at looking at all the time,

looking down over the North Pole.

Here you've got the Arctic Ocean,

it's about 14 million square kilometers, and it's surrounded

by land on all of its sides, but then you have these regions

where water can flow in and out in here

through the Bering Strait, into the North Pacific, and then here

through Fram Strait and the Davis Strait

into the North Atlantic.

In the winter, the temperatures across the ocean on average get

down to about minus 30.

And because of these cold temperatures,

the sea waters freeze and it forms a layer

of ice known as sea ice.

And over average, the ice is about 2,

2 and half meters thick,

and below it is kilometers of the ocean.

And in the winter, that ice completely fills the Arctic

basin and even reaches

out through the Bering Strait into the Bering Sea.

Now in the summer, that ice cover starts to retreat

as the temperatures get warmer and in the summer.

The average temperature across the Arctic Ocean is

around 0 degrees, and this video

that you've been watching here shows the summer September ice

extent minimum.

It's from the National Snow and Ice Data Center in the USA,

and some of you might be familiar

within the press every year around September,

you see headlines about we've reached another minimum arctic

sea ice extent, and it's a downward trend

since the satellite records began in 1979.

So, and then you often get kind of speculation

about when the arctic ice cuff is going to be ice-free

in this summer, and whether the Northwest Passage will be open.

And we can go back to PowerPoint now.

The [background noise], this is in the melting and freezing

of the ice [inaudible], that the ice kind of changes.

The ice is also moved around by the wind, it's dynamic.

And what you're looking at here is a video

that I took while I was on an ice company arctic.

And its two icebergs, basically being blown by the wind

and they crush in together, there's actually the sound

that you could hear, just before they lowered the volume,

was the sound of actually the ice [inaudible], the ice pushing

up against each other.

So as the ice is being moved around,

it's forming these ridges and it's thickening dynamically.

The other thing that happens as the ice moved is

that it splits apart, and again this was a picture I took during

an ice camp.

We've gone in for dinner into our mess tent and came out,

kind of-- just as the sun was setting and we were greeted

by this view about 100 meters from where we were camping.

The ice plain we were on had actually split in two,

so we're quite glad it hadn't happened straight on the center

of our camp, but it was an amazing view

and what happens here in the winter, because the warm--

the ocean is a lot warmer than the atmosphere.

It would lose heat to the atmosphere

and then ice-- new ice will form.

So these areas of open water become production areas

for the formation of ice.

So those figures I shared, or that animation I showed you

at the beginning, with the ice extent changing from year

to year, it's changing for 2 reasons,

one because it's melting or it's refreezing,

and the other thing 'cause it's changing dynamically.

So we don't just need to know how the area is changing,

we also really need to know how the ice thickness is changing,

'cause then we can work out how the ice volume maybe changing.

Now sea ice has kind of an important role

in our climate system.

Something that you maybe familiar with it is this--

is the idea of the ice-albedo feedback mechanism.

So albedo is ma-- measure of the amount

of solar radiation that's reflected back in to space.

So sea ice covered by layer

of fresh snow has a very high albedo, about 0.9.

That means it reflects more radiation back into space

than the open water does and absorbs more radiation.

So when you have more [inaudible] from water,

more radiation is absorbed, you get more heating that can go

on to melt more ice and so on.

And of course the converse is true.

When you have more ice cover,

you get more radiation reflected,

and you can get cooling.

But this is isn't the only way the sea ice affects our climate.

It forms on the ocean so it forms a barrier

between the atmosphere and the ocean.

On the open ocean, the winds can free, they can't move the water,

but this isn't the case in the Arctic.

So that's another effect that it can have on,

on our climate system.

The third thing is that when it melts,

it adds fresh water into the ocean.

When it freezes, it add salt into the ocean.

So that can affect the density of the water,

but it's not just sea ice that's an important component

of the fresh water in the Arctic.

Now this diagram here describes a kind

of simple structure of the Arctic Ocean.

The sea ice actually fits in a very cool fresh layer

and that's separated from warm, salty Atlantic waters beneath,

by I think called the halocline.

So "halo" means salty and "cline" means slope.

It's a steep density gradient that's controlled by salinity

that separates these 2 very distinct water masses.

Now, I siad the sea ice forms contribution to that fresh water

in the top, but it's not just that.

In the beginning, you saw the map of the Arctic

and it was surrounded by continents.

And as you move in to the summer, these--

the rivers in those continents thaw and the river runoff runs

into the Arctic Ocean, and that provides another source

of fresh water, will also got cha--

changes from fresh water from precipitation and evaporation,

and also exchanges through those outlets that I showed you

into the North Pacific and North Atlantic.

