Current-Stopping Climate Change
2018, High School, Prose
We portray climate change as a generational thing, a gradual change that humans may not see with the naked eye in real time, like tectonic plates inching along Earth’s crust. In due time, plate movement may amount to a noticeable shift. But what if one effect of climate change was sudden, not generational – an unexpected earthquake of rapid change that Earth’s ecosystems and humans cannot adapt quickly to? This is the threat a disruption in global ocean currents poses on our biosphere. Those currents, collectively called thermohaline circulation, are “the great regulator” of Earth, keeping global temperatures, oceanic nutrient balances, weather patterns, etc., stable and consistent.
Like tectonic plates, layers of water can independently glide over each other. In macro trends, two general forms of ocean water emerge: a nutrient-deprived, low-salt surface layer heated by the sun; and a cooler, denser, solute-laden deep layer (“Ocean on the Move,” 2018). How these layers transverse the oceans and interact are what form the basis of thermohaline circulation. These layers meander from ocean to ocean in a conveyor belt of water that stretches the globe. In the North Atlantic, masses of warm surface water release heat to the air, become saltier through evaporation, and sink to assimilate with deep water below them. By forming the North Atlantic Deep Water (NADW), aerated surface waters “bury” gases in deep water, both keeping carbon dioxide, a harmful greenhouse gas, out of the atmosphere and providing an influx of oxygen that allows deep marine life to breathe (Withgott & Laposata, p. 424–426, 2015). An inverse process occurs in the Pacific Ocean. The El Niño–Southern Oscillation (ENSO), among other factors, allows solar warming to bring cold deep layers up to intermix with surface waters. This process brings crucial nutrients, such as potassium, nitrogen, and phosphorus, up to the surface where algae and cyanobacteria (keystone species of many ocean ecosystems) can use them (Withgott & Laposata, p. 425, 2015).
In addition to ecosystems, this cycle has a profound effect on terrestrial and oceanic climates. The heat released by the transition from hot surface water to NADW collectively forms a wall of warm air. This mass of air flows over western Europe, providing the entire region with a relatively mild and stable climate that temperature-sensitive species depend on. Similarly, the looping, gentle swoops of thermohaline currents decrease the risk of damaging hurricanes that stress coastal communities. Without thermohaline circulation, ocean waters would be more prone to erratic currents bearing large surface temperature variabilities—in other words, the breeding grounds for freak storms (Marotzke, 2000).
The great regulator of worldwide stasis is in fact still very fragile. An external influx of water into the NADW region can slow down thermohaline circulation. If large enough, the process could halt altogether from density and temperature imbalances. Today, we are beginning to see signs of thermohaline slowdown, which may threaten shutdown.
The NADW is adjacent to the large Greenland ice sheet, a massive reservoir of solid freshwater. As rising global temperatures break record after record, meter by meter, outlying Greenland ice caps and glaciers melt away into the ocean. In 2017, Autumn Arctic temperatures ran 20-30°F above the normal, severely accelerating the rate of glacial retreat (Gillis, 2018). By older 2014 estimates, Greenland loses about 280 gigatons of ice mass per year (Velicogna, Sutterley & van den Broeke)! One gigaton is equivalent to one trillion kilograms of ice. Multiply that by 280 and that is how much fresh meltwater invades oceans and interferes with circulation annually.
This mass of meltwater directly disrupts the formation of deep water in the North Atlantic. Meltwater is just freshwater, water bearing low density and nominal salt content. When this intermixes with surface water, its properties resemble freshwater more and more and saltwater less and less, making the water layer less likely to sink as deep, salty water and maintain circulation (Withgott & Laposata, p. 425, 2015). Unfortunately, this is not a localized problem. A slowdown in one stage of an assembly line indirectly slows other processes; in this case, ENSO and average current velocity are stalled in other regions of the globe.
Scientists predict that at some point, thermohaline circulation may collapse entirely. This event would be a flashpoint in the Anthropocene, the human era of history, leading to climactic change in our environment. Stouffer et al. used various climatic models to predict if Earth could recover from a thermohaline shutdown, ultimately concluding that the reestablishment of circulation is highly unlikely (2006). Crucial ecosystem services such as nutrient balancing, oceanic CO2 uptake, mild temperatures, and the like would be little maintained by global phenomena, precipitating the rapid cooling of Europe. In the context of our rapidly warming planet, that doesn’t sound unfortunate, but our ecosystems cannot bear the stress of an even more rapid and more drastic temperature change. Around 12,000 years ago, when thermohaline circulation slowed down by about 50-80% due to receding ice sheets, rapid cooling made regions inhospitable for life, leading to a series of extinction events (Eisenman, Bitz & Tziperman, 2009). Should we approach 100% slowdown in the modern day, ecosystems and human societies may face greater consequences.
