Kia ora, koutou
Ko Richard Levy toko ingoa. Ke Te Pou Ao me Te Herenga Waka e mahi ana.
Hello, all. I'm Richard Levy.
I work at GNS Science and the University of Wellington -
Te Herenga Waka.
I co-lead the New Zealand Sea Rise programme, Te tai pari o Aotearoa
with my colleague here.
Kia ora, I'm Tim Naish
I'm also at Te Herenga Waka,
Victoria University of Wellington.
Today we're going to talk a little bit about the importance
of vertical land movements when it comes to understanding
and predicting future sea level. In places like New Zealand
that sit across convergent plate boundaries where there's uplift
and where there's earthquakes and where there are vertical land movements,
this has to be taken into account when predicting future sea level.
So vertical land movements can be quite significant
and they can be as large as the global mean sea level signal itself.
So right now, global mean sea level is about
3.5 millimetres per year of sea level rise.
That's the average rate around the world.
And in some places around the New Zealand coastline,
the land is subsiding twice that much
or it is rising twice that rate.
And so
when the land
is subsiding at the same rate sea level is rising, you essentially double
the amount of sea level rise and where the land is uplifting
at the same rate sea level is rising,
you cancel out the global sea level rise.
So vertical land movements or VLMs
as we call them, matter when predicting
sea level rise.
We're using
a couple of different instruments to make these estimates.
Global positioning satellites that are sitting in some of the hills
behind us attached to the rocks, accurately measuring change
in the position of the rock through time
is one method we're using, but we're also using
radar systems that are
based on satellites that are orbiting the Earth
above us right now.
These satellites repeat their orbits and fire a radar beam
down to the surface of the Earth, hits the earth, reflects back
and can actually measure the elevation of the land at sub-millimetre scale.
And as those orbits repeat and additional measurements are made
you can actually detect change in the surface elevation of the Earth
at very high resolution.
So we're able to use those satellite interferometry
those satellite radar data, and the GPS data to combine
to get accurate measurements of changes in the surface elevation.
And we're doing this every two kilometres around
the 15,000 kilometres of New Zealand's coastline, making these measurements
so that we can incorporate them into our sea level projections.
When the land is going up,
we can show that the amount of sea level rise will be less than you would predict
if you weren't considering those vertical land movement estimates.
Where the land is sinking, it's going to amplify the amount of sea level
rise in the future.
Because New Zealand is sitting on a plate boundary,
you've got the Australian plate and the Pacific plate
basically crashing into each other and that creates uplift and mountains.
So a lot of New Zealand in the long term historically has gone up.
Something interesting is happening off the eastern margin of the North Island
of New Zealand, and that is where the Pacific plate
is going down, being pushed down under the Australian plate
and it locks, it grips, and it pulls the land down.
So in the lower North Island and on the eastern side of the
lower North Island, the land is actually subsiding,
going down
at quite high rates of up to eight millimetres
per year. Right here on the south coast of Wellington
at Lyall Bay, the subsidence is three to four millimetres per year,
which is effectively doubling the amount of sea level rise
we can expect right now and in the coming decades and centuries.
And so, on short timescales of decades to centuries
the land in this region - the lower North Island and the
eastern lower North Island, is being pulled down, but it does pop back up again
on the long timescale.
So an example of that is if you look out here to Turakirae Head,
you can see these lovely flat surfaces, these terraces
which are old shorelines, hundreds of thousands of years old,
and they're now uplifted because of that long term uplift.
But right now, and what matters on human timescales
and what matters for projecting the impacts of sea level rise
in this part of New Zealand, it's the short term subsidence
that really matters, and has to be factored into our sea level projections.
This part of New Zealand is exposed to some fairly
big swell waves that get generated down around Antarctica.
So this part of New Zealand already gets big storm events
that at a high tide causes coastal flooding, and these buildings around us
are often in the news and getting damaged by those storm events.
Now the problem with the subsidence is that it's
really amplifying the amount of sea
level rise we will get over the coming decades.
So, by 2050, globally we would expect an average sea level
rise of about 30 centimetres. You can double that for this spot.
So that'll be 50 to 60 centimetres in the coming decades,
and that cannot be avoided.
That is already baked in, from the global warming so far.
So what that means is that the hundred year
coastal flooding event we currently get now that does a lot of damage to the roads
and the houses around this part of Wellington, will become an annual event
with just 30 centimetres of global sea level rise,
but 60 centimetres right here because of the land going down.
We're sitting
here on the south coast of Wellington on a pretty
nice day. There's very little wind, weather's fine, there's a little bit of a swell
and we can sort of see the level of of the ocean.
You sort of might wonder, why do we really need to worry
about a slight change in the level of the ocean?
The seagulls sitting on the beach
might have to migrate up the beach a little bit to sit where they are.
Who cares right?
But you imagine this place
on a day where the swell is large and there's a big storm coming in
and we already know that, when that happens, the road behind us
is inundated by waves that crash across the road.
Add 20 centimetres to the average sea level and put that same storm on top of it,
and those waves are going to inundate even further.
So the bottom line is that
relatively small increases in average sea level matter
when you put those big storms on top of it.