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Many biological processes are temperature dependent and would be adversely affected by even a few degrees of temperature change (Harley, 2006). Higher temperature waters, such as those in the tropics tend to have less primary production. This results from the fact that a a strong temperature gradient prevents the upwelling of cool, nutrient rich waters, such as is common off the coasts of Peru and Chile during the negative periods of the ENSO cycle (Harley, 2006). A drop in the amount of primary production causes drops in higher trophic levels as well. Therefore, in many cases, it is reasonable to believe that global warming will cause fish stocks to decrease (Harley, 2006).
Another adverse effect from global warming of the most visible effects of climate change is coral bleaching. When hermatypic corals are stressed by high water temperatures or other effects, they expel their symbiotic zooxanthelle from their tissues. This process deprives corals of the color, as well as of their primary source of nutrition. If corals are without their symbionts for too long they can perish from starvation. The impact of coral death can spread then spreads through the reef ecosystem. Secondary effects are most obvious in fish, especially among those that feed specifically on corals, such as butterfly fish. These Studies have indicated that such fish were gradually starving to death and the that their decline in numbers indicated they had also failed resulted from a failure to breed in the months and years following the destruction of their reef. As it stands today, more than 30% of coral reefs throughout the world are already severely degraded and up to 60% of corals may be lost by 2030 due to temperature induced bleaching (ARC, 2007). Coral reefs are also threatened by the sea level rise. For risFor example, when slow growing corals cannot grow quickly enough to counteract the rise in sea level, reefs can fall below the photic zone depths at which photosynthesis can occur and perish (Harley, 2006).
It is also likely that climate change will have severe, direct effects on humans. Thermal expansion of seawater alone is expected to cause a rise of of between 0.09 to and 0.37 m meters over the next century (IPCC, 2001). This modest sounding rise is nevertheless enough to threaten many coastal cities , and in some cases entire island nations. It is also predicted that storms, such as monsoons and hurricanes, may increase in number and intensity as a result of global warming (IPCC, 2001). Global warming can affect land-based agriculture in certain areas by changing patterns of precipitation. Desertification For example, desertification is a major threat in areas such as the southwestern United States, while excessive flooding is the threat in other regions (IPCC 2001). Increased carbon dioxide levels will also alter the growth rates of crops and weeds. In certain environments, changes in the productivity of traditional, land agriculture could lead to a changes in fish demand.
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