Global warming may chill planet

by Charles Gardner | 4/14/03 5:00am

The continued warming of earth's atmosphere threatens to trigger a dramatic change in ocean circulation that could paradoxically plunge much of Europe and North America into bitter cold within the next few decades, oceanic expert Robert Gagosian said to a packed crowd on Friday.

Earth's oceans, which hold vastly more heat than the atmosphere, are responsible for transporting and redistributing this heat across the globe through a linked series of giant, slow-moving currents known as the "great ocean conveyor," according to Gagosian, who is President and Director of the Woods Hole Oceanographic Institute.

The Gulf Stream, which carries warm equatorial waters north, moderating the climate in Europe and the North Atlantic, is one of the best known of these currents. Data show that the waters of the North Atlantic have become steadily less salty over the past 30 years, a trend which could lead to the slowdown or complete cessation of the Gulf Stream conveyor as cold Arctic waters become increasingly buoyant and remain at the ocean's surface, preventing warmer waters from flowing north.

The observed freshening of North Atlantic waters has been "arguably the biggest change that has ever been measured in the global ocean," Gagosian said, citing as possible causes the continued melting of the Arctic ice shelf and increased outflow from Eurasian rivers.

Though scientists do not know what precise level of salinity will cause the current to stop, Gagosian emphasized that such a change could occur extremely rapidly -- in contrast to current thinking that tends to see climate change in gradual terms -- and that the resultant effects of the projected 10 degree Fahrenheit drop could have a significant impact on a region "where 60 percent of the world's economy is based."

The colder climate could "send alpine glaciers advancing, disrupt shipping lanes and cause energy needs to soar," Gagosian said. Furthermore, the global effects of the cooling could produce widespread and severe droughts leading to famine, mass migration and displacement of human populations.

Such drastic and rapid climate shifts have occurred several times throughout the last few thousand years, Gagosian said, including a recent cold period in Europe and North America known as the "Little Ice Age" that ended only around 150 years ago.

Additionally, scholars have recently attributed the demise of the ancient Mayan civilization of Central America in large part to a series of "megadroughts," Gagosian said, while the Akkadian Empire in Mesopotamia and the Anasazi culture in the American southwest also may have met their demise at the hands of extreme drought.

Current climate simulations estimate that the conveyor will cease operating in around 40 to 50 years, according to Gagosian, though it is impossible to rule out a change occurring much sooner.

"We are walking towards the edge of a cliff blindfolded," he said. A change within the next 10 years would likely have drastic effects, Gagosian said, while if estimations are off and the warm current ceases flowing in a century, the effects of the cooling would be felt in the context of a much warmer globe, allowing Europe and North America to counter the effects of warming and return to a climate much like today's.

Though the freshening of the North Atlantic may be an irreversible trend, Gagosian emphasized that "we can still take steps to adapt to change," particularly by establishing a more comprehensive series of monitoring instruments to better document subtle changes in the oceans.

Gagosian, a marine geochemist by training, was in 2002 appointed to the Science Advisory Panel of the presidentially-appointed U.S. Commission on Ocean Policy and is also a fellow of the World Economic Forum.