By Juliana Marini Marson
Illustration by Joana Ho
These days, we hear a lot about climate change. Although the headlines focus on the increasing air temperature, the entire climate system - atmosphere, ocean, cryosphere, vegetation and land area - is being affected, since its components are linked by complex interactions. For example, as a result of the current atmospheric warming, glaciers are melting at an accelerated rate. As a result, a huge volume of fresh water that was stored within these glaciers on land is now entering the ocean. In addition to the subsequent rise in sea level, questions also arise as to how this input of fresh water may affect climate. This was one of the questions that motivated my doctoral thesis.
The ocean, just like the atmosphere, is constantly moving. In addition to the wind and tides, an important force that generates ocean movement is density differences between water masses. Observe the experiment in the video below. In this video, an ice cube (blue) and a small opened bottle with warm water (red) are gently placed in a tank full of room temperature water. The cold, blue water sinks to the bottom of the tank while the warm, red water stays close to the surface. Therefore we can say that the cold water is “heavier” (denser) than the warm water.
Salinity is also important in determining the density of water masses in the ocean. Salinity can be lowered with rain, snow, or continental ice entering the ocean; salinity rises with evaporation and the formation of marine ice. How can this be, you may wonder. First you say that ice lowers salinity and then you say that it makes the ocean saltier? There is an important distinction to make here about how ice is formed. Continental ice, the ice that forms glaciers, is made from freshwater; it is formed on land through the accumulation and compaction of snow (freshwater). Marine ice is the result of the freezing of seawater. Although it is slightly salty, most of the salt in the water is expelled as it freezes. Therefore, the salt that was in that parcel of water ends up in the water below the ice, making it more saline. Because the salt molecules are “heavier” than the water molecules, salt water is “heavier” than the saltless water (fresh).
So, warm fresh water is “lighter” than cold salt water, that is why the former tends to be above the latter. In this search for stability (less dense above, denser below), seawater circulates like a treadmill: the hot tropical waters are transported to higher latitudes where they lose heat and receive salt (by the formation of marine ice). They are then denser than when they entered the cold polar regions and sink. This then forms deep water masses, which originate in the North Atlantic (close to Greenland) and in the Southern ocean (especially in the Atlantic). These deep water masses are exported from the Atlantic to other oceans and eventually return to the surface, where they’re heated and return to the poles, restarting the cycle. This circulation is known as Meridional Overturning Circulation (MOC), a process that has a fundamental role in heat distribution around the Earth.
Schematic of Meridional Overturning Circulation (Source: Wikimedia Commons in public domain)
Many glaciers are located in these polar areas where deep water is formed. These glaciers are losing mass quickly, and the resulting meltwater makes the surface less salty and therefore less dense and able to sink. If little dense water is formed in high latitudes, the MOC is weakened, affecting global heat distribution. Warm, tropical water would then not be efficiently transported to the poles, which would ultimately make the mid- and high latitude regions (Europe, for example) experience overall lower temperatures. This is why it is important to study the impact of meltwater in ocean circulation. But how can we do this?
The climate has always and will always be in a state of change on Earth. Factors that affect climate in long time scales include astronomical parameters like the tilt of the Earth’s axis and orbital eccentricity, the amount of ice covering the planet, variation in vegetation, and the concentration of naturally occurring greenhouse gases in the atmosphere. We can therefore use data on past climate changes to understand and try to predict future responses of the planet due to changes. 21,000 years ago, North America and part of Europe were covered by large mantles of ice in a period known as the Last Glacial Period. The average temperature of Earth was approximately 4°C (compared to today’s average of 14°C). Due to an increase in atmospheric heat insolation on Earth, the last glaciation came to an end and those mantles started to melt. In these last 21,000 years, this melted ice has caused approximately 120 meters of sea level rise. That is A LOT of freshwater entering the ocean! Therefore, this period serves a nice model to understand how the ocean’s circulation responds to the addition of meltwater.
