Unless you live right at the coast, most of us really only think of the ocean when we have a beach-trip planned. Of course, we know that it is a useful part of the planet when we want fish tacos, but otherwise it’s mostly a bit of an inconvenience for travel and shipping. And there are jellyfish.
But did you know that the ocean is at least as important as the atmosphere in transporting heat around the planet?
The primary purpose of circulation in the atmosphere and the ocean is to move heat from the equator to the poles. If this didn’t happen, the tropics would keep getting warmer and the poles would get colder. In the tropics, oceans account for around 50% of heat transport towards higher latitudes.
This turns out to have huge implications for parts of the globe. For example, parts of Britain are about at the same latitude as southern Hudson Bay, and at this latitude, you’d expect sea ice to form all around the UK in the winter. But it doesn’t of course. The Gulf Stream keeps the northeast US, parts of Europe, and Britain, if not toasty warm, at least reasonably equable in the winter. It’s possible to grow hardy varieties of palm trees in England.
This all happens via a series of interconnected currents often called the ocean ‘Conveyor Belt’ - a term coined by Wally Broecker, who died in 2019. He was a geochemist (who famously refused to use email), and I had the privilege of hearing him lecture on several occasions, including for a whole week at a summer school I attended while at graduate school. He was quite the legend, and we owe a lot of our understanding of the relationship between the ocean and climate change to him.
Also called the thermohaline circulation (less catchy), differences in heat and saltiness drive the currents around the globe - partly on the surface, and then sinking to move cold water around in the deeper ocean. When the colder water resurfaces as part of the conveyor belt, it is full of nutrients and drives much of our food web.
Also, one of the most important tasks of the ocean in the current era is to absorb carbon dioxide. About 30% - 50% of the carbon dioxide we emit into the atmosphere gets absorbed by the ocean (some by directly dissolving in the ocean and some is used by algae and phytoplankton). This is very handy - without it, we would be in WAY more trouble with climate change.
It comes at a cost though. The ocean is becoming more acidic, and there are already signs of impact in shellfish. The calcium carbonate of most shells is susceptible to more acid in the ocean. It’s also possible that as we warm the ocean, its efficiency to absorb CO2 will decrease. Cold water can ‘hold’ more carbon than warm water (think about a can of Coke - it emits more bubbles of CO2 when warm). This is a feedback loop we definitely don’t want to explore in real life.
The other challenge with climate change is at the ‘top’ of the conveyor belt. You can see in the image below (from NASA) that water becomes colder and saltier (denser) at the northern end of the Atlantic. The dense water then sinks and is drawn back south at a deeper, colder level. This is a very important driver of the thermohaline circulation.
However, increased meltwater from the Greenland ice sheet, flowing into the northern Atlantic, makes the seawater less salty and therefore less dense. So there is concern that at some point, the water won’t sink, or at least the sinking zone may move further south, disrupting the conveyor belt. This would be bad news for more than the palm trees in England.
So, whichever way you look at it, the ocean is a true mover and shaker.
Our global distribution of heat and nutrients, our levels of carbon dioxide, as well as the existence of basically all living beings, are all driven and supported by the ocean.
This seems at least as important as beach trips and fish tacos.
Plus, I truly believe it is a frontier we have only just begun to explore biologically, chemically, geologically and more. A beautiful treasure trove of global gifts - beginning right there at the water’s edge.
I think we should keep learning, exploring, and protecting it this World Ocean Day.
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The National Park Service has some ideas for taking personal action to support ocean health.
And the World Wildlife Fund.