Disturbing deep-water discoveries: Weddell waters warming

Beyond sea level rise, it often seems that most people don’t really consider the way warming will affect the planet’s oceans, or the ways in which that will affect us in turn. Sea level rise is an easy concept to grasp, and the effects are obvious and dramatic. I also think sea level rise is likely to end up being one of the easiest parts of global warming for us to deal with, even when it comes those changes happening within the oceans.

After rising waters, the next set of changes we’ll notice are those in the fisheries. Rising temperatures and rising acidity are already changing both the health and behavior of various forms of sea life. Beyond the efforts of marine biologists, the fact that a sizable portion of humanity’s protein intake comes from the ocean means that a drop in fish populations will be noticed around the world pretty quickly.

There’s another factor that’s likely to be harder for the layperson to detect, until it reaches the point of causing pretty dramatic changes at a global level. A network of huge currents wraps around the planet, moving heat, food, and oxygen not just from one part of the world to another, but also from the surface to the depths and back. The Arctic and Antarctic regions are where water cools, gains density as ice formation increases salt concentration, and drops down to the sea floor to run along the bottom.

These currents are the main reason that western Europe is as warm as it is. Without the constant flow of heat up from the equator, I’d be facing temperatures like those in the northern half of Canada, probably with a huge amount of snow from being so close to the ocean. Instead, thanks to the Gulf Stream, it’s unlikely to get cold enough for any snow to stick at all here.

If the current slows or stops, western Europe would experience a dramatic – though temporary – drop in average temperature, the ocean deeps could lose the constant supply of oxygen from the surface, and oceanic ecosystems the world over would undergo major upheaval.

That’s why it’s been worrying that so much of the heat trapped by our CO2 increase has been absorbed by the oceans, and why it’s very worrying that researchers have confirmed dramatic warming specifically in the deeper parts of the Weddell Sea, where the Atlantic meets Antarctica:

Over the past several decades, the world’s oceans have absorbed more than 90 percent of the heat trapped in the atmosphere by greenhouse-gas emissions, effectively slowing the rise in air temperatures around the globe. In this regard, the Southern Ocean is pivotal. Though it only accounts for 15 percent of the world’s oceans in terms of area, because of the overturning that takes place there, it absorbs roughly three-fourths of the heat.

Until recently, very little was known about what happens to this heat in the depths of the Southern Ocean, due to the lack of sufficiently long time series. In order to trace the development down to the seafloor, researchers relied on regularly repeated ship-based measurements taken with ‘CTD’ probes (Conductivity, Temperature and Depth). These probes have now become so precise that they can measure changes in water temperature down to the nearest ten-thousandth of a degree Celsius. The data they gather can also be used to determine the water masses’ density and salinity.

For the past 30 years, AWI oceanographers have been taking these temperature and salinity readings during expeditions to the Weddell Sea on board the German research icebreaker Polarstern — always at the same sites, always from the surface to the seafloor, and always with extremely high accuracy. By doing so, the researchers have produced the only time series of its kind on the South Atlantic and the Weddell Sea, which has now allowed them to precisely reconstruct the warming of the Weddell Sea and identify potential causes.

Only the water below 700 metres is growing warmer

Their findings are surprising. “Our data shows a clear division in the water column of the Weddell Sea. While the water in the upper 700 metres has hardly warmed at all, in the deeper regions we’re seeing a consistent temperature rise of 0.0021 to 0.0024 degrees Celsius per year,” says Dr Volker Strass, an AWI oceanographer and the study’s first author.

These values may seem minuscule at first glance. But, as Strass explains, “Since the ocean has roughly 1,000 times the heat capacity of the atmosphere, these numbers represent an enormous scale of heat absorption. By using the temperature rise to calculate the warming rate in watts per square metre, you can see that over the past 30 years, at depths of over 2,000 metres the Weddell Sea has absorbed five times as much heat as the rest of the ocean on average.” Through the formation of bottom water in the Weddell Sea, this heat is then distributed to the deep basins of the world’s oceans.

Potential effects on global circulation

In the Weddell Sea, which represents the southern extension of the Atlantic Ocean and is roughly ten times the size of the North Sea, tremendous water masses cool down. In the course of sea-ice formation they take on salt, sink to deeper water layers as cold and heavy Antarctic Bottom Water, and then spread to the great ocean basins as a deep-sea current. This overturning is considered to be an important motor for the global ocean circulation. The warming of the depths of the Weddell Sea could weaken that motor, since warmer water has a lower density. Consequently, it is lighter and could fill higher layers of the water column.

“Our field data already shows a temperature-related loss in density in the deeper water masses of the Weddell Sea. This change is most pronounced in the Bottom Water,” says co-author and AWI oceanographer Gerd Rohardt. Whether or not the Antarctic Bottom Water will continue to fulfil its function as the deepest limb of the global ocean overturning circulation chiefly depends on how the density of the water masses above it changes.

This is all very concerning of course, but there’s one possible result that this article didn’t discuss. Seafloor conditions in various parts of the world have a combination of low temperatures and high pressure that lead to the creation of methane deposits generally called clathrates or hydrates. These are basically methane trapped in a sort of latticework of ice, buried in ocean sediments. The exact amount of methane trapped in this form on the sea floor is uncertain, but it is estimated to be at least double the quantity in all other known natural gas sources, which has been a source of some worry for climate scientists. Methane is a much more effective greenhouse gas than CO2, and while it has a smaller total effect on the planet’s temperature right now, that’s due to there being so much less of it in the atmosphere. A big increase would cause a dramatic spike in global temperature, and clathrates are known to be unstable formations. A drop in pressure or an increase in temperature can cause them to destabilize, releasing the methane, which then bubbles up into the atmosphere.

The Weddell Sea doesn’t seem to have a whole lot in the way of clathrates, but other parts of Antarctica do, and as the article said, the heat is spreading out along the bottom of the sea. Even if there aren’t similar pockets of warming in other places, it seems likely that this could result in a destabilization of a significant clathrate deposit at some point in the near future, leading to an acceleration in warming.

This doesn’t change what we need to be doing, really, but it does underscore the urgency of the situation we’re in, and the importance of acting with the assumption that things could get a lot warmer very quickly, and somewhat unpredictably. At this point it seems like it’s a matter of “when”, not “if”, but we may not know an event like that has started until it’s already underway.

I think it’s reasonable to say, all evidence considered, that we cannot rely on things like the current political structure of the United States, let alone “market forces” to take action quickly enough to avoid massive problems. It is vitally important for everyone reading this to get involved in local and regional organizing and networking. This can be the kinds of things I’ve discussed here, through existing mutual aid efforts, or through things like the Transition Town Network. Ideally, I would say all of the above – the more the various networks being formed right now have multiple points of overlap and contact with each other, the more resilient we become as a species, and the better able we will be to use those networks not just for meeting our material needs when times get tough, but also to exert coordinated political power to get government action.

Despite everything happening in the world right now, life goes on, and I’m still required to spend money in order to live. My work is supported by a group of wonderful people over at patreon.com/oceanoxia, and I would be immeasurably grateful if you would consider joining their ranks. How much you give, and for how long are entirely under your control, and every little bit helps a great deal, as my household is very short on money right now. Thank you for reading, and take care of yourselves.


  1. says

    The Bay of Bengal is nearly an anoxic zone and there are anoxic zones in the Gulf of Mexico and off the Oregon coast. Sea level rise is a problem but having the fisheries collapse and the entire chain or marine life unravel – that’s a) terrifying b) happening right now.

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