Arctic Ocean current temperatures traced back to Roman Empire

Using as tiny climate sensors the miniscule bodies of hard-shelled protozoa deposited on the sea floor over millennia, an international research team has made a startling discovery off the coast of Greenland.

A northbound current flowing far beneath the surface of the Atlantic Ocean through Fram Strait has been sloshing the warmest water of the past 2,000 years into the Arctic Ocean -- indirectly contributing to the thinning and shrinkage of sea ice near Alaska and across the Arctic.

This unprecedented flow, almost 3.6 degrees Fahrenheit warmer than when Augustus ruled Rome, has now become a key factor in the decades-long shift toward ice-free summers in the Arctic, as its heat spreads out and slowly mixes with colder waters at the surface, according to a new study published recently in the journal Science.

"We find that early-21st-century temperatures of Atlantic water entering the Arctic Ocean are unprecedented over the past 2,000 years and are presumably linked to the Arctic amplification of global warming," wrote lead investigator Robert Spielhagen, a palaeoceanographer at the Leibniz Institute of Marine Sciences in Kiel, Germany, along with eight coauthors from Colorado, Germany and Norway.

The most stunning increase in the current occurred over the last 120 years, the scientists said, coinciding with other indicators of human-triggered climate change. This particular stream of water is now about 2.5 degrees-F warmer than it was during the so-called Medieval Warm Period between 900 and 1300, when Vikings planted crops in Greenland and vineyards supposedly spread across southern England.

"Such a warming of the Atlantic water in the Fram Strait is significantly different from all climate variations in the last 2,000 years," Spielhagen said in a story posted by the University of Colorado about the study.

"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," added study co-author Thomas Marchitto, a fellow at CU-Boulder's Institute of Arctic and Alpine Research, in the same story. "We found that modern Fram Strait water temperatures are well outside the natural bounds."