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Dive deep into short essays about the ocean and what its problems mean for people & policy.

IPC(Sea): Part 5

Ocean circulation, AKA climate science's blind spot

Diana Haemer

Apr 1, 2022

Ever been to the beach on a calm sunny day? Looking out at the horizon, the water looks shiny and placid. Occasionally white caps appear, adding texture to the smooth silk scarf that is the sea surface. But spend a couple hours at the seaside and you’ll that our ocean is in constant motion. Tides and winds move water either towards or away from you, yet these forces only affect the very top of the water column. Their influence dwindles the farther you move from the ocean’s surface, eventually dissipating altogether after 100m. So, what does cause the ocean to move?

Although each area of Earth has its unique circulation patters, every drop of water is governed by the global ocean conveyor belt. Standing at the shore, it’s mind-boggling to realize that the water now flowing between your toes has travelled from the Indian, Arctic, or Southern Oceans. The ocean is comprised of layers of different water masses that vary by temperature, salinity, and density. These blobs of water work together to distribute heat, nutrients, and CO2 across the ocean. But climate change is putting the stability of this cycle at risk.

The IPCC does not make definitive predictions about the extent of climate change’s effect on ocean circulation. In the Johari Window of climate science, we can file “changes in ocean circulation” under a known unknown, also known as a blindspot. That is, scientists do know something will change, but they do not know by how much.

think of humanity as the Self and our climate as the Other

So what do we know, what don’t we know, and what can we predict about changes in ocean circulation? As always, let’s start with some background information.

What is the global ocean conveyor belt?

As mentioned above, the ocean contains water masses with different temperatures and salinities. The water column is divided into layers, with the lightest water on top and the heaviest water at the bottom, like the “rainbow in a jar” science experiment:

These differences form when surface water is cooled by the atmosphere, causing some of it to freeze into sea ice and the rest of it to sink. This process is known as deep water formation, or “downwelling,” and it makes up a significant part of the conveyor belt. Take the Atlantic Ocean: when warm water from the Caribbean flows northward via the Gulf Stream, it is naturally cooled by the frigid Arctic atmosphere. It also gets saltier because when sea ice forms, most of the salt does not freeze and is left behind in the surrounding water. The cold water is now denser and sinks toward the ocean floor, creating North Atlantic Deep Water (NADW). In order to replace this sinking blob of water, new surface water moves in, creating a current.

This GIF (courtesy of NOAA) seems to imply that the subsurface ice causes water to flow downwards — this is not the case. The ice is made of lighter, less-salty water, which means the surrounding water becomes very cold, very salty, and very dense. It therefore sinks, and when new water that flows into the area it also freezes and sinks so the cycle begins again.

NADW then flows southward towards Antarctica, where it is cooled even further. It then sinks deeper down the water column to become Antarctic Bottom Water (AABW). It then circles the Antarctic and splits off into the Indian and Pacific Oceans. As these two deep water currents move, they rub against other currents (which are moving at different speeds, causing friction), becoming warmer, lighter & less salty and rise to the surface in a process called “upwelling.” These masses of surface water then loop back towards the South Atlantic and eventually return to the North Atlantic, where the cycle begins again. At each stage of the conveyor belt, the sinking or rising water is constantly replaced by new water masses with the same properties as the old water mass (i.e. when AABW rises, a new batch of AABW slides in to take its place at the bottom of the water column).

You can see that upwelling occurs in the open ocean off the coasts of Alaska and India.

This entire process is known as thermohaline circulation because it is controlled by changes in temperature (thermo) and salinity (haline). According to NOAA, it takes about 1,000 years for water to complete the journey along the conveyor belt. Other masses of water are formed along the way, like Antarctic Intermediate Water & Mediterranean Intermediate Water, but since the IPCC report does not discuss them I will leave any further analysis for another time.

Why is thermohaline circulation so important?

Without it, our waters would overheat, become stagnant & stratified, and lose the ability to overcome environmental stressors (acidification, for example). The conveyor belt transports heat, salt, carbon dioxide, and nutrients. Warm surface waters are depleted of nutrients and carbon dioxide, but they are enriched again as they travel through the conveyor belt into its deeper layers. Vertical mixing and upwelling are key factors that affect primary productivity, which is the base of the world’s food chain. The life cycle of phytoplankton and plants uses up excess CO2 in the water: if there are more plants photosynthesizing, there will be less CO2. Furthermore, deep water formation allows for CO2 to be stored in the lowest levels of the ocean. As seawater absorbs more & more CO2, it will slowly but surely get sequestered into the deep so long as the global ocean conveyor belt continues to run.

The most important leg of the global conveyor belt is called the Atlantic Meridional Overturning Circulation (AMOC). This is when warm water flows up the Gulf Stream and is cooled when it reaches the Labrador Sea off Greenland and the Norwegian Sea off Scandinavia. And as mentioned above, this process forms NADW. The formation of NADW kickstarts the global conveyor belt — without the AMOC, there would be slow to no thermohaline circulation.

So what’s happening now?

As Newton’s 1st Law states, an object in motion stays in motion unless acted upon by an outside force. Thermohaline circulation occurs naturally because of the temperature contrasts between the ocean & the atmosphere as well as those occurring naturally within the water column. If any changes are made to either the temperature, salinity, or density of sea water, the global ocean conveyor belt could slow or even halt completely.

