Effects of climate change on ocean dead zones

Transcription

1 Effects of climate change on ocean dead zones Isabelle Hurley, Undergraduate Student, Dalhousie University Abstract Over this century ocean dead zones are expected to dramatically increase in number. This paper reviews articles describing how climate change will impact ocean dead zones. These studies show that there are many aspects of climate change that affect dead zones. Primarily, an increase in temperature on dead zones, examined in depth in this review, will to lead to the expansion of dead zones through mechanisms such as stratification. Other aspects of climate change, such as changes in patterns of precipitation and changes in ocean circulation, will also affect ocean dead zones, though currently not enough research exists to say definitively how. Overall, the studies reviewed suggest that climate change will cause dead zones to spread globally. 1. Introduction This review paper discusses the effects of climate change on the phenomena of ocean dead zones. Ocean dead zones are areas of water that are hypoxic, meaning they have very low levels of oxygen, and thus can sustain very little marine life. Dead zones are found both in lakes and in costal ocean waters; however, this paper will only discuss dead zones that occur in oceans. The cause of dead zones is generally accepted to be from excess nutrients in the runoff of industrial agriculture. However, some researchers are suggesting that climate change will soon be a factor of great importance for the creation of dead zones. Statements like this rely heavily on predicting how Earth s temperature will change in the far off future, which has led to some skepticism. Carstensen et al. (2014) argue that too little information is known to make large assumptions on the future effects of climate change on dead zones. Additionally, Diaz and Rosenburg (2008) note that climate change is associated with an increase in tropical storms, that could in fact disrupt dead zones. Ocean dead zones destroy marine ecosystems, forcing organisms to either flee or perish. As the area occupied by dead zones expands, more and more ecosystems will be lost (Schrope 2006). If the spread of ocean dead zones continues, as it has for decades, there will be massive financial consequences for fisherman, who will have to seek work elsewhere, and for consumers, as seafood prices rise. If scientists do not take into consideration all factors that will contribute to their spreading or intensification, namely the effects of climate change, then they may not be adequately informed to counsel politicians attempting to combat the spread of dead zones on what decisions to make and this could result in the underestimation of what an appropriate response is. This paper synthesizes peer- reviewed articles discussing the potential effects of climate change on ocean dead zones, while acknowledging that there is always some room for debate when considering 17

2 predictions. Clarifying this debate and synthesizing existing knowledge in this field will benefit future studies and inform policy in new ways. 2. Formation and observation of dead zones Most dead zones are caused by the presence of large amounts of nitrogen and phosphorus in the water, which triggers a process called eutrophication (Schrope 2006). The nutrients enable an unnaturally large algae bloom to occur. When the algae dies, it sinks to the sea floor where it decomposes in a process that uses up a significant amount of oxygen, leaving very little for organisms in that region. Hypoxic is a term used to describe this lack of oxygen; specifically, a hypoxic area has less than 2 millilitres of dissolved oxygen per litre. Ocean dead zones are hypoxic regions. In 2003, a United Nations report found that the number of ocean dead zones had doubled every decade since 1960 (Dybas 2005). To determine whether a dead zone is present, many research papers measure parts per million of oxygen. Sediment core analysis and water column analysis are two primary research methods. Mathematical models are also used, in order to predict future spreading trends of dead zones, temperature increase and other factors related to climate change. The methods used in the papers reviewed here vary. Altieri and Gedan (2015), as well as Diaz and Rosenburg (2008), performed no direct research and instead assembled known information on the subject into coherent review papers. Howarth et al. (2000) performed various experiments on the Hudson River estuary to examine eutrophication; such as calculating Gross Primary Production and the rate of fresh water discharge into the Hudson. Carstensen et al. (2014) analyzed over one hundred years of data on stratification and oxygenation of the Baltic Sea. 3. Climate change effects on dead zones associated with temperature increase In 2008 a survey of the world s oceans found that over 400 dead zones existed globally. Cross- referencing this information with the United Nations study in 2003, Altieri and Gedan (2015) determined that ocean dead zones are spreading exponentially. Using the information laid out by the 2008 survey, a map was created illustrating the location of known dead zones and categorizing the oceans based on the expected sea surface temperature increase by the end of this century (Figure 1). Figure 1 shows that 94% of all ocean dead zones were found in areas that were expecting an increase of 2 degrees Celsius by the end of the century. It is predicted that an increase in sea temperature will affect ocean dead zones by further strengthening them and thus encouraging them to spread. Altieri and Gedan (2015) suggest that in order to understand the effects of temperature increase on ocean dead zones, one must first examine the most basic relationship between water and oxygen. That is, that the warmer the water the less oxygen can be dissolved in it. Altieri and Gedan (2015) and Diaz and Rosenburg (2008) both conclude that as its temperature increases the ocean will be able to 18

