Exploring the Possibility of Altered Ocean Circulation Patterns Using the Second Law of Thermodynamics

1 Adrienne Propp ES 112 Semester Project 9 May 2016 The Day After Tomorrow: Exploring the Possibility of Altered Ocean Circulation Patterns Using the Second Law of Thermodynamics Climate change, one of the most urgent and universal issues of our time, involves a complicated web of interrelated processes, many of which are quite complicated, themselves. As a result, although there is general consensus that climate change is occurring, it is difficult to determine what its effects will ultimately be or when they will be realized. As this has potentially critical consequences for society, a deeper understanding of how human actions affect the climate system must be pursued. Ocean circulation, one of the major processes affecting the climate system, is believed by some to be undergoing a fundamental change. Specifically, warmer temperatures and higher levels of atmospheric CO2 are believed to be contributing to the weakening of what is colloquially called the “great ocean conveyor belt,” or Earth’s network of ocean currents (Marshall, 2012; Weijer, Maltrud, Hecht, Dijkstra, & Kliphuis, 2012). If true, this could be indicative of the level of severity of anthropogenic climate change, and have potentially disastrous consequences. In this paper I will first provide a brief overview of ocean circulation’s role in the global climate system. I will then discuss the nature of the reports being made. Finally, I will present ocean circulation in the context of the Second Law of Thermodynamics, and discuss an investigation of the validity of these claims using this thermodynamic perspective. This approach is valuable because “the sequence of all natural processes is determined by the principle of entropy increase” (Fenn, 1982, p. 242) – in other words, the Second Law of Thermodynamics determines the course that a system will actually follow when multiple courses satisfy the First Law of Thermodynamics, or conservation of energy. Ocean Circulation and the Global Climate System Earth’s climate is influenced by many factors, including solar radiation, wind, ocean currents, and the interactions between them. As the oceans cover about 71% of the Earth, they are unsurprisingly a major component of the global climate system. Indeed, the oceans are both responsible for and responsive to many changes in environmental conditions. (Pidwirny, 2007) 2 More than their sheer size and volume, the influence exerted by oceans comes largely from their circulation patterns. Ocean currents transport enormous amounts of heat around the world and absorb gases in the atmosphere. In fact, oceans are estimated to transport a maximum of heat just under 3 petawatts, and to have absorbed up to half of all of the CO2 produced by the burning of fossil fuels since the beginning of the industrial revolution (Bollmann et al., 2010). Therefore, whether the climate will change in the future, and by how much, is strongly linked to ocean circulation. Ocean circulation is both mechanically and thermally driven. It is mechanically affected by wind stresses, waves, and the like. Thermally, ocean circulation is affected by the sun, via radiative heating, and the core of the Earth, via geothermal heating. (Bollmann et al., 2010) (Rahmstorf S. , Thermohaline Ocean Circulation , 2006) Overall, most of the large-scale circulation is driven by density, which depends on salinity and temperature. Thus, this characterization of ocean circulation is often called thermohaline circulation. Figure (1) gives a broad overview of what is colloquially called the “great ocean conveyor belt”, or the manifestation of this thermohaline circulation. Density, a measure of how tightly packed together molecules are, governs the direction, location, and depth of currents (Ocean and Figure 1: The "Great Ocean Conveyor Belt" Climate - The Odd Couple). Warm water and fresh water rise due to low density, while cold water and salty water sink due to high density. Furthermore, water in regions with high concentrations of heat and salt diffuses into regions with low concentrations, dissipating the unequal distributions of heat and salinity, and thus diminishing the density gradients. These unequal distributions of heat and salinity are caused by precipitation and evaporation, as well as differential heating between the polar and equatorial regions. Overall, there are net gains of heat and salt in the equatorial regions, and net losses of heat and salt in the polar regions (Shimokawa & Ozawa, 2002). This flux imbalance results in the inhomogeneous distribution of temperature and salinity at the ocean surface that is often considered to be the driving force behind global-scale thermohaline circulation. One manifestation of this is the water mass produced by these convective processes in the Arctic, termed the North Atlantic Deep Water (NADW). Warm, salty surface water spreads from the tropics into the North Atlantic. In the Labrador and Greenland seas, the cold 3 temperature and formation of ice increases the region’s water density, pulling it down to rest on a layer of even