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 arecord 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.