giant atmospheric waves which behave in a similar way

to crashing waves in the sea.

When one of these waves breaks,

it generates something called sudden stratospheric warming.

Adam's discovered that these events have occurred

during some of our recent cold winters.

So two out of three of the recent very cold winters that we've had

have occurred in conjunction with sudden stratospheric warming.

This initially occurs really high in the atmosphere,

50 kilometres or 30 miles above the surface and it happens when a wave

in the atmosphere breaks at really high altitude.

That breaking wave actually pushes the wind opposite

to its normal direction.

Normally the winds in the stratosphere blow

in the same direction as the jet stream, from west to east,

but as this enormous stratospheric wave breaks,

it pushes these winds in the opposite direction towards the west.

These winds then burrow their way down through the stratosphere

until they hit the jet stream.

And because the jet stream flows in the opposite direction,

these winds act like a brake - slowing it down.

The whole process of this burrowing down through the atmosphere

can occur on a timescale of a few days, maybe a week or two, until it

reaches the jet stream and at this point, it kind of switches off the

jet stream and blows cold air from Siberia in towards the UK and leads

to those dramatic cold snaps that we've experienced in recent winters.

So a sudden stratospheric warming slows the jet stream

which can produce a big meander.

As a result, the polar continental air mass is dragged in from the east

across Britain and with it comes bitterly cold air from Siberia.

Since these events can influence our winters so dramatically,

scientists want to know if there's a pattern to their occurrence.

But it isn't so simple.

Now there is no regular pattern to when these events occur.

On average, they're every two years but just like tossing a coin,

you could get three heads in a row,

sudden stratospheric warmings can occur in runs of winters

or you can have long periods, like the 1990s, when there were

no sudden stratospheric warmings for several years on end.

The thing that's a bit frustrating about these sudden

stratospheric warming events is that there's no clear pattern to them

but as humans, we're always looking for patterns.

If a coin falls heads lots of times in a row,

we start to ask why but sometimes that's just the luck of the draw

and so it may well be that we've had some cold winters

and we're looking for a pattern, but really there isn't one.

So it looks as if at least two factors could have caused

some of our recent cold winters by weakening the jet stream.

Sudden stratospheric warming...

..and Arctic amplification.

We still don't know which one will dominate in the future...

..or indeed whether other factors like the behaviour of the oceans

or changes in solar activity could play a role too

in influencing our winter weather.

All this makes it a challenge to know

whether we face cold or wet, stormy winters in the future.

But despite this uncertainty,

there may be something we can say about our future winters.

Once again, it all comes back to the jet stream.

For me, the strongest signal to emerge

as we struggle to understand the recent extreme weather is the idea

that the jet stream can become stuck in certain configurations.

At one end of the spectrum, a very straight, fast jet stream

which brought the storms of last winter.

At the other end of the spectrum, a much slower, meandering jet stream

which has brought the recent run of particularly cold winters.

But either end of the spectrum is capable of bringing prolonged

and extreme weather and perhaps this is something

we should expect more of in the near future.

Scientists are continuing to improve their understanding of the jet stream

but, even now, there's a lot we can do to prepare for our future.

We don't control the weather but we're not helpless

and being uncertain about the future isn't the same as knowing nothing.

I think the science here is in a really good state.

There's lots of debate, lots of different ideas, lots of evidence

that's available for everyone to see and I feel optimistic.

I think that we're really getting to grips

with the science of these extreme weather events.

And that means we can also begin to get to grips with how to deal

with our changing climate.

When I was a boy, I lived on the side of a hill

and I'd sit overlooking the valley

and I'd watch the weather coming my way and that was my world really

and as far as I was concerned,

the weather was contained within the valley.

We all have a tendency to think locally but we have to look beyond

the parochial confines of our valley, our country, our ocean even.

We have absolutely no control over the weather but what we can do is

understand it and adapt our society for the changes which lie ahead.