Thus, the purpose of my work was to diagnose changes in ocean circulation under the influence of fresh water from melting continental ice. To achieve this goal, we used results from a numerical model (similar to those used in weather forecasting) that simulated the variation of the Earth's climate over the last 21,000 years. The model was generated by scientist Feng He at the University of Wisconsin-Madison (USA) and encompasses the atmosphere, the ocean, the Earth's surface, and the ice and vegetative cover. In the simulation, Feng He informed the model how and when the astronomical parameters varied, the concentration of greenhouse gases, and where, when, and how much melt water may have entered the ocean. This is estimated by using data obtained through the analysis of geological records (for example, gas bubbles trapped in deep ice sheets in Antarctica and Greenland). It is important to note that a numerical simulation, however detailed, is not a complete representation of what happened in the past. Simulations do however, take into account both physical laws and conditions known from the past - so they are not in any way “guesses” or “hunches.” In this particular simulation, the evolution of the air temperature is very similar to that reconstructed from geological records. Thus, it can be considered a good approximation of what happened.
In this numeric scenario, we observed that the introduction of polar melt water in the North Atlantic really weakens the MOC. This weakening is associated with cold periods in the Northern Hemisphere. Conversely, when the influx of fresh water was abruptly interrupted, the MOC was intensified and warm periods were observed. Additionally, the warm water masses of the Atlantic were very different 21,000 years ago from those we see today. The water masses formed around the Antarctic were considerably saltier, possibly due to the greater formation of sea ice, encouraged by the low temperatures of that time. These salty waters occupied much of the Atlantic. On the other side of the world, the waters formed in the North Atlantic did not reach as great of depths as today, nor were they transported so far to the south. The nucleus of the water mass that originated in the North Atlantic reached 1000-2000 meters down and would stay essentially contained in the North Hemisphere, while today it reaches 3500-4000 m in depth and latitudes around 40°S.
Meridional Circulation seen vertically (cutting the Atlantic Ocean in half, North-South). Schematic of how circulation was 21,000 years ago (top panel) and how it is today (bottom panel).
The effects of meltwater entering the North Atlantic was also observed far away; In the tropical Indian Ocean, the discharge of meltwater is associated with changes to atmospheric circulation, which leads to changes in the intensity of monsoons, typical of the region.
From this, we conclude that the melting of continental ice, induced by the rise of air and sea temperature, leads to changes in oceanic circulation and in the distribution of water masses in the Atlantic. This may eventually be reflected in the air temperature, creating a cycle. Rahmstorf and collaborators published an article in the magazine Nature Climate Change showing a weakening in the MOC in the 20th century, especially after 1970. They point to the accelerated melting of the Greenland ice sheet as one of the primary reasons for this weakening. (It is important to emphasize that these cause-and-effect relationships in climate systems are very complex and are far from being taken as definitive. Many of them are still not completely clear, and all we can do is infer if they are in agreement with what the data shows.)
Detailed information about this study can be found here:
Marson, J.M., Wainer, I., Mata, M.M., and Liu, Z. (2014). The impacts of deglacial meltwater forcing on the South Atlantic Ocean deep circulation since the Last Glacial Maximum. Climate of the Past, 10(5), 1723-1734. http://www.clim-past.net/10/1723/2014/
Marson, J.M., Mysak, L.A., Mata, M.M., and Wainer, I. Evolution of the deep Atlantic water masses since the Last Glacial Maximum based on a transient run of NCAR-CCSM3. Climate Dynamics, DOI: 10.1007/s00382-015-2876-7.
Stefan Rahmstorf, Jason E. Box, Georg Feulner, Michael E. Mann, Alexander Robinson, Scott Rutherford & Erik J. Schaffernicht, 2015. Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation. Nature Climate Change, DOI: 10.1038/NCLIMATE2554). Link para o artigo:
About Juliana Marini Marson:
Born in a small town, away from the coast, I fell in love with marine science when I was 12 years old, after participating in an intensive course on the ocean and environmental conservation. I graduated with a Bachelor’s degree in Oceanology and a Master’s degree in Physical Oceanography from the Universidade Federal do Rio Grande (FURG). I obtained my Doctorate title at the Universidade de São Paulo (USP). My focus was always on the study of the polar oceans’ physics and its interactions with climate. Throughout my academic career, Antarctica was my main field of study. Currently, I am a Postdoctoral fellow at the University of Alberta (Canada), where I am learning more deeply about the ocean on the other side of the world - the Arctic.