And this diagram here is taken from a paper

and it shows the mean distribution

of that liquid fresh water in the Arctic.

And you'll notice it-- the red colors,

basically show we've got more of that fresh water

and that's predominantly in the Western Arctic.

So here you've got Greenland,

and this is the Canadian archipelago.

This is an area, notice the Canada basin

and it contains the Beaufort Gyre,

which is something I'm going to talk a bit more about later on.

Now we're interested in the storage and distribution

of this fresh water because if it is released even in parts,

it has the potential to disrupt the thermohaline circulation,

which then could have a knock-on effect

to our climate in Northern Europe.

So this slide is basically to summarize UCL's heritage

with working wit the European Space Agency to use satellites

to look at the changes in the Arctic.

This photo here was taken of the remote sensing group

at the Mullard Space Science Laboratory which is also known

as MSSS-- MSSL, and it's part of UCL.

This photo was taken about 20 years ago,

but actually our heritage with this kind of work starts,

even earlier than that, around the early 1980s, 1982 to 1983.

MSSL let us study for the European Space Agency,

looking at the feasibility of using satellites

to monitor changes in the Arctic.

But it wasn't really until the launch

of the Earth remote sensing satellites, which name is ERS1.

That was launched in 1991.

We can use data from 1993 onwards.

It wasn't until those satellites we launched

that we're actually able to have observations over the Arctic.

Previously, the satellites didn't go up that high,

we can actually see-- take data from there,

and it was really work done during this time

by Seymour Laxon who is sitting in the audience here

that pioneered the method that we used to calculate

or estimate sea ice thickness from space.

And I'm going to go on to the next few slides

and describe actually what this technique is

and how we're actually doing it.

So all of those satellites you saw were on the last slide,

and you carry an instrument known as a radar altimeter.

So the first bit of that is radar.

Now radar's a really useful tool for Earth observation.

This diagram here shows the opacity of the atmosphere

to different wavelengths of radiation.

And I'm sure you're all familiar with the idea

that an x-ray can see through your skin

and it can make a map with your bones.

Well, radar can see through our atmosphere.

It can see through the clouds and it can monitor what's going

on at the surface of the earth.

So that's why we use that frequency in Earth observation.

It also doesn't rely on having daylight,

which if you're using sort of an invisible wavelength,

then when it's dark, you're not going to see anything.

So it's useful for getting year-round measurements

over the earth.

Now the second word I mentioned was an altimeter.

Now this slide describes the measurement principle

for an altimeter.

And if you cast your mind back to your school days,

I'm sure you'll remember the relationship speed equals

distance over time, and that's basically what we're doing here.

The altimeter transmits a pulse of radiation, it travels to down

to the surface of the earth, it reflects to the surface,

and travels back up to the satellite.

And we measure the time taken for that pulse of radiation

to travel from the satellite to the earth and back again.

Now we know how fast the radiation is traveling.

We know the time, so from that we can work out the elevation

of the satellite above the surface that we've looking at.

So over the Arctic Ocean, we have the--

those areas of open water, the leads.

So we measure the elevation to the leads,

and then we measure the elevation

to the ice slopes next to them.

And if we take the difference between those measurements;

we can calculate the free board of the ice, so that's the amount

of ice, that sticky arc above the ocean surface.

Now, to kind of simply explain what we do next

to estimate ice thickness from this measurement,

the ice is floating and roughly nine tenths

of the ice is below the water level.

In reality, it's a little bit more complicated from this,

we have to take into consideration things

like the snow depth and density that's sitting on the top

of the ice and how it weights the ice down.

We have to consider the ice density

and the water density as well.

But that basically describes the principle of the measurement

and what were looking at.

Now, as I said today, I want

to present you our most recent results,

and I don't really have time to go into what we've looked

in the past, and though Seymour and myself had looked

at how the Arctic ice thickness has been changing,

but today just going to take the measurements over the ocean,

and this was actually started a couple of years ago

when we started looking at these ocean graphic measurements

on their own, and we noticed something was going on.

Now this is a video that's being made using our data.

The reds mean that the sea surface height is getting

higher, and we were looking at data between 1995 and 2010,

and what we could see is the sea surface height getting higher

and higher.

This area is in the Western Arctic, that area I pointed

out to you earlier where you get the largest storage

of fresh water.

So obviously after seeing this in the--

our data, we wanted to find out what was going on.

So our first port of call was to get and actually have a look

in the literature, and see what other people had been observing.

Now this figure here is from some work by [inaudible]

and what they've done is taken in situ measurements,

say measurements that should be made for being

on the Arctic Ocean, so from a ship or being in ice kind

of pool having a mooring floating around