The biggest threat to thermohaline circulation is the melting of sea ice, glaciers, and land-based ice sheets. Meltwater changes the makeup of sea water, making it less saline and diluting the nutrients (if you have more water but the same amount of calcium and salt, for example, the solution will be weaker). Any change in the density and/or temperature of the water column will trigger “tipping points” in the polar regions. Water that is less dense will be unable to sink to its previous depths — that is, if it is heavy salty & cold enough to sink at all.

It is important to note that in order for the warm water to be cooled by the atmosphere, it must release heat into the atmosphere. As ocean waters warm, more heat will be released from the ocean into the atmosphere, but as the atmosphere warms, more heat will be absorbed by the ocean. Eventually, Earth will become so hot that these temperature differentials will equalize and there will be no heat exchange between the ocean and atmosphere, making it impossible for deep water to form. This effect of climate change is what is threatening to shut down the AMOC. Four years ago, Nature magazine published a report stating that the circulation system of the North Atlantic Ocean weaker now than it has been in the past 1,600 years.

Does the IPCC have any specific predictions about changes to the conveyor belt?

In a word, no. I searched through the source material for all the chapters and found only six(!) studies that focused on thermohaline circulation. The IPCC admits this topic is vastly understudied; scientists did not begin to observe on the AMOC until the 2000s, so there is not enough data to make quantitative predictions about the current’s rate of change in the decades to come. Yet they predict there will be “relatively large, abrupt and sometimes irreversible changes in systems [that will be] caused by global warming.” Essentially, they know that climate change will cause thermohaline circulation to slow down, but they do not know the magnitude or even the timing of this change.

Many reports about climate change highlight its effects on marine life. It is important to remember that sea creatures would not survive without the mixing of water between the layers of the ocean. Global warming will progressively reduce marine animal biomass (the number of animals in the ocean) because of extinction of habitats, fewer nutrients, and higher competition for habitable areas of the ocean. And as land & sea ice melts, there will be more light penetrating into the Arctic Ocean, which according to the IPCC will increase primary productivity but in the wrong part of the ocean. Some specifics outlined by the report are listed below; these are slated to happen by mid-century and beyond if thermohaline circulation is further disrupted:

Less carbon will be stored in the deep ocean because less water will be sinking. Or if water is sinking, it will not settle as deep as it did previously, meaning that CO2 is closer to the surface of the ocean. This will exacerbate ocean acidification.
Seafood is more likely to be contaminated by harmful algal blooms, which are caused by excessively warm & nutrient-rich waters. Algae thrives in warm water, but deep nutrient-rich NADW and AABW are too cold for phytoplankton to bloom. When previously cold areas of the ocean become warmer, invasive organisms will migrate to those areas and use up the nutrients.
Coral bleaching will worsen because nutrient supply from upwelling can increase the mortality risk of bleaching events. If the water that rises to the surface is less saturated with nutrients, the zooxanthellae will not survive as they wait to be readmitted to their corals.

The tipping point for thermohaline circulation will occur when the AMOC slows so considerably that deep water forms at a slower rate or does not become cold enough to fully sink. Scientists have low confidence in their models that predict the exact degree of change in the AMOC; they do, however, agree decisively that the AMOC will decline over the 21st century. On the bright(?) side, IPCC scientists are fairly confident that the AMOC will not collapse outright until the 22nd century.

Yikes! Is there anything we can do to stop this from happening?

If the planet warms by 4°C, the effects of crossing a tipping point become irreversible. The one place in the report where circulation changes is discussed in depth is the chapter on polar regions. The polar seas account for one fifth of the world’s oceans, or 14% of the earth’s surface. The IPCC notes that the Arctic Ocean is experiencing the highest rates of warming and acidification — and is undersaturated with nutrients. These conditions will spread down the water column as NADW is formed. This water will then be disperse throughout the earth’s oceans, and since it takes a thousand years for the conveyor belt to complete, the effects of acidification and warmer masses of deep water will continue to manifest themselves in the centuries to come.

The only way to avoid reaching a tipping point is to stop the melting of sea ice and ice sheets. However, as ice melts the earth loses its ability to reflect the sun’s heat; this is known as a positive feedback loop, a phenomenon I mentioned in my Annexing the Arctic newsletter. Click that link if you’d like to read my explanation, or examine the graphic below:

“temperature” in this case refers to both the atmosphere and the ocean water

The IPCC concludes that the Arctic will soon become sea-ice free in the summers, and that this will occur for the first time before 2050. If warming remains below 2°C, sea ice melt and surface-level ocean acidification are reversible. If not, a significant dearth of sea ice could persist well into the next century. The report says that “multi-level ocean governance” is a feasible and effective way to adapt to climate change. In other words, international governments and local Indigenous communities must work together and create a more sustainable world.

That being said, the one thing we CAN do in the present moment is fund further research on ocean currents. As mentioned above, the IPCC only drew from six studies about thermohaline circulation when developing conclusions about the future of the AMOC and other currents. Scientists do not have enough data in order to make definitive predictions, so much of what I wrote above is based on the educated guesses of career climatologists and oceanographers. Indeed, a simple Google Scholar search yields mostly informational pages. While there are (hopefully) long-term studies currently underway, this blind spot will continue to hinder our ability to adapt to climate change — knowledge is power, after all. Until we know more, stopping greenhouse gas emissions is truly the only way to stop our oceans crossing a tipping point.