The National Science Foundation (NSF) funded study also finds that if a shutdown persisted for a long enough time, the entire Northern Hemisphere would eventually cool.
The computer model simulations of ocean and atmosphere processes used in this study imply a similar phenomenon has the potential to occur in the future due to freshwater additions from increased rain and snow caused by global climate change.
"For the first time, it is shown that realistic additions of glacial meltwater into the North Atlantic would have shutdown North Atlantic Deep Water production over a period of a few hundred years if the initial ocean circulation was somewhat weaker than that of today," said David Rind, lead author of the study and a senior climate researcher at NASA's Goddard Institute for Space Studies in New York, NY. The study appears in the November 16 issue of Journal of Geophysical Research - Atmospheres.
When Rind and his colleagues entered realistic estimates of freshwater from melting glaciers into their model, they found the North Atlantic circulation stopped completely after some 300 years. When the model was adjusted to make the circulation weaker than it is today, cessation of the Gulf Stream took only 150-200 years, matching current estimates based on paleo-climate records .
Freshwater additions into the ocean through the St. Lawrence River have a profound effect on the ocean circulation.
"The more freshwater you add, and the longer you add it, the greater reduction in the North Atlantic circulation," Rind said. "According to our model, this is a linear response."
When the Gulf Stream moves warm surface water from the equator north through the Atlantic, the water cools, gets saltier due to evaporation and becomes very dense. By the time it approaches the coast of Newfoundland, or further northeast in the Norwegian Sea, it becomes dense enough to sink. This process is called overturning. The dense water then slowly travels through the deep water southward into the Southern Hemisphere, with the return flow to the north occurring at the surface.
But when freshwater gets mixed with the salty water in the North Atlantic, it makes the water less dense and slows the overturning process and the ocean circulation.
While the study finds that freshwater input could slow and stop overturning, this would not stop the Gulf Stream entirely. That's because the stream is partially pushed by winds. As a result, the model shows the reduced Gulf Stream would only transport about half as much heat northward, thereby cooling Western Europe. Were this to occur in a global warming scenario, it would act to partly counter the effects of projected greenhouse warming in parts of Western Europe.
Many scientists suspect more rainfall in parts of the Northern Hemisphere during this century as a result of greenhouse warming. That's because warmer temperatures increase the atmosphere's capacity to carry water. "The North Atlantic circulation may already be weakening due to freshwater rainfall additions associated with global warming," Rind said.
But the model shows a number of inconsistencies with previous studies on the last ice age. Those studies speculate that once freshwater stopped flowing, the ocean circulation would return within only a few decades, matching a rapid warming seen in the climate record. The model finds that deepwater circulation does not return for at least hundreds of years when the freshwater additions end. Also contrary to observations, the model showed cooling throughout the Northern Hemisphere; during the last ice age, the majority of the United States land mass did not appear to cool.
"It's hard to understand how parts of the Northern Hemisphere might have cooled to the magnitude suggested, but not North America," Rind said. "That seems to imply that either the paleo-records are being misinterpreted, or something else went on, something major that is not being accounted for. This isn't necessarily the end of the story."