How does rising atmospheric CO2 affect marine organisms?

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A 21st-Century Weakening of the Thermohaline Circulation?
Gent, P.R.  2001.  Will the North Atlantic Ocean thermohaline circulation weaken during the 21st century?  Geophysical Research Letters 28: 1023-1026.

What was done
A number of researchers have run global climate model simulations into the future in an attempt to determine whether or not CO2-induced global warming will significantly affect the thermohaline circulation of the world's oceans, with some of the models predicting a significant weakening of this global ocean-water "conveyor belt."  In a new study of this subject, Gent uses the Climate System Model -- a coupled general circulation model with atmosphere, ocean, land and sea-ice components -- to evaluate the strength of the thermohaline circulation through the 21st century.

What was learned
The model simulations showed the Northwest Atlantic becoming warmer and more saline, which changes had little net effect on the surface water density in this part of the world and, hence, led to little net change in the rate of deep water formation in this important deep-water source region.  Thus, the author concluded there is "no evidence of a significant weakening of the thermohaline circulation" over the 21st century.

What it means
As with all model studies, there are a number of caveats that must be attached to the findings of this report; and the author lists several that could affect the outcome of his modeling exercise, stressing that determining the behavior of the thermohaline circulation in the 21st century is "a very demanding question to ask of current state-of-the-art coupled climate models."  Yet, it is an important question to ask, because the thermohaline circulation is a major contributor to world climate and that of Europe in particular.  Clearly, however, a firm understanding of this phenomenon has yet to be achieved, which suggests that the ultimate impact of the ongoing rise in the air's CO2 concentration is far from known with any degree of certainty.