[Vision2020] "Nature Geoscience" 6-17-12: "Hydrologic cycling over Antarctica during the middle Miocene warming"

[Ted Moffett]

This 2012 peer reviewed study is further paleoclimate evidence of polar
amplification from greenhouse warming.  Atmospheric CO2 levels between 400
to 600 ppm during the Miocene, is the range where anthropogenic CO2
emissions will force our contemporary atmosphere in a matter of a few
decades.  NASA news story explicating this study lower down:

http://www.nature.com/ngeo/journal/v5/n8/full/ngeo1498.html
Hydrologic cycling over Antarctica during the middle Miocene warmingNature
Geoscience 5,557–560(2012)doi:10.1038/ngeo1498 Received 25 August 2011
Accepted 15 May 2012 Published online 17 June 2012 From 20 to 15 million
years (Myr) ago, a period of global warmth reversed the previous ice growth
on Antarctica, leading to the retreat of the West Antarctic Ice Sheet and
the contraction of the East Antarctic Ice
Sheet1<http://www.nature.com/ngeo/journal/v5/n8/full/ngeo1498.html#ref1>,
2 <http://www.nature.com/ngeo/journal/v5/n8/full/ngeo1498.html#ref2>.
Pollen recovered from the Antarctic shelf indicates the presence of
substantial vegetation on the margins of Antarctica 15.7 Myr
ago3<http://www.nature.com/ngeo/journal/v5/n8/full/ngeo1498.html#ref3>.
However, the hydrologic regime that supported this vegetation is unclear.
Here we combine leaf-wax hydrogen isotopes and pollen assemblages from Ross
Sea sediments with model simulations to reconstruct vegetation,
precipitation and temperature in Antarctica during the middle Miocene.
Average leaf-wax stable hydrogen isotope (δD) values from 20 to 15.5 Myr
ago translate to average δD values of −50‰ for precipitation at the margins
of Antarctica, higher than modern values. We find that vegetation persisted
from 20 to 15.5 Myr ago, with peak expansions 16.4 and 15.7 Myr ago
coinciding with peak global
warmth4<http://www.nature.com/ngeo/journal/v5/n8/full/ngeo1498.html#ref4>and
vegetation growth
5 <http://www.nature.com/ngeo/journal/v5/n8/full/ngeo1498.html#ref5>. Our
model experiments are consistent with a local moisture source in the
Southern Ocean6<http://www.nature.com/ngeo/journal/v5/n8/full/ngeo1498.html#ref6>.
Combining proxy measurements with climate simulations, we conclude that
summer temperatures were about 11 °C warmer than today, and that there was
a substantial increase in moisture delivery to the Antarctic coast.
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http://www.nasa.gov/topics/earth/features/antarctica20120617.html

Study Finds Ancient Warming Greened Antarctica
06.17.12

PASADENA, Calif. -- A new university-led study with NASA participation
finds ancient Antarctica was much warmer and wetter than previously
suspected. The climate was suitable to support substantial vegetation --
including stunted trees -- along the edges of the frozen continent.

The team of scientists involved in the study, published online June 17 in
Nature Geoscience, was led by Sarah J. Feakins of the University of
Southern California in Los Angeles, and included researchers from NASA's
Jet Propulsion Laboratory in Pasadena, Calif., and Louisiana State
University in Baton Rouge.

By examining plant leaf wax remnants in sediment core samples taken from
beneath the Ross Ice Shelf, the research team found summer temperatures
along the Antarctic coast 15 to 20 million years ago were 20 degrees
Fahrenheit (11 degrees Celsius) warmer than today, with temperatures
reaching as high as 45 degrees Fahrenheit (7 degrees Celsius).
Precipitation levels also were found to be several times higher than today.

"The ultimate goal of the study was to better understand what the future of
climate change may look like," said Feakins, an assistant professor of
Earth sciences at the USC Dornsife College of Letters, Arts and Sciences.
"Just as history has a lot to teach us about the future, so does past
climate. This record shows us how much warmer and wetter it can get around
the Antarctic ice sheet as the climate system heats up. This is some of the
first evidence of just how much warmer it was."

Scientists began to suspect that high-latitude temperatures during the
middle Miocene epoch were warmer than previously believed when co-author
Sophie Warny, assistant professor at LSU, discovered large quantities of
pollen and algae in sediment cores taken around Antarctica. Fossils of
plant life in Antarctica are difficult to come by because the movement of
the massive ice sheets covering the landmass grinds and scrapes away the
evidence.

"Marine sediment cores are ideal to look for clues of past vegetation, as
the fossils deposited are protected from ice sheet advances, but these are
technically very difficult to acquire in the Antarctic and require
international collaboration," said Warny.

Tipped off by the tiny pollen samples, Feakins opted to look at the
remnants of leaf wax taken from sediment cores for clues. Leaf wax acts as
a record of climate change by documenting the hydrogen isotope ratios of
the water the plant took up while it was alive.

"Ice cores can only go back about one million years," Feakins said.
"Sediment cores allow us to go into 'deep time.'"

Based upon a model originally developed to analyze hydrogen isotope ratios
in atmospheric water vapor data from NASA's Aura spacecraft, co-author and
JPL scientist Jung-Eun Lee created experiments to find out just how much
warmer and wetter climate may have been.

"When the planet heats up, the biggest changes are seen toward the poles,"
Lee said. "The southward movement of rain bands associated with a warmer
climate in the high-latitude southern hemisphere made the margins of
Antarctica less like a polar desert, and more like present-day Iceland."

The peak of this Antarctic greening occurred during the middle Miocene
period, between 16.4 and 15.7 million years ago. This was well after the
age of the dinosaurs, which became extinct 64 million years ago. During the
Miocene epoch, mostly modern-looking animals roamed Earth, such as
three-toed horses, deer, camel and various species of apes. Modern humans
did not appear until 200,000 years ago.

Warm conditions during the middle Miocene are thought to be associated with
carbon dioxide levels of around 400 to 600 parts per million (ppm). In
2012, carbon dioxide levels have climbed to 393 ppm, the highest they've
been in the past several million years. At the current rate of increase,
atmospheric carbon dioxide levels are on track to reach middle Miocene
levels by the end of this century.

High carbon dioxide levels during the middle Miocene epoch have been
documented in other studies through multiple lines of evidence, including
the number of microscopic pores on the surface of plant leaves and
geochemical evidence from soils and marine organisms. While none of these
‘proxies’ is as reliable as the bubbles of gas trapped in ice cores, they
are the best evidence available this far back in time. While scientists do
not yet know precisely why carbon dioxide was at these levels during the
middle Miocene, high carbon dioxide, together with the global warmth
documented from many parts of the world and now also from the Antarctic
region, appear to coincide during this period in Earth’s history.

This research was funded by the U.S. National Science Foundation with
additional support from NASA. The California Institute of Technology in
Pasadena manages JPL for NASA.

For more information about NASA programs and projects, visit:
http://www.nasa.gov/ .
Alan Buis 818-354-0474
Jet Propulsion Laboratory, Pasadena, Calif.
Alan.buis at jpl.nasa.gov

J. D. Harrington 202-358-5241
NASA Headquarters, Washington
j.d.harrington at nasa.gov

Robert Perkins 213-740-9226
University of Southern California, Los Angeles
perkinsr at usc.edu

Zac Lemoine 225-578-1399
Louisiana State University, Baton Rouge
jlemo26 at lsu.edu

2012-179
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Vision2020 Post: Ted Moffett
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