New analyses of ancient ice from
Antarctica and the air contained inside it are extending the history of Earth's
climate records and expanding researchers' understanding of how the planet has
changed over the last 3 million years.
The findings, published in two
papers in the journal Nature, show the long-term cooling of Earth's
climate during this period has been accompanied by only a modest decline in
heat-trapping greenhouse gases in the atmosphere.
Scientists have known that Earth
was much warmer and the sea level much higher as recently as 3 million years
ago, since the first discoveries more than 100 years ago, of temperate and
subtropical forest fossils in Alaska and Greenland and ancient stranded beaches
stretching from Georgia to Virginia.
But the cause of that period of
global warmth and subsequent cooling has remained a mystery, in large part due
to the difficulties in accurately reconstructing global temperatures and the
levels of heat-trapping atmospheric greenhouse gases.
The new research was led by
scientists with the National Science Foundation Center for Oldest Ice
Exploration, a nationwide collaboration exploring Antarctica for Earth's oldest
ice, headquartered at Oregon State University.
The two studies, led by Julia
Marks-Peterson, a doctoral student at OSU, and Sarah Shackleton, then a
postdoctoral fellow at Princeton University and now a professor at Woods Hole
Oceanographic Institution, use recently discovered archives of multi-million-year-old
ice from Allan Hills, at the margin of the East Antarctic ice sheet.
Ancient Antarctic ice as a time capsule
Allan Hills is an unusual setting
where ice from the Antarctic interior is stranded in mountain ranges at the
edge of the continent. Flow patterns deform the originally horizontal layers,
making it difficult to find continuous records of climate. Instead, the data
provide "snapshots" that indicate average environmental conditions at
discrete time periods.
"Those snapshots extend
climate records from ice much further than previously possible," said
COLDEX Director Ed Brook, a paleoclimatologist in OSU's College of Earth,
Ocean, and Atmospheric Sciences.
"These longer records are also
now raising new questions about Earth's climate evolution and how far back in
time we might be able to go with ice core data."
Tracking 3 million years of cooling
Using precise measurements of the
ratio of different noble gases in air trapped in the ice, which reflect ocean
temperature changes, Shackleton and
colleagues showed that the average temperature of the ocean has declined by 2 to
2.5 degrees Celsius over the past 3 million years.
Previous studies have extensively
examined changes in ocean surface temperatures, which also show cooling over
this period, but the new work shows that the timing of cooling is different
between the ocean surface and the ocean depths.
"The noble gases in ice
provide a unique way to look at ocean temperature change," Shackleton
said. "Other methods can give you information about ocean temperature at a
single site, but this gives a more global view."
For example, a large fraction of
the mean ocean temperature cooling happened early, starting 3 million years ago
and continuing for about a million years, during the time that ice sheets began
to form in the Northern Hemisphere.
In contrast, surface temperatures
cooled gradually until about 1 million years ago. In the paper, Shackleton and
her co-authors suggest that these discrepancies involve changes in how heat is
transferred between the surface and deep ocean.
New greenhouse gas records from old ice
Using the same ice core
samples, Marks-Peterson and her
co-authors identified the first direct records of the levels of two of the most
important atmospheric greenhouse gases, carbon dioxide and methane, over the last 3 million years.
The data show that long-term
average atmospheric carbon dioxide levels have likely remained below 300 parts
per million over this time; measured carbon dioxide levels were 250 parts per
million 2.7 million years ago and declined modestly by about 20 parts per
million until 1 million years ago. Long-term average levels of atmospheric
methane remained unchanged at 500 parts per billion.
Some previous work using the
chemistry of ancient sediments has suggested higher values of carbon dioxide
than the new data indicate, but not all such studies agree, underscoring the
need for ice core data going back as far as possible, the researchers noted.
Modern levels of carbon dioxide and
methane have increased dramatically in the last two centuries, with carbon
dioxide averaging 425 parts per million in 2025 and methane averaging 1,935
parts per billion in 2025, according to the National Oceanic and Atmospheric
Administration.
What the findings mean for climate
The implications of the results are
that the cooling of the last 3 million years probably involves, in addition to
the key role of heat-trapping greenhouse gases, important contributions from
other components of the climate system such as Earth's reflectivity, variations
in vegetation and/or ice cover and ocean circulation.
"Our hope is that this work
will refine our view of past warmer climates and sharpen our understanding of
how different elements of the Earth system interact," said Marks-Peterson.
The work has led to new research
questions, many of which are currently being investigated by others in NSF
COLDEX, Brook said. COLDEX researchers recently discovered ice as old as 6
million years at the bottom of one of their cores and are currently developing new
data from these older samples.
Recently completed drilling of new ice cores should access additional old ice. Researchers are also investigating methods to test carbon dioxide reconstruction, studying other gases in the ice cores and developing a deeper understanding of the conditions that lead to preservation of very old ice, which should help identify new targets for drilling.
Source: New ice core studies expand histories of greenhouse gases and ocean temperature to 3 million years
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