denser, deeper water produced by convection in the Antarctic, the Antarctic Bottom Water (AABW), that extends up the entire length of the Atlantic Ocean. (Bollmann et al., 2010) (Gordon, 2004) Approximately one third of the world’s ocean water is involved in thermohaline circulation, or about 400,000 cubic kilometers of water (Bollmann et al., 2010). Although it transports about 20 million cubic meters of water per second, there may be hundreds of years between sinking and returning to the surface (Conkling, Alley, Broecker, & Denton, 2011). Indeed, oceans react very gradually to change. As a result, though ocean circulation exerts a huge influence on, and is a powerful indicator of, the state of the global climate, the impacts of climate change evident in the oceans today do not yet reflect the total extent of climate change already caused by human activity (Bollmann et al., 2010). Thus, the decisions made today may have consequences that extend far into the future. Are Humans Altering the Course of Ocean Circulation? In Florida and much of the southeastern United States, citizens anxiously await predictions of the severity of the approaching hurricane season each spring. This year, there is growing concern that the presence of a “cold blob,” or a region of unusually cool water, in the North Atlantic will affect Atlantic Ocean currents. This could potentially initiate a transition from El Niño to El Niña and increase the likelihood of a severe hurricane season (MacMath, 2016). Scientists’ concern reaches even farther – some climate models have predicted that the Atlantic turnover process will weaken by about 25% by the end of this century (Bollmann et al., 2010). The 2004 film, The Day After Tomorrow, may have been inspired by, and probably aggravated, public concern over the issue. Indeed, there is evidence that ice sheets in the North Atlantic are melting at an increasing rate, discharging large amounts of cold fresh water into the ocean (e.g. Frauenfeld, Knappenberger, & Michaels, 2011; Marshall, 2012). Furthermore, rising Arctic temperatures inhibit the formation of sea ice, diminishing the level by which salinity is increased in this region. As a result, the increase in water density that usually occurs in this region is diminished, weakening the convective forces that pull the water down, driving circulation. As a result, many believe the Atlantic Meridonal Overturning Circulation (AMOC) will be weakened, affecting global climate patterns as well as the ocean’s uptake of CO2, likely contributing to a positive feedback cycle. (Bollmann et al., 2010) These changes are significantly tied to human activity, such as the burning of fossil fuels. However, information about past environmental conditions, drawn from ocean floor sediment and paleo-data, indicates that shifts in oceanic circulation patterns have occurred in the past, and corresponded to shifts in overall climatic conditions. Scientists believe that certain cold climate episodes, occurring over a few decades or even less, were caused by abrupt disturbances in the ocean currents of the North Atlantic. (Bond et al., 1993 as cited in Rahmstorf, 1997) The 4 significance of this is multifaceted. On one hand, it is concerning that a major shift in ocean circulation patterns is indeed a possible scenario. On the other hand, changes in ocean circulation are a naturally occurring phenomenon that may be out of our control. That said, there is evidence that this naturally occurring phenomenon is encouraged by the very changes we are exerting on our environment today. For this reason, a holistic and thorough investigation of the likelihood of this occurring is critical to ensuring that we are prepared for the potentially inevitable consequences of our actions. One Method of Investigation: The Second Law of Thermodynamics Ocean Circulation in the Context of the Second Law As it turns out, thermohaline circulation is a fantastic example of the second law of thermodynamics, manifested in the dissipation of heat and salt gradients. In this context, the second law of thermodynamics will be considered with the ocean as an open dissipative system – it is open because the system exchanges heat and salt with the surroundings, and dissipative because the system is not at equilibrium. Fluxes of heat and salt between the ocean and its surroundings (most notably the atmosphere) produce gradients of temperature and salt concentration, which processes like thermohaline circulation act to diminish. In effect, these processes occur in an attempt to bring the system closer to equilibrium, towards a state of higher entropy. The rate at which the ocean system approaches equilibrium is described as the rate of entropy production. (Ozawa & Shimokawa, 2000) From the Second Law of Thermodynamics, we know that the overall entropy of the universe must increase. 1 ��!"# ≡ ��!"## + ��!"! ≧ 0 , where �� + �� − � �� = !"# = ! ! ! � � Therefore, for any spontaneous process to occur, it must result in an overall increase of entropy. We also know that any spontaneous process in any isolated system always results in an increase in the entropy of that system (Fenn, 1982).

Load more