3 hold less and less oxygen. This is a serious problem, as it will increase the strength of dead zones by further decreasing the already shockingly low amounts of dissolved oxygen. Figure 1: Map of current dead zones overlaid with predicted sea surface temperature increase for the end of the century (Altieri and Gedan 2015). Altieri and Gedan (2015) and Doney et al. (2011) both find that with an increase in air temperature comes an increase in sea surface temperature. Warmer water is more buoyant and thus it is less likely to mix with the lower layers of the ocean. This layering of water based on density, a factor controlled primarily by temperature and salinity, is called stratification. The surface water is the part of water that receives oxygen but in the presence of stratification it is unable to mix and spread oxygen to the bottom because that would require convection to occur. Convection does not occur in stratified columns because the surface water is warmer and less dense and thus does not sink. The more stratification there is, the stronger the dead zone (Altieri and Gedan 2015; Doney et al. 2011). As the ocean bottom is often where the worst of hypoxia occurs the predicted increase in ocean temperature will magnify hypoxia in areas that already suffer from it. Diaz and Rosenburg (2008) point out that though climate change, more specifically temperature increase, will lead to ocean stratification, it will also lead to an increase in tropical storms, like hurricanes, which mix the ocean as they move, thus disrupting stratification. The Gulf of Mexico dead zone serves as a good example of how ocean dead zones can be affected by hurricanes. In 2005, four distinct hurricanes passed over this dead zone distorting the stratification. While the stratification resumed as soon as the storms passed, the area of the dead zone was still reduced. Diaz and Rosenberg (2008) go on to state that this possible positive effect of climate change is dwarfed by the many other potential negative effects climate change will have on ocean dead zones and their spreading. It should also be 19

4 noted that debate still exists in the scientific community as to whether climate change will lead to an increase in the number of hurricanes formed. Carstensen et al. (2014) argue that the global increase in hypoxic regions is due mainly to excess nutrient input into coastal regions and only partially due to climate change. Their study included statistically modeling the oxygen and stratification conditions in the Baltic Sea for the previous 115 years. Carstensen et al. (2014) acknowledge that in the past two decades temperature increase has led to a decrease in oxygen solubility, but states that there is not enough information currently available to determine whether temperature increase, leading to decreasing oxygenation, is comparable in importance to the increase in nutrients in coastal areas for the formation and maintenance of dead zones. Though decreasing oxygenation propelled through temperature increase may not be by itself as influential a variable as nutrient input, it is important to remember that the decrease in oxygen solubility is not the only consequence of an increase in temperature. There are many more outcomes that stem from an increase in temperature such as the expected increase in respiration or in the duration of dead zones annually, which will be discussed later in this paper. Altieri and Gedan (2015) also explain that coastal regions are more effected by an increase in temperature because these areas tend to be shallower and thus as a whole are more effected by the air temperature around them. This explains why shallow areas show a greater rate of temperature increase in comparison to the open ocean. Altieri and Gedan (2015) refer to Chesapeake Bay s 0.03 degree Celsius increase every year since 1960 as an example of this. As most dead zones are found in coastal regions this observation is worth noting. Furthermore, with the expected sea level rise, shallow coastal regions such as bays and estuaries will increase in water volume and thus more water will be found in coastal areas that are extremely susceptible to forming dead zones due to the fact that agricultural runoff is found in its greatest concentration along the coast (Altieri and Gedan 2015). Currently, dead zones are often seasonal events, peaking in the summer months, as they require a certain temperature to form (Altieri and Gedan 2015). The high temperature allows stratification to occur and, as described above, stratification often leads to hypoxia. However, with the predicted increase in temperature in the next century many dead zones could become year- round phenomena. 4. Climate change effects on dead zones not associated with temperature increase Climate change is not solely an increase in temperature. It involves numerous factors; climate change will also entail changes in global patterns of precipitation and ocean circulation. Both of these factors are predicted to influence dead zones. Several studies have explored these factors and their potential effects on dead zones. 20

