The last of the four powerful forces to be studied in Dr Iain Stewart’s look at the power of the planet was the oceans. Next week he’ll be wrapping up with a look at how unique the Earth is and how it was turned from a barren rock to a place burgeoning with life (in other words, look forward to a series of repeats of the graphics we’ve already seen) but for now he was content to stand on the prow of a small boat cutting through the vast Atlantic, and tell us to look with awe at the majesty of the oceans.

In the next breath, he was showing us just how fragile and small this cocoon of water is compared to the rest of the planet. Armed with more clever graphics Dr Stewart sucked all the water off the surface and formed it into an orbiting droplet. Compared to the Earth it did indeed look small and vulnerable. And that, it seems, was the point.

Like some incredible superhero, the oceans have many forces. They have wave power which, it turns out, is actually wind power that’s collected out at sea and then traverses the deep oceans, only turning into breakers when it hits the shore.

This force is responsible for carving away our coastlines, from the really hard jobs it does at the basalt coasts of NW America, to the relatively simple task of knocking down the muddy cliffs of Eastern England where the coastal erosion takes a kilometre of Blighty into the sea every 500 years.

The oceans also have tidal power, which normally means the moon but can get extra powerful when moon and sun align to cause tidal bores, travelling up to 25km up-river and washing away tons of river bank.

And they have heat power, but more on that later. First: where did the oceans come from? Well about half the water fell as rain once the steam that was ejected from the embryonic Earth’s guts had cooled sufficiently to fall back to Earth. This was one heck of a rainstorm mind you. It’s estimated to have taken thousands of years for the rain to stop. Easy job being a weatherman back in those days, but it was 4 billion years ago and there weren’t any weathermen on hand to take advantage of it.

The other half came from outer space in the shape of comets. Carrying millions of cubic litres of water, they crashed repeatedly onto the surface and gave up their water in the process. Once this was finished and the oceans were full, they stayed pretty much at the same level from that day to this. Except of course they’ve moved around a bit as a result of plate tectonics. In fact Doctor Iain showed us a remarkable shot of a new ocean forming. OK it looks just like any other small hole in the ground at the moment, but trust me, it’s a baby ocean.

The reason we know that, is the hole is forming on the edge of two plates that are moving apart. It’s in Ethiopia, close to the Suez Canal. Those land masses are moving apart, and during the next few million years the gap will fill with water and become a new ocean. How cool is that? Well, not very. It’s pretty hot down there.

Oceans can disappear too. Well, seas at least. The Mediterranean to be precise. The Med evaporates at a rate greater than it is replenished by rain and rivers, so it depends on its connection to the Atlantic to stay … well … in existence. Some time ago Africa crashed into Europe. What an insurance claim that must have been, but the point is the Straits of Gibraltar closed up, and 2,000 years later the Med had all evaporated. We know that because of the huge salt deposits in the salt mines underneath. Salt mines so vast that the rate of extraction (500,000 tonnes per year) can be kept up for the next million years.

But let’s get back to the oceans, because they’re full of phytoplankton and without them there’d be no life on Earth. There has to be something at the bottom of the food chain, and they’re it. They might be small, but they’re visible from space, like green clouds floating round the oceans. Which just goes to show, like the old Trades Union advert on the telly, that it doesn’t matter if you’re small as long as there’s a lot of you.

Oh and currents too. There’s this thing called the Great Ocean Conveyor that’s driven across the middle of the Earth by the sun, but also relies on the polar icecaps to make the hot water cool and sink, and therefore capable of making the return journey. It carries nutrients and oxygen all around the planet, so without it: no phytoplankton. And without them: no life on Earth. In fact this happened once. A long, long time ago. 250 million years actually, before the dinosaurs in the last *really serious* global warming epidemic. The ice caps disappeared, the cold water couldn’t sink, the Great Ocean Conveyor stopped conveying, and everything died.

You have been warned. (I’m not kidding. I was scared.)