Sudden shifts in temperature over Greenland and tropical rainfall patterns during the last ice age have been linked for the first time to rapid changes in the salinity of the north Atlantic Ocean, according to research published Oct. 5 in the journal Nature. The results provide further evidence that climate change can have a direct and rapid impact on ocean circulation and chemistry.
"It's a very complicated system," said lead author Matthew Schmidt, who carried out the work as a graduate student at the University of California, Davis, and is now a visiting NOAA Climate and Global Change Postdoctoral Fellow at the Georgia Institute of Technology. "But when it responds, it responds big time."
Schmidt, Maryline Vautravers of Cambridge University in England, and Howard Spero, professor of geology at UC Davis, reconstructed a 45,000-to-60,000-year-old record of ocean temperature and salinity from the chemical traces in fossil shells of tiny planktonic animals recovered from deep sea sediment cores. They compared their results to the record of abrupt climate change recorded in ice cores from Greenland.
At that time, much of North America and Europe was a frigid sheet of ice. But the ice records show repeated patterns of sudden warming, called Dansgaard-Oeschger cycles, when temperatures in Greenland rose by five to 10 degrees Celsius over a few decades.
Those cycles were matched by rapid changes in surface-water salinity in the north Atlantic, the researchers found. The Atlantic got saltier during cold periods, and fresher during warm intervals. The freshening likely reflects shifts in rainfall patterns, mostly in the tropics, Spero said.
"Suddenly, we're looking at a record that links moisture balance in the tropics to climate change," he said.
Close to the tropics, warm, moist air forms a zone of heavy tropical rainfall, called the Intertropical Convergence Zone, which dilutes the salty ocean with fresh water. Today, the tropical rainfall zone reaches into the northern Caribbean, but during the colder periods of the ice age it was pushed much farther south, toward Brazil. That kept fresh water out of the northern Atlantic, so it became more salty, Spero said.
"The most striking thing is that a measurable transition is happening over decades," Spero said.
The circulation, or gyre, in the north Atlantic moves warm, salty water north, keeping Europe relatively temperate. The deep ocean circulation is very sensitive to the saltiness of north Atlantic surface waters, Spero said. Warming climate, higher rainfall and fresher conditions can alter the circulation. During glacial times, reduced circulation caused climate to cool.
The new paper shows that as the climate cooled in Greenland, salinity rapidly increased in the North Atlantic subtropical gyre. The build-up of salt during these cold intervals when the conveyor circulation was reduced would have primed the system to quickly restart on transitions into warm intervals, Schmidt said. However, the actual trigger that caused Atlantic circulation to restart during the ice age is still unknown, he said.
Once warming began, melting ice sheets would have contributed fresh water to the Atlantic, but this would have been partly buffered by the elevated saltiness of the Atlantic.
The research looked at changes during the last ice age, when global temperatures were much lower than today. But the results show that ocean salinity is very sensitive to climate change, and could change rapidly -- over a matter of decades, Spero said.
"The salinity of the north Atlantic is the canary of the climate system," Spero said.
The work was supported by the National Science Foundation.
Journal
Nature