High-fidelity record of Earth's climate history puts current changes in context

Date:
September 10, 2020
Source:
University of California - Santa Cruz
Summary:
Scientists have compiled a continuous, high-fidelity record of
variations in Earth's climate extending 66 million years into
the past. The record reveals four distinctive climate states,
which the researchers dubbed Hothouse, Warmhouse, Coolhouse,
and Icehouse. These major climate states persisted for millions
and sometimes tens of millions of years, and within each one the
climate shows rhythmic variations corresponding to changes in
Earth's orbit around the sun.



FULL STORY ========================================================================== [View of Planet Earth | Credit: (c) tsuneomp / stock.adobe.com] View of
Planet Earth (stock image).

Credit: (c) tsuneomp / stock.adobe.com [View of Planet Earth | Credit:
(c) tsuneomp / stock.adobe.com] View of Planet Earth (stock image).

Credit: (c) tsuneomp / stock.adobe.com Close For the first time, climate scientists have compiled a continuous, high- fidelity record of variations
in Earth's climate extending 66 million years into the past. The record
reveals four distinctive climate states, which the researchers dubbed
Hothouse, Warmhouse, Coolhouse, and Icehouse.


========================================================================== These major climate states persisted for millions and sometimes tens
of millions of years, and within each one the climate shows rhythmic
variations corresponding to changes in Earth's orbit around the sun. But
each climate state has a distinctive response to orbital variations,
which drive relatively small changes in global temperatures compared
with the dramatic shifts between different climate states.

The new findings, published September 10 in Science, are the result of
decades of work and a large international collaboration. The challenge was
to determine past climate variations on a time scale fine enough to see
the variability attributable to orbital variations (in the eccentricity
of Earth's orbit around the sun and the precession and tilt of its
rotational axis).

"We've known for a long time that the glacial-interglacial cycles are
paced by changes in Earth's orbit, which alter the amount of solar
energy reaching Earth's surface, and astronomers have been computing
these orbital variations back in time," explained coauthor James Zachos, distinguished professor of Earth and planetary sciences and Ida Benson
Lynn Professor of Ocean Health at UC Santa Cruz.

"As we reconstructed past climates, we could see long-term coarse
changes quite well. We also knew there should be finer-scale rhythmic variability due to orbital variations, but for a long time it was
considered impossible to recover that signal," Zachos said. "Now that
we have succeeded in capturing the natural climate variability, we
can see that the projected anthropogenic warming will be much greater
than that." For the past 3 million years, Earth's climate has been in
an Icehouse state characterized by alternating glacial and interglacial periods. Modern humans evolved during this time, but greenhouse gas
emissions and other human activities are now driving the planet toward
the Warmhouse and Hothouse climate states not seen since the Eocene epoch, which ended about 34 million years ago.

During the early Eocene, there were no polar ice caps, and average global temperatures were 9 to 14 degrees Celsius higher than today.



==========================================================================
"The IPCC projections for 2300 in the 'business-as-usual' scenario will potentially bring global temperature to a level the planet has not seen
in 50 million years," Zachos said.

Critical to compiling the new climate record was getting high-quality
sediment cores from deep ocean basins through the international
Ocean Drilling Program (ODP, later the Integrated Ocean Drilling
Program, IODP, succeeded in 2013 by the International Ocean Discovery
Program). Signatures of past climates are recorded in the shells of
microscopic plankton (called foraminifera) preserved in the seafloor
sediments. After analyzing the sediment cores, researchers then had
to develop an "astrochronology" by matching the climate variations
recorded in sediment layers with variations in Earth's orbit (known as Milankovitch cycles).

"The community figured out how to extend this strategy to older time
intervals in the mid-1990s," said Zachos, who led a study published in
2001 in Science that showed the climate response to orbital variations
for a 5-million-year period covering the transition from the Oligocene
epoch to the Miocene, about 25 million years ago.

"That changed everything, because if we could do that, we knew we
could go all the way back to maybe 66 million years ago and put these
transient events and major transitions in Earth's climate in the context
of orbital-scale variations," he said.

Zachos has collaborated for years with lead author Thomas Westerhold at
the University of Bremen Center for Marine Environmental Sciences (MARUM)
in Germany, which houses a vast repository of sediment cores. The Bremen
lab along with Zachos's group at UCSC generated much of the new data
for the older part of the record.



========================================================================== Westerhold oversaw a critical step, splicing together overlapping segments
of the climate record obtained from sediment cores from different parts
of the world. "It's a tedious process to assemble this long megasplice
of climate records, and we also wanted to replicate the records with
separate sediment cores to verify the signals, so this was a big effort
of the international community working together," Zachos said.

Now that they have compiled a continuous, astronomically dated climate
record of the past 66 million years, the researchers can see that the
climate's response to orbital variations depends on factors such as
greenhouse gas levels and the extent of polar ice sheets.

"In an extreme greenhouse world with no ice, there won't be any feedbacks involving the ice sheets, and that changes the dynamics of the climate,"
Zachos explained.

Most of the major climate transitions in the past 66 million years
have been associated with changes in greenhouse gas levels. Zachos has
done extensive research on the Paleocene-Eocene Thermal Maximum (PETM),
for example, showing that this episode of rapid global warming, which
drove the climate into a Hothouse state, was associated with a massive
release of carbon into the atmosphere. Similarly, in the late Eocene,
as atmospheric carbon dioxide levels were dropping, ice sheets began to
form in Antarctica and the climate transitioned to a Coolhouse state.

"The climate can become unstable when it's nearing one of these
transitions, and we see more deterministic responses to orbital forcing,
so that's something we would like to better understand," Zachos said.

The new climate record provides a valuable framework for many areas of research, he added. It is not only useful for testing climate models,
but also for geophysicists studying different aspects of Earth dynamics
and paleontologists studying how changing environments drive the evolution
of species.

Coauthors Steven Bohaty, now at the University of Southampton, and Kate Littler, now at the University of Exeter, both worked with Zachos at UC
Santa Cruz. The paper's coauthors also include researchers at more than
a dozen institutions around the world. This work was funded by the German Research Foundation (DFG), Natural Environmental Research Council (NERC), European Union's Horizon 2020 program, National Science Foundation of
China, Netherlands Earth System Science Centre, and the U.S. National
Science Foundation.


========================================================================== Story Source: Materials provided by
University_of_California_-_Santa_Cruz. Original written by Tim
Stephens. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* Graphic_showing_past_and_future_trends_in_global_mean_temperature
spanning_the_last_66_million_years,_showing_four_distinctive_climate
states ========================================================================== Journal Reference:
1. Thomas Westerhold, Norbert Marwan, Anna Joy Drury, Diederik
Liebrand,
Claudia Agnini, Eleni Anagnostou, James S. K. Barnet, Steven
M. Bohaty, David De Vleeschouwer, Fabio Florindo, Thomas Frederichs,
David A.

Hodell, Ann E. Holbourn, Dick Kroon, Vittoria Lauretano,
Kate Littler, Lucas J. Lourens, Mitchell Lyle, Heiko Pa"like,
Ursula Ro"hl, Jun Tian, Roy H. Wilkens, Paul A. Wilson, James
C. Zachos. An astronomically dated record of Earth's climate and
its predictability over the last 66 million years. Science, 2020
DOI: 10.1126/science.aba6853 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200910150313.htm

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