Climate change made ocean circulation switch direction

The circulation of water in the Atlantic Ocean reversed its direction less than 20,000 years ago, a study has found.

Nowadays, warm currents such as the Gulf Stream travel from the tropics to the subpolar North Atlantic, where they cool. Crucially for the world’s climate, they bring warmth with them.

But according to researchers led by the Universitat Autònoma de Barcelona, things were very different in the past.

The team investigated the distribution of isotopes in the Atlantic Ocean. These are generated from the natural decay of uranium in seawater, and are distributed with the flow of deep waters across the Atlantic basin, indicating where the waters carrying them originated.

They found that there was a period during the ice age 20,000 years ago, when the flow of deep waters in the Atlantic was reversed, when the climate of the North Atlantic region was substantially colder and deep convection was weakened.

At that time, the balance of seawater density between the North and South Atlantic was shifted in such a way that deep water convection was stronger in the South Polar Ocean, with warm waters flowing southward rather than northward. 

The authors say the study shows that the Atlantic circulation pattern in the past was very sensitive to changes in the salt balance of Atlantic Ocean currents. The Southern Ocean was then much saltier than it is now.

Similar changes in seawater salt concentration are expected to take place in the North Atlantic in the course of climate warming over the next 100 years, leading to the possibility that the ocean circulation could change again.