I’m the first to admit that my main interests in geosciences are in what some might think are the more glamorous — certainly the more newsworthy — areas, specifically earthquakes and volcanoes. In this context it’s sometimes easy to overlook the holistic nature of the Earth system. It includes everything — mountains, oceans, trees, weather, you name it — and everything is interrelated.
This week the theme of the digest is the oceans. With over two-thirds of the planet’s surface covered with water, it’s surprising I haven’t come to it before. But the oceans have been in the news this week, with the so-called Pacific Garbage Patch, and you don’t have to look very far to find a other ocean-related stories to go with it.
The Pacific Garbage Patch: What, Where and Why?
We’ve been talking a lot about plastics recently. Plastic is part of the Earth system, too (didn’t I say that everything was?) and that means that you can’t just throw it, or anything else, away. When you eject it from your part of the Earth’s system, it has to go into another part, just like everything else.
Many things — even rock — do eventually decompose and become subsumed into another part of the system, but many sorts of plastic, designed to be durable, just hang on in there, not just for years but sometimes for decades or centuries. So when you throw them out they might end up in landfill, or in the oceans.
This is where the ocean circulation comes in. Driven by the sun’s heat, convection currents form in the ocean giving rise to a generally steady, though sometimes varying, pattern of movement through the oceans. This is the subject for a textbook in itself, so I’ll keep it simple. These circulations form massive loops, known as gyres, at various points and anything which gets caught in one of those gyres remains there.
That’s what happens to plastic. (And, in fairness, to a whole load of other junk that finds its way into the sea.)
Oceanographers and others have known about their existence for years, but this week they leapt to prominence with the revelation that one of them, to the north west of Hawaii and known as the ‘Great Pacific Garbage Patch’ is, to quote The Guardian: “more than twice the size of France and is up to 16 times larger than previously estimated”. (France, by the way, has an area of 643,801 square km.)
I don’t propose to go on at length about what’s in it, or any of the other oceanic rubbish dumps that float about on the surface of our planet. There’s plenty of information and opinion out there if you care to look. But I will reiterate what I said above. The Earth is a closed system and anything that is in it will stay there, in one form or another, unless it’s sent into space.
The North Atlantic and Ocean Circulation
The oceanic garbage patches are essentially two-dimensional, moving about from one point to another on the surface of the water. But ocean circulation is three-dimensional, with water sinking in some areas when it becomes cold and upwelling in others where it becomes warm. Again this is a phenomenally complicated system and little of it is understood, but we do know that because the ocean currents are enormous transporters of heat energy, they are tightly linked to climate.
So, back to that point again. A change in the climate will have an impact on ocean circulation and vice versa.
The ‘driver’ of the ocean circulation is thought to lie in the North Atlantic, where cold water sinks and effectively forms a pump. This leads to a pattern of circulation at deep and intermediate levels which is different from that at the surface. The downward movement of denser water in the pump is driven by cold salt water at the surface, associated with formation of sea ice Arctic ice. So what happens when the patterns of ice in the Arctic change?
With an increase in global temperatures, there’s an increase in melting of the Greenland ice cap. Water from this source is cold but, unlike sea ice, it’s fresh rather than salt. That means it’s less dense and so doesn’t sink so easily.
Quite what impact this injection of fresh water will have isn’t clear. The website Physics.org reports that: “researchers say it’s too soon to say whether the decline is due to an influx of freshwater from melting ice on land or sea, or part of a natural, longer term cycle” while the New Scientist runs the story under the headline: “Polar melt may shut down the Atlantic current that warms Europe”. These two headlines are not mutually exclusive, but they certainly give off different messages.
I said above that we don’t fully understand the ocean circulation, and that’s true. Like the climate, it has natural variations over long time periods that may be the cause of the changes we see, or they may be caused, or accentuated, by anthropogenic climate change. But it seems certain that major changes in the ocean circulation, whatever their cause, will have significant impacts elsewhere.
The Mediterranean and the Mile-High Waterfall
I’m coming to this piece of news less in the spirit of gloom which will almost inevitably accompany the first two sections, but rather in a spirit of awe. Earth scientists have to think on large scales — thousands of miles, hundreds of millions of years, mega-eruptions — and this week I saw a reference to something that made me sit up — a megaflood in the Mediterranean.
First, I should fill in some background. The Mediterranean is notoriously salty, the legacy of a time when it was cut off from the rest of the ocean system by lower sea levels and a higher sea bed at its eastern margin. At this stage, the ocean almost completely — possibly totally — dried up.
At some stage the connection between the dried-out basin and the sea was re-established. This has been known for some time, as has the fact that the volume of water flowing into the Mediterranean via Gibraltar was enormous: “about a hundred times larger than the Victoria Falls” as one source has it.
This week I’ve seen reports of a study (actually published back in January) into the detailed mechanism of the refilling of the Mediterranean has suggested that the flood, which must initially have come from the west, actually filled the eastern Mediterranean via a second enormous waterfall (for want of a better term), cascading over a ‘step’, located west of Sicily.
They don’t give volumes to the extent of this event (or, indeed, say that the waterfall was a mile high, as an article in Scientific American suggests), but they conclude that it was: “a Mediterranean-wide catastrophic flood … and suggest that the identified sedimentary body is the largest known megaflood deposit on Earth”.