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We are all familiar to varying degrees with the atmosphere’s complex role in global heat distribution. We call it weather.
Underwater ocean weather is less well known and less visible.
Surface circulation of the oceans creates the familiar currents such as the Gulf Stream. These currents are shallow, driven by the wind, and circulate in gyres that are concentrated on the western edges of the oceans.
Deep and surface circulation systems are linked in what is called the Great Ocean Conveyor because the water moves in a three-dimensional, circuitous path through the Atlantic, Pacific and Indian oceans.
Because the conveyor carries one-third of all of the heat transferred poleward from the equator, even small changes in the heat transport play an important role in controlling global climate.
Deep currents are driven by density, which is controlled by temperature and salinity. The driving force for the conveyor is ultrasaline and super-cold water that forms at the surface near Greenland and Antarctica. The water there is cold because of the latitude, and because of the flow of cold, dense water from the Arctic Ocean that flows southward around Greenland.
The water is saltier than normal because sea ice is fresh water that leaves the water saltier when it forms. Being denser, the cold, salty surface water sinks to drive the deep part of the conveyor.
This system is apparently very sensitive to climate. If sea ice does not form during an unusually warm period, the water does not increase in salinity and does not sink. On the other hand, melting ice contributes more fresh water and decreases salinity, thereby making surface water less dense and also preventing it from sinking.
We do not know the details of the connection, but we do know that major climate shifts have occurred in the past when the global ocean circulation was disrupted by moving continents.
When the Isthmus of Panama closed off less than
2 million years ago, it disrupted the flow of both surface and deep currents that formerly circulated from the Atlantic into the Pacific. The effect was to divert the water and the heat it was carrying northward.
The climate change that would become the great Ice Age began shortly thereafter.
We do not know whether that was a cause of the Pleistocene glacial period, but the effect is just what one would expect. When any system that is near equilibrium is disturbed, it responds to correct, but overshoots and then oscillates around a new equilibrium position.
There have been four major advances and retreats of the continental glaciers. The last retreat was a mere 15,000 years ago in a series where advances and retreats occurred on the average of 80,000 years and 50,000 years respectively.
We are still in the Ice Age, albeit an interglacial stage, and there is little doubt that we live in a time of climate change. It may or may not be anthropogenic, but climate will change. The one thing we can be sure of is that we won’t like it.
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