Greenland Ice Not A Reliable Model For Younger Dryas Period

Brett Smith

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Ice samples that profile Greenland glaciers have long been used to give climate scientists historical temperature data, but those samples could be misleading, according to a new study published Monday in the Proceedings of the National Academy of Sciences.

A team led by University of Wisconsin-Madison researchers found that the data gathered from the ice cores around Greenland varies greatly from other records of Northern Hemisphere temperatures during the Younger Dryas. Also referred to as the “Big Freeze” — the Younger Dryas was a period of abrupt cooling that began nearly 13,000 years ago.

“In terms of temperature during the Younger Dryas, the only thing that looks like Greenland ice cores are Greenland ice cores,” said Anders Carlson, a UW-Madison geosciences professor.

“They are supposed to be iconic for the Northern Hemisphere, but we have four other records that do not agree with the Greenland ice cores for that time. That abrupt cooling is there, just not to the same degree.”

While collaborating with UW–Madison climatologist Zhengyu Liu and scientists at the National Center for Atmospheric Research (NCAR), Carlson could reliably recreate temperatures in the Oldest Dryas, a cooling period about 18,000 years ago. But when using the same model to reconstruct Younger Dryas data, the program breaks down around data culled from Greenland ice cores.

In an attempt to understand why the Greenland samples did not match expectations, researchers found that oxygen isotope data did not account for the sort of crash climate change occurring during the Younger Dryas. They also realized their assumption that prevailing winds and jet streams are dumping precipitation from the Atlantic Ocean on Greenland was incorrect.

“The Laurentide ice sheet, which covered much of North America down into the northern United States, is getting smaller as the Younger Dryas approaches,” Carlson said. “That’s like taking out a mountain of ice three kilometers high. As that melts, it allows more Pacific Ocean moisture to cross the continent and hit the Greenland ice sheet.”

The two oceans have distinctly different ratios of oxygen isotopes, thus snow that originated from the Pacific will appear much different than Atlantic snow.

“We ran an oxygen isotope-enabled atmosphere model, so we could simulate what these ice cores are actually recording, and it can match the actual oxygen isotopes in the ice core even though the temperature doesn’t cool as much,” Carlson said. “That, to us, means the source of precipitation has changed in Greenland across the last deglatiation. And therefore that the strict interpretation of this iconic record as purely temperature of snowfall above this ice sheet is wrong.”

Based on the study’s findings, the researchers said Greenland temperatures may not have cooled as momentously as climate headed into the Younger Dryas relative to the Oldest Dryas, because of the increased levels in atmospheric carbon dioxide that had occurred since the Oldest Dryas.

The study also showed that climate change is the result of a confluence of factors and determining historical temperature trends is a highly nuanced science.

“Abrupt climate changes have happened, but they come with complex shifts in the way climate inputs like moisture moved around,” Carlson said. “You can’t take one difference and interpret it solely as changes in temperature, and that’s what we’re seeing here in the Greenland ice cores.”