Since the upper ocean takes up much of the heat added to the earth system by anthropogenic global warming, one would expect that global warming would lead to an increase in stratification and a decrease in the ventilation of the ocean interior. However, multiple simulations in global coupled climate models using an ideal age tracer 5 which is set to zero in the mixed layer and ages at 1 yr/yr outside this layer show that the intermediate depths in the low latitudes become younger under global

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... rming. This paper reconciles these apparently contradictory trends, showing that a decrease in upwelling of old water from below is responsible for the change. Implications for global biological cycling are considered. 25 Given the evidence that the oxygen minimum zones have changed in the past, it is 806 OSD 3, 2006