New study tells three million-year old story of the Arctic

  • 09 May 2013, 19:20
  • Roz Pidcock

The Arctic wasn't always as cold and icy as it is now. Three million years ago, carbon dioxide levels were similar to today - but summer temperatures were eight degrees Celsius higher and Greenland was almost ice-free. A new study has scientists speculating whether earth's distant past can provide a window into future climate change.

Ancient lake

Scientists can glean clues about the history of Earth's climate by examining plants and animals preserved in ancient sediments. Fossilised pollen can reveal the vegetation that existed at different times, which in turn paints a picture of temperature and rainfall.

Published today in the journal Science, a new study uses the longest sediment core ever collected from the Arctic to reconstruct polar temperature between 2.2 and 3.3 million years ago.

The international team of scientists analysed fossilised pollen from layers of sediment below an 18 kilometre-wide lake in the northeast Russian Arctic, which formed when a meteorite hit earth nearly 3.6 million years ago.

Arctic _sediment _core

Co-authors Julie Brigham-Grette and Pavel Minyuk with the drill corer that collected sediment hundreds of metres below lake El'gygytgyn in north eastern Russia. Credit: Tim Martin

The sediment in the 300-metre long core is remarkably undisturbed. Lead researcher Julie Brigham-Grette from the University of Massachusetts explains that previous research has offered clues about earth's distant past but the new study provides the longest continuous temperature record. She says:

"As if reading a detective novel, we can go back in time and reconstruct how the Arctic evolved with only a few pages missing here and there."

Hot summers

About 3.4 to 3.6 million years ago, during a period known as the Pliocene, the researchers found Arctic summers were eight degrees warmer than today, and had three times more rainfall. This radically different climate meant forests extended to the Arctic Ocean coastline, and Greenland was almost completely ice free.

The researchers say both polar regions were "substantially warmer than present". This is despite atmospheric carbon dioxide levels being fairly similar to today's, at between 380 and 450 parts per million.

According to the data, temperatures gradually cooled over the next million years or so. But summers stayed warmer than today until about 2.2 million years ago, after ice sheets first started to appear in the northern hemisphere.

Window into the future?

With similar carbon dioxide levels today, the Pliocene is often thought of as an analog for current conditions. But why were Arctic temperatures so much higher?

The precise mechanisms are not well known. Brigham-Grette tells us most models underestimate Arctic temperatures during the Pliocene, so there must be interactions between different parts of the climate system that scientists don't yet fully understand.

One factor could be that during the Pliocene, the Arctic was sea ice-free in summer. As ice melts, sunlight that previously would have been reflected by ice is instead absorbed by the ocean leading to further warming. This leads to Arctic temperatures rising faster than the rest of the world, known as Arctic amplification.

Looking back in time can help scientists unravel how earth might respond to future warming - because the same feedbacks are relevant today. Arctic sea ice has declined rapidly in the last few decades and recent research suggests summers could be almost entirely ice-free before 2050.

So can we expect Greenland to become ice-free too? Brigham-Grette tells us it's feasible. But, she adds:

"[M]ost scientists would agree that an ice-free Greenland would take many thousands of years to achieve. I have seen estimates of at least 3 to 5,000 years. It would be a very long process."

The Arctic has been closely tied to global climate throughout earth's history. Knowing how and why Arctic climate has changed in the past helps narrow the uncertainties in future projections of climate change - although the timescales over which changes take place may prove to be significantly shorter.

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