Climate changes in the northern and southern hemispheres are linked by a phenomenon by which the oceans react to changes on either side of the planet. A research team from the Universitat Autònoma de Barcelona and the Cardiff University has shown for the first time that ocean circulation in the southern hemisphere has, in the past, adapted to sudden changes in the north. The research published today in Science will enable more accurate forecasts to be made on how the oceans will react to climate change.
The scientists have observed that at several periods in history when the temperature has increased in the northern hemisphere, the southern hemisphere has entered a cooling period, which creates a decrease in the amount of deep water transported to the Atlantic Ocean from the south. The opposite effect also took place: when the climate cooled in the North Atlantic, the southern hemisphere entered a warmer period, causing water to be transported northwards.
These mechanisms linking the two hemispheres had already been observed in computer climate simulations, but this is the first time they have been confirmed with detailed data obtained from scientific experiments using weather records from the past. This is the first evidence showing that waters in the southern hemisphere play an active role in sudden climate changes.
Today's climate in Europe and North America is greatly influenced by the gulf stream. This ocean current carries warm water from the Gulf of Mexico northwards along the Florida coast, eastwards across the Atlantic and southwards along the west coast of Europe, bringing a mild climate. The strength of the current is dependent on the salinity of the water travelling from the south. If the salinity decreases, the current weakens. Scientists predict that global warming could cause part of the Greenland ice sheet to melt, giving rise to increased levels of freshwater in the Atlantic Ocean. This could reduce the strength of the gulf stream, creating a cooler, dryer climate in Europe and North America.
However, according to the authors of this latest study, the Atlantic Ocean could already be adapting to the changes brought about by global warming in the same way as it adapted to climate changes in the past. The waters in the southern hemisphere are less salty than those in the northern hemisphere, and this freshwater in the south sinks to the ocean floor and is transported to the rest of the Atlantic, reducing the salinity of the North Atlantic Ocean and strength of the gulf stream. Nevertheless, the researchers have observed a decrease in the volume of freshwater sinking to the floor of the South Atlantic Ocean. According to Rainer Zahn, "although we don't know where global warming will take us, this could be a sign that the oceans are already adapting to the changes".