Arctic climate more vulnerable than thought, maybe linked to Antarctic ice-sheet behavior

Further, these extreme inter-glacial warm periods correspond closely with times when parts of Antarctica were ice-free and also warm, suggesting strong inter-hemispheric climate connectivity, say the project's three co-chief scientists. The Polar Regions are much more vulnerable to change than once believed, they add. The team was led by Martin Melles of Germany's University of Cologne, Julie Brigham-Grette of the University of Massachusetts Amherst and Pavel Minyuk of Russia's Northeast Interdisciplinary Scientific Research Institute in Magadan.

Brigham-Grette, the lead U.S. scientist says, "What we see is astonishing. We had no idea that we'd find this. It's astonishing to see so many intervals when the Arctic was really warm, enough so forests were growing where today we see tundra and permafrost. And the intensity of warming is completely unexpected. The other astounding thing is that we were able to determine that during many times when the West Antarctic ice sheet disappeared, we see a corresponding warm period following very quickly in the Arctic. Arctic warm periods cluster with periods when the Western Antarctic ice sheet is gone."

Data reported by Melles, Brigham-Grette, Minyuk and colleagues come from analyzing sediment cores collected in 2009 from under ice-covered Lake El'gygytgyn in the northeast Russian Arctic. "Lake E" was formed 3.6 million years ago when a huge meteorite hit Earth and blasted out an 11-mile (18 km) wide crater. It has been collecting layers of sediment ever since. Luckily, it is located in one of the few areas in the Arctic not eroded by continental glaciers, leaving the thick sediment record remarkably undisturbed and continuous. Cores from Lake E reach back in geologic time nearly 30 times farther than Greenland ice cores covering the past 110,000 years.

In their first paper reporting results, Brigham-Grette and colleagues discuss four warm phases in detail; two of the oldest warm interglacials from about 1.1 million years ago and 400,000 years ago, and two of the youngest from 125,000 and about 12,000 years ago.

Working with 2.5-inch (6.6 cm) diameter mud cores, they analyzed such variables as pollen assemblages and number of diatoms, which allow estimates of changing vegetation type, coverage, summer average temperatures and rainfall, for example. Other analyses yield quantitative physical properties including elemental carbon, nitrogen and sulfur, whether organic material is present and in what concentrations.

The pollen-based climate reconstructions suggest that summer temperatures and annual precipitation during the exceptional interglacials were about 4 to 5 degrees C warmer and about 12 inches (300 mm) wetter than in other interglacials, they say. Modeling and sensitivity tests for these warm periods also suggest it's virtually impossible for Greenland's ice sheet to have existed in its present form at those times.

Simulations using a state-of-the-art climate model show that the high temperature and precipitation during the "super interglacials" cannot be explained by Earth's orbital parameters or variations in atmospheric greenhouse gases alone, which geologists typically see driving the glacial/interglacial pattern during ice ages. This suggests additional climate feedbacks are at work. The Lake E researchers suspect the trigger for intense interglacials might be in Antarctica. Earlier work by the international ANDRILL program discovered recurring intervals when the West Antarctic Ice Sheet melted. The Lake E study shows that some of these events match remarkably well with the super interglacials in the Arctic.

Brigham-Grette and colleagues discuss two scenarios for future testing that could explain inter-hemispheric climate coupling. First, reduced glacial ice cover and loss of ice shelves in Antarctica could have limited formation of cold water masses that flow into the north Pacific and well up to the surface, resulting in warmer surface waters, higher temperatures and increased precipitation on nearby land. Alternatively, disintegration of the West Antarctic Ice Sheet likely led to a significant global sea level rise and allowed more warm surface water into the Arctic Ocean through the Bering Strait.

Not only do results shed light on natural variability of the Arctic climate, but this view of the past may be a key to understanding climate in future centuries, the researchers say."We have a lot more to learn," says Brigham-Grette. "But our results mesh with what glaciologists are seeing today. Seven of the 12 major ice shelves around the Antarctic are melting or are gone. We suspect the tipping point for the gradual de-glaciation of Greenland and the Arctic may be lower than glaciologists once thought."

The international Lake El'gygytgyn Drilling Project was funded by the International Continental Drilling Program (ICDP), the U.S. National Science Foundation's Division of Earth Sciences and Office of Polar Programs, the German Federal Ministry for Education and Research, Alfred Wegener Institute, GeoForschungsZentrum-Potsdam, the Russian Academy of Sciences Far East Branch, the Russian Foundation for Basic Research and the Austrian Ministry for Science and Research.