5 Howarth et al. (2000) and Altieri and Gedan (2015) both agree that precipitation is arguably the most important factor controlling the formation of dead zones, due to its relationship with agricultural runoff. Precipitation is necessary to bring the nutrients to the streams and rivers that carry them to coastal waters where eutrophication can occur and cause dead zones. Howarth et al. (2000) state that due to this relationship there is no doubt that change in precipitation patterns will have a great effect on ocean dead zones, but that as of yet there is not enough research predicting how climate change will affect precipitation frequency. However, it is suggested that with the increase in temperature we may see less precipitation in the summer and more in the winter. Perhaps this will lead to a reversal of the current dead zone season (Howarth et al. 2000). Altieri and Gedan (2015) and Doney et al. (2011) both recognize change in ocean circulation as another consequence of climate change. As with precipitation, not enough data currently exists to be able to accurately predict how circulation will change. However, it is likely that these changes will take one of two forms when concerning dead zones. Either circulation will change so as to introduce oxygen rich water to presently hypoxic areas or it will bring hypoxic water to areas that are already hypoxic strengthening the dead zone. Thus, both studies conclude that dead zones will be impacted by climate change through changes in ocean circulation. All papers reviewed here agreed that ocean dead zones are spreading globally (Altieri and Gedan 2015; Carstensen et al. 2014; Diaz and Rosenberg 2008; Doney et al. 2011; Dybus 2005; Howarth et al. 2000; Schrope 2006). They also all accepted excess nutrients, like nitrogen, as the primary cause of dead zones. Additionally, most papers recognized that an increase in temperature just above the sea surface can drastically change a region s susceptibility to becoming a dead zone. Thus, a consensus exists stating that if climate change continues as it is predicted to, or it surpasses the expectations of predicted temperature rise, then there will be an increase in ocean dead zones globally. While every paper reviewed here agreed that climate change has an effect on dead zones, they all also acknowledged the limitations of their research as it is based heavily on prediction (Altieri and Gedan 2015; Carstensen et al. 2014; Diaz and Rosenberg 2008; Doney et al. 2011; Dybus 2005; Howarth et al. 2000; Schrope 2006). However, it is impossible to ignore the vast amount of evidence that shows that dead zones will be altered in the next century. Certain factors, such as ocean circulation, are unclear as to whether they will end up decreasing or increasing the number and severity of ocean dead zones, but unfortunately that is an unavoidable limitation of prediction. Either way it is clear that climate change will have great impact on hypoxic areas. 5. Conclusion Climate change is and will continue to have an effect on ocean dead zones. This conclusion was drawn through the review and synthesis of current knowledge and 21

6 debate in the literature surrounding the influence of climate change on ocean dead zones. Factors that will directly lead to this impact include a global increase in temperature, as well as changes in precipitation and ocean circulation. The majority of the research reviewed suggested that climate change will increase the severity and number of dead zones within this century. A dissenting view that is skeptical of the impact of climate change on dead zones continues to be held by a select few, suggesting that future research is important. To further explore the potential effects of climate change on ocean dead zones more research must be done on the predicted change in precipitation and ocean circulation, so as to be able to model the expected change by the end of this century. This could be accomplished with methods such as reconstructing past ocean circulation and precipitation patterns to see how they evolved with an increase in temperature. Another future study could focus on comparing the relative effects of nutrient input versus temperature increase on ocean dead zones. This might answer questions raised by Carstensen et al. (2014). As well, any organizations attempting to come up with solutions to the problem of ocean dead zones must take into consideration the effects of climate change on ocean dead zones. Whether that be to ensure that they do not over compensate if they have ocean circulation working in their favor or to make sure that they do not undercompensate by not taking the global increase in temperature under consideration when forming a plan of action. References Altieri A, Gedan K Climate Change and Dead Zones. Glob Change Bio. 21: Carstensen J, Andersen J, Gustafsson B, Conley D Deoxygenation of the Baltic Sea during the last century. Proc Natl Acad Sci USA. 111: Diaz R, Rosenberg R Spreading Dead Zones and Consequences for Marine Ecosystems. Science. 321(5891): Doney S, Ruckelshaus M, Duffy E, Barry J, Chan F, English C, Galindo H, Grebmeier J, Hollowed A, Knowlton N et al Climate Change Impacts on Marine Ecosystems. Annu Rev Mar Sci. 4: Dybas, C Dead Zones Spreading in World Oceans. Bioscience. 55(7): Howarth R, Swaney D, Butler T, Marino R Rapid Communication: Climatic Control on Eutrophication of the Hudson River Estuary. Ecosystems. 3(2): Schrope, M The Dead Zones. New Sci. 192(2581):