Unlike more clear-cut environmental problems such as oil spills or CFC emissions, which governments may have direct regulatory control over, the melting of Greenland’s ice sheet and thermohaline shutdown arise from nonpoint sources of pollution. It is a macro trend, the final result of decades of greenhouse gas emissions, deforestation, ice-albedo feedback, and other issues that raise global temperatures. Appropriately, strategies to prevent slow/shutdown are interdisciplinary and often international in scale. To slow the slowdown of thermohaline circulation, cooperation between governments and between people are crucial. While governments, especially those in western Europe and Scandinavia who have most to lose with thermohaline shutdown, have repeatedly funded new climate models, accuracy and consistency with other established models is hard to come by. As one professor put it, “we must pursue good decisions, not good predictions” (Murray, 2018).
Policy recommendations and analyses are broad. The general consensus from geoscientists and international affairs experts is that countries need to A. stop greenhouse gas emissions, and B. accelerate the transition to renewable energy on a large scale. A and B in tandem would likely have the greatest effect in preserving thermohaline circulation (Keller et al., 2000). That said, large scales require both sweeping governmental policy and personal resolve to be effective; we need to win the war on both fronts. Individuals must commit to reducing their carbon footprints, while supporting policymakers and policies that commit to reducing emissions and promoting renewables. Again, this is not just Europe’s problem, it is worldwide. The oceans connect the continents. Hurricanes and typhoons that slam the Florida Keys or the Philippines originate from the destabilization of the North Atlantic Deep Water thousands of miles away.
While a thermohaline collapse may not be reversible, a 30-50% slowdown may be recoverable. So long as we move in the right direction, thermohaline may once again speed up. This of course can help mitigate issues tied to oceanic instability, including coral bleaching, hurricane intensity, etc. (not to mention the other profound environmental improvements of reducing greenhouse gas emissions). Humans stand to benefit as well: coastal/tropical communities can spend less on storm fortifications, fish stocks will be more abundant/regular from dissolved nutrients, etc.
Thermohaline circulation is perhaps one of our most valuable ecosystem services because it directly regulates or influences so many others. Preserving ocean health in this era of rapid climate change and the sixth mass extinction has focused on more popular issues such as eutrophication, coral reef health, and acidification. However, should we focus on preventing thermohaline slowdown and ending the risk of shutdown, we may see significant improvements on many sub-issues. Truly, a recovery in circulation is a recovery in our global environment and human society.
Eisenman, I., Bitz, C., & Tziperman, E. (2009). Rain driven by receding ice sheets as a cause of
past climate change. Paleoceanography, 24 (4). doi: 10.1029/2009pa001778
Gillis, J. (2018). Earth Sets a Temperature Record for the Third Straight Year. Retrieved from https://www.nytimes.com/2017/01/18/science/earth-highest-temperature-record.html
Keller, K., Tan, K., Morel, F. M., & Bradford, D. F. (2000). Preserving the ocean circulation: implications for climate policy. Climatic Change, 47 (1-2), 17-43.
Marotzke, J. (2000). Abrupt climate change and thermohaline circulation: Mechanisms and predictability. Proceedings of The National Academy of Sciences, 97 (4), 1347-1350. doi: 10.1073/pnas.97.4.1347
Murray, T. (2018). GLIMPSE project. Retrieved from http://www.swansea.ac.uk/research/pretrash/impact/features/glimpse/
Ocean on the Move: Thermohaline Circulation | UCAR Center for Science Education. (2018). Retrieved from https://scied.ucar.edu/ocean-move-thermohaline-circulation
Stouffer, R., Yin, J., Gregory, J., Dixon, K., Spelman, M., & Hurlin, W. et al. (2006). Investigating the Causes of the Response of the Thermohaline Circulation to Past and Future Climate Changes. Journal of Climate, 19 (8), 1365-1387. doi: 10.1175/jcli3689.1
Velicogna, I., Sutterley, T., & van den Broeke, M. (2014). Regional acceleration in ice mass loss from Greenland and Antarctica using GRACE time-variable gravity data. Geophysical Research Letters, 41 (22), 8130-8137. doi: 10.1002/2014gl061052
Withgott, J., & Laposata, M. (2015). Environment (5th ed., pp. 424-426). Pearson Education.
In the modern world, we are inundated with messages about the dangers of climate change, including a few select issues on the oceans. They most often include coral bleaching, eutrophication, oil spills, and other events we can directly see with the naked eye. Beneath all of that is a larger (literally) issue that is under-reported by the media and underfunded in solutions by the government: the destabilization of thermohaline circulation. This process -- the cyclical, global movement of surface and deep ocean waters -- regulates so many facets of our environment that directly support ecosystems and support human institutions; it’s a shame this isn’t a central issue in the public’s view of climate change. Thermohaline circulation is to credit for the stability of fishing stocks and for western Europe’s friendly and mild temperature. Yet we cannot see it. In that sense, it is out of sight and out of mind for many people, including myself. I had no idea what the implications of global ocean currents could be before beginning this paper. I realize now that there are so many different facets to climate change that warrant addressing. Some, such as this one, require international and interdisciplinary cooperation, like many environmental issues in order for positive change to be affected. Thermohaline circulation, unlike some more local climate change problems, has wide-ranging effects, and we must address them as such. Fixing circulation at the global level may alleviate environmental problems at more local scales.