It doesn’t stop at massive droughts and flooded cities

One of the difficulties that many climate models face is predicting the future. Environmental scientists collect tons of data, and in some instances can foretell the level of certain gases in the future. Yet, understanding how the world will respond is tricky. Wind patterns, ocean currents, and weather are incredibly dynamic systems that influence each other and are influenced by many external forces. Fully modeling how the climate behaves in response to exponentially increasing carbon levels has the equivalent complexity of predicting where a toothpick will land after being dropped in a tornado.

This uncertainty is why, even climate change scientists, tend to downplay what the possible outcomes of climate change are. There simply doesn’t exist an answer with a low enough uncertainty. It is for these reasons, discussion of the possible Thermohaline shutdown, while being incredibly terrifying, are often eschewed. However, based on observation, history, and safe assumptions scientists in publications have voiced their concerns. So let’s explore this possibility.

To start, the Thermohaline circulation is a term used to describe the ocean current phenomena on Earth. The circulation travels across the Earth’s oceans, moving water between all the hemispheres. The currents in this circulation are facilitated by varying density of salt concentration throughout the oceans and the effects of temperature (Hence the name Thermo-haline). Near the equator, the salt concentration is higher due to evaporated water. When the ocean region that contains higher salinity contacts a region with lower salinity, the higher density region is swallowed and submerged by the lower density region. This submersion creates a current that starts at the equator.

This current has a tremendous impact on the climate of regions it travels through. As warm equatorial water travels northward, the water heats up the overall climate in the central eastern United States and European regions. This is the reason these regions have more temperate climates compared to similar latitudinal located regions that are land locked. Additionally, These ocean currents are responsible for moving sea organisms (and human waste) across the hemispheres.

Over the last two decades, there has been a growing concern from the scientific community that the Thermohaline circulation is experiencing a slow down. As global temperatures pass the one degree Celsius mark, the Arctic sheets and regions of Greenland have begun to melt at an alarming rate. The Arctic, which contains roughly 70% of all fresh water on Earth, dilutes the salt concentration in the ocean as the sheets melt.

The decrease in salt concentration impedes on the aforementioned density gradient influenced current. Not surprisingly, Bryden et al. noted that the net flow of the Thermohaline circulation has slowed by 30% since the 1950’s. A slowdown may explain the decrease in temperatures in certain climates. While overall global temperatures increase, the absence of warm currents in naturally occurring regions will result in sinking temperatures. There is still much uncertainty over the affects of cooling currents. If temperatures drop slightly, they might simply counteract the effects of global warming in regions like Europe. This is not to say the rest of the world will be so lucky. In a more grim scenario, a drastic reduction in Thermohaline currents can cause temperatures to greatly drop in regions. If a slowdown continues, Europe and regions dependent on warm currents for their climate can expect an ice age.

A more concerning outcome of a Thermohaline shutdown is the potential triggering of a anoxic event. Anoxic events have been associated with halting of ocean currents and global warming events in Earth’s prehistoric period. As oceans become more stagnant, the life below becomes momentarily more active. Microbial ocean organisms, such as plankton, are given the opportunity to reproduced in large numbers. This is the same idea why drinking running water is safer than still water. Running water is less likely to foster bacterial growth.

As the biomass of the ocean explodes, the oxygen content of the ocean begins to decline. Ocean life needs oxygen to survive, yet with too much organic life, obtaining oxygen can become difficult. Regions that have low oxygen content can develop into dead zones, areas in which much of marine life cannot survive in. Kump et al. demonstrated that during such Anoxic events in the past, large quantities of Hydrogen Sulfide gas were released from the Oceans. This noxious gas was most likely related to the large die-off of organic sea life.

This would explain why previous mass extinctions were associated with Thermohaline shutdowns. Much mammalian and plant life cannot survive with hydrogen sulfide gas present in the atmosphere. The same researchers also demonstrated that the release of this gas would have damaged the Ozone layer. Their theory was further supported by fossil records that showed ultraviolet (UV) radiation related scarring. Massive amounts of UV radiation would further facilitate the extinction of land organisms. Human life, as we know it, in these environmental conditions will be impossible.

While the direct causation and links between many of these events are ambiguous, one specific trend is consistent. In all cases in which mass extinctions and Thermohaline shutdowns occurred, the Earth was experiencing record global temperatures and carbon levels. During the Permian-Triassic extinction, atmospheric carbon levels had reached 1000 ppm (Ward, page 71). Current concentrations are at 410.92 ppm. The Earth is still far from reaching cataclysmic carbon levels, yet this is no reason for apathy.

It should be understood that once the Thermohaline circulation is shutdown it cannot be reasonably reversed. Earth has recovered from such events in past, yes. Full recoveries often take a little less than a million years.

I believe I would not be doing this article scientific justice if I did not emphasize the uncertainty of Thermohaline trends in the scientific community. While a Thermohaline shutdown is frightning, data on Thermohaline slowdown is a reason for skepticism. The circulation is not constant and fluctuates in net flow. So while Bryden et al. may have noted a slow down, in such a short time frame (50 years is a short time frame relative to any global events) it’s not clear whether this event is correlated to global warming.

This article by no means declares what will happen. It simply explores one of the more frighting possibilities proposed.