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.
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
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.
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."
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
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
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.