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The introduction of fresh water from melting ice caps can also affect thermohaline circulation. Since fresh water is less dense than salt water, it floats on the surface in high latitude regions. This cap prevents the sinking of water in regions of downwelling, thus weakening or stopping the overturn of the ocean. Current models predict that a shutdown of downwelling in the North Atlantic could occur soon and lead to a shutdown of the ocean conveyor (Gagosian 2007). Historically, such events have resulted in build-ups of toxins in the ocean that have been linked to mass-extinction events (Bralower 2002). A shutdown of global thermohaline circulation is also likely to cause rapid and severe changes in climate, with similar changes in temperature to what weve seen over the past century occuring on the scale of decades (Gagosian 2007).
Warming of the atmosphere is expected to result in intensified atmospheric pressure gradients. There is already some evidence that this has resulted in increased storm frequency and intensity over recent years. Atmospheric conditions are largely responsible for surface currents, which transport water in the surface layers of the ocean where most of the biomass resides (Harley 2006). Modeling predicts that advection will increase as a result of global warming, especially in eastern boundary currents. Increased advection is generally linked to decreased biomass (Harley 2006).
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