Slowdown in Atlantic Could Impact Sea Levels, Ecosystems

This illustration depicts the overturning circulation of the global ocean. Throughout the Atlantic Ocean, the circulation carries warm waters (red arrows) northward near the surface and cold deep waters (blue arrows) southward. (NASA/JPL)

This illustration depicts the overturning circulation of the global ocean. Throughout the Atlantic Ocean, the circulation carries warm waters (red arrows) northward near the surface and cold deep waters (blue arrows) southward. (NASA/JPL)

By ecoRI News staff

BRISTOL, R.I. — Roger Williams University professor Scott Rutherford is one of several researchers on a global team who recently published research on the slowdown of the Gulf Stream system, which could have major implications for marine ecosystems, sea levels and weather systems.

The Atlantic overturning is one of the planet’s most important heat transport systems, pumping warm water northward and cold water southward. Atlantic meridional overturning circulation (AMOC) is responsible for the mild climate in northwestern Europe. A team of researchers from Roger Williams University, Potsdam Institute for Climate Impact Research, Geological Survey of Denmark and Greenland, and Pennsylvania State University have now found evidence for a slowdown of the overturning — multiple lines of observation suggest that in recent decades, the current system has been weaker than ever before in the past millennium.

The gradual but accelerating melting of the Greenland ice sheet, being caused largely by manmade global warming, is a possible major contributor to the slowdown, according to the researchers’ paper. Further weakening could impact marine ecosystems, sea level and weather in the United States and Europe.

As part of the team of researchers, Rutherford’s role was to reconstruct past climate using different types of proxy data such as tree rings, ocean or lake sediments and cave deposits. He reconstructed the temperature in the North Atlantic subpolar gyre, the area just south of Greenland, and compared its temperature to the hemisphere’s mean temperature, the difference gives an indication of the magnitude of Atlantic Ocean overturning.

These temperature reconstructions allowed the team to see how unusual the 20th-century slowdown is compared to the past 1,000 years, according to the article’s authors.

“It’s important that we continue to follow these new patterns in the Atlantic overturning because they have a significant impact on the distribution of heat on the planet and our fundamental understanding of how the climate system works,” Rutherford said. “These changing dynamics are one of many factors that affect larger-scale weather systems, potentially changing weather and precipitation patterns that we have come to expect and rely on for a variety of our global resources.”

Because long-term direct ocean current measurements are lacking, the scientists mainly used sea-surface and atmospheric temperature data to derive information about the ocean currents, exploiting the fact that ocean currents are the leading cause of temperature variations in the subpolar North Atlantic.

From so-called proxy data — gathered from ice cores, tree rings, coral, and ocean and lake sediments — temperatures can be reconstructed for more than a millennium back in time. The recent changes found by the team are unprecedented since the year 900 AD, strongly suggesting they are caused by manmade climate change, according to the article.

The Atlantic overturning is driven by differences in the density of the ocean water. From the south, warmer and lighter water flows northward, where the colder and heavier water sinks to deeper ocean layers and flows southward.

“Now fresh water coming off the melting Greenland ice sheet is likely disturbing the circulation,” Jason Box of the Geological Survey of Denmark and Greenland said, noting this fresh water is diluting the ocean water. “So the human-caused mass loss of the Greenland ice sheet appears to be slowing down the Atlantic overturning — and this effect might increase if temperatures are allowed to rise further.”

“Common climate models are underestimating the change we’re facing, either because the Atlantic overturning is too stable in the models or because they don’t properly account for Greenland ice sheet melt, or both,” said Michael Mann of Penn State.

If the slowdown of the Atlantic overturning continues, the impacts might be substantial, according to the researchers. They said disturbing the circulation will likely have a negative effect on ocean ecosystems, fisheries and the associated livelihoods of many people in coastal areas. A slowdown also adds to the regional sea-level rise affecting cities such as New York and Boston, they said.