Hello! Today we're on the Castlecliffs beach. We're here with our third year geology students
and you can see they're having a good time looking at the rocks. This really is some
of the most spectacular geology in the world in terms of recording past sea level change.
What we're looking at here is mudstone - very fine-grained marine sediment that you'd find out
on the on the shelf today, offshore from where we are. But this is old. This is 800
000 years old this mudstone, and it was probably representing a water depth of about 100 metres.
Okay we go up and we see more mudstone and then we come into this rather impressive shell bed.
So we've gone from 100 metres water depth here to zero. The fossils in here
live in the intertidal zone. They live pretty much at sea level on the beach. And
you can see they're all smashed up, they've been reworked by waves so this is an old shoreline
where there was waves moving material around. We go up a bit higher and it's very sandy up here,
and this is very shallow - sandy, and if we look further up we start to get a few more shells
but progressively the sediments get deeper water again, they get finer grained. By the time we get
three metres above my head we're back in about 50 to 100 meters of water depth. So this is a classic
sea level cycle where we've gone from deep water to the beach back to deep water again. Well what
it's telling us is sea level has been high and sea level has fallen again and it's done that
many times in the past, the rocks tell us that story. And what's special about this coastline is
you can walk along it and you can see 20 sea level cycles that span the last million years or so. So
what we're seeing is a window into earth sea level change. As sea level went up and down it moved the
shoreline backwards and forwards. You can think of it as sort of a tape recorder - a sedimentary tape
recorder. These rocks are recording the changes in the shoreline as sea level went up and down.
800 000 years ago this was underwater so why is it now up? So what's happened is there's been
uplift of the land, the land has gone up and it's gone up just in the last few hundred thousand
years because of earthquakes and tectonic forces that have shaped the North Island.
Okay! Well we're here in the Rangitikei river valley. It's in flood at the moment. We're
lucky to be sitting on the shell bed. It's it's absolutely spectacular - It's one of
the most impressive shell beds in the whole of Wanganui basin. It represents the time at which
the Earth cooled and the big ice sheets first formed on the Northern Hemisphere. So that was
two and a half million years ago. We're on a major river - the Rangitikei River,
and it has been cutting through the cliffs here. And we really have a spectacular example
of sediments that record sea level changes over the last two and a half million years.
It is the best place to go in the world to see the complete record of global sea level changes
since the first glaciations began about two and a half million years ago. So
what does all this mean? Two and a half million years ago this was the sea floor and it was
thriving with life. Now one of the shells tells us that it was cold up here, and it marks the
beginning of the Quaternary period of geological time. And what is special about the Quaternary:
we had the first major glaciations of the Northern Hemisphere and humans our species began to evolve.
And so what caused the Northern Hemisphere glaciations was a natural fall in carbon
dioxide - the planet cooled, big ice sheets formed on the Northern Hemisphere and they've
come and gone regularly. That is a natural rhythm over hundreds of thousands and millions of years
in our climate system. So what's vitally important for us to know: What is the natural pattern? Where
are we in the natural cycle and where should we be heading? And we know that, we know that
we are currently in an interglacial warm period and within a few thousand years
we should be going back into another glaciation. Is the warming unusual in the context of the last
few thousand years, millions of years? Absolutely! The amount of greenhouse gases
and the temperature warming we've seen over the last century is unprecedented
for three million years. We have not seen that level of greenhouse gas or that level of warming.
Furthermore if the warming continues we are unable to naturally go back into another cold period.
We now know that we put enough carbon dioxide in the atmosphere that we can no longer go back into
a natural ice age. So this is a very amazing thing humans have done through the burning of fossil
fuels. We've warmed our climate to a state that we have changed the natural pattern of the ice ages
and the interglacials. And that is said with 100 per cent confidence by the Intergovernmental Panel
on Climate Change based on the science, knowing what the world was like when we had more carbon
dioxide in the atmosphere and now knowing that we can no longer get cold enough to have another
glacial unless we get our emissions back to about 350 parts per million. Then we have a
chance of restoring the natural balance, the natural rhythm of the ice ages. We,
through all the greenhouse gases we have put in the atmosphere, have managed to take the Earth
off its natural climate cycle, and now we're into a world we call the Anthropocene. Now we are the
drivers of climate change: humans are controlling what our future climate will look like.
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