How do we measure sea level rise?

So you mentioned measuring the height of something that's moving.

You can see that.

That can't be easy.

How do you actually go about starting to do that?

Originally, it was just literally a big ruler.

Like, you know, those giant sort of gauges you see kind of go up to a few meters.

So any given sort of coastal spot where knowing sort of the tide level or the sea level was important, there'd just be a big ruler with markings on it and people would sort of note them down at different times of the day.

So that's a measurement that's gone back hundreds of years.

Over the last century, we've gotten a lot more mechanical with it.

So we use these things called tide gauges.

Obviously, the big ruler was a really, really primitive tide gauge.

And then we started doing more elaborate things to record sea level.

So there was for a while they used these floats attached to pencils, attached to these kind of rotating streams of paper.

And the pencil would show you the sea level.

So you'd get a record of it over the course of the day without someone having to sort of sit there and mark the whole thing down.

Now what most places use are these tide gauges which are attached to like a wall or a wharf or something and they have these acoustic sensors in them so they send out a little sound signal which bounces off the ocean and they time the amount of time it takes for that signal to come back and then that tells them the height and they can do that every couple of seconds so you've got this kind of continuous measure of what the sea levels are doing at any given spot.

Now there are some limitations though with the tide gauges, such as being connected to land.

Talk to us more about those.

Yeah, so if you want to use a tide gauge to figure out what the sea level is, it's got to be connected to something solid.

So it has to be connected to like a wall or a wharf.

It's basically got to be attached to land.

So that means out in the open ocean, it's really, really difficult to establish what sea level is.

And the sea level does change around the world.

There are points of the ocean where it's much higher than other points.

Obviously, if you're out sort of in the really, really deep sea, it's like six kilometres deep.