Stalagmites like these from northern Borneo are the ice cores of the tropics. Credit: Adkins/Caltech

There
is an old trick for remembering the difference between stalactites and
stalagmites in a cave: Stalactites hold tight to the ceiling while
stalagmites might one day grow to reach the ceiling. Now, it seems,
stalagmites might also fill a hole in our understanding of Earth’s
climate system and how that system is likely to respond to the rapid
increase in atmospheric carbon dioxide since preindustrial times.

   

Many
existing historical climate records are biased to the high
latitudes—coming from polar ice cores and North Atlantic deep ocean
sediments. Yet a main driver of climate variability today is El Niño,
which is a completely tropical phenomenon. All of this begs the
question: How do we study such tropical climate influences? The answer:
stalagmites.

   

“Stalagmites
are the ice cores of the tropics,” says Jess Adkins, professor of
geochemistry and global environmental science at the California
Institute of Technology (Caltech). He and geochemist Kim Cobb of the
Georgia Institute of Technology led a team that collected samples from
stalagmites in caves in northern Borneo and measured their levels of
oxygen isotopes to reconstruct a history of the tropical West Pacific’s
climate over four glacial cycles during the late Pleistocene era (from
570,000 to 210,000 years ago).

   

The results appear in the May 3 issue of Science
Express. The lead author of the paper, Nele Meckler, completed most of
the work as a postdoctoral scholar at Caltech and is now at the
Geological Institute of ETH Zürich.

   

Throughout
Earth’s history, global climate has shifted between periods of glacial
cooling that led to ice ages, and interglacial periods of relative
