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Over the past century or so, the earth has seen a significant rise in average global temperatures (see Figure 2). Studies show that average surface temperatures have risen at the rate of approximately 0.1°C/decade, which is significant when compared to estimates of historical values (IPCC, 2001). Regardless of whether this temperature increase is primarily a result anthropogenic causes, such as as emission of greenhouse gases, or natural fluctuations, global warming will have a profound effect upon the oceans and should therefore be of great concern to anyone in charge of managing global fisheries. It is also very likely that global warming will accelerate in the near future due to positive feedback mechanisms, including the lowering of the Earth's albedo due to melting of polar caps (IPCC, 2001). Climate change is quite difficult to monitor, and even more difficult to predict accurately Despite this, research on current systems, as well as research into past global warming events provides a general idea of what can be expected in future years. Knowledge of these general trends will allow us to better understand the effects and, consequently, better manage fisheries.
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In the next century, models predict average water temperatures will increase by 1 to 7 degrees celsius Celsius (IPCC, 2001). Many life processes of marine animals and plants are dependent on water temperature, and could be significantly altered by a rise of even a few degrees in temperature (Harley, 2006). Higher temperature waters, such as those in the tropics, have less primary production in the form of phytoplankton. This is mainly due to the fact that a greater temperature gradient causes more intense stratification of the water, thus weakening the upwelling of cool, nutrient-rich water to the surface. A decrease in primary production causes a decrease in the numbers of individuals at higher trophic levels, including fish that depend on phytoplankton for food (Harley, 2006). Therefore, in many cases, it is reasonable to believe that warming of seawater will cause fish stocks to decrease (Harley, 2006).
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It is also likely that climate change will have effects on the human aspects of many fisheries. Sea level rise may threaten many coastal areas. Thermal expansion of seawater is expected to cause a rise of 0.09 to 0.37 m over the next century (IPCC, 2001). This may threaten coastal cities, and infrastructure of the fishing industry in some areas. It is also predicted that storms such as monsoons and hurricane my hurricanes may increase in number and intensity as a result of global warming (IPCC, 2001). Flooding and storm surges could also result in damages to fishing infrastructure in locations prone to these disasters. Terrestrial effects of global warming could effect agriculture in certain areas by the changes in timing and intensity of droughts and flooding, as well as the effect of increased carbon dioxide on the growth rates of crops and weeds. There are also likely to be effects on agriculture by sustained changes in temperature and precipitation, such as the desertification of the southwestern United States (IPCC, 2001). A decrease in agricultural yield could increase the demand for fish.
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