The water flowing from the North Atlantic into the Arctic Ocean is the warmest it's been in 2,000 years, say scientists.
An international study has shown that water from the Fram Strait, which runs between Greenland and northern Norway, has warmed about 3.5 degrees Fahrenheit in the last century. It's now about 2.5 degrees F warmer than during the Medieval Warm Period.
The team believes that this increased heat transfer is what's reposnsible for the rapid warming of the Arctic and recent decrease in Arctic sea ice.
"Such a warming of the Atlantic water in the Fram Strait is significantly different from all climate variations in the last 2,000 years," says Robert Spielhagen of the Academy of Sciences, Humanities and Literature in Mainz, Germany.
According to study co-author Thomas Marchitto of CU-Boulder's Institute of Arctic and Alpine Research, the new observations give a context to the current warming trend of the North Atlantic.
"We know that the Arctic is the most sensitive region on the Earth when it comes to warming, but there has been some question about how unusual the current Arctic warming is compared to the natural variability of the last thousand years," said Marchitto. "We found that modern Fram Strait water temperatures are well outside the natural bounds."
Since continuous meteorological and oceanographic data for the Fram Strait reach back only 150 years, the team drilled ocean sediment cores dating back 2,000 years to determine past water temperatures. The researchers used microscopic, shelled protozoan organisms called foraminifera - which prefer specific water temperatures at depths of roughly 150 to 650 feet - as tiny thermometers.
They also examined the chemical composition of the foraminifera shells to reconstruct past water temperatures in the Fram Strait.
"Cold seawater is critical for the formation of sea ice, which helps to cool the planet by reflecting sunlight back to space," said Marchitto. "Sea ice also allows Arctic air temperatures to be very cold by forming an insulating blanket over the ocean. Warmer waters could lead to major sea ice loss and drastic changes for the Arctic."
The rate of Arctic sea ice decline appears to be accelerating due to positive feedbacks between the ice, the Arctic Ocean and the atmosphere, Marchitto said.
"We must assume that the accelerated decrease of the Arctic sea ice cover and the warming of the ocean and atmosphere of the Arctic measured in recent decades are in part related to an increased heat transfer from the Atlantic," said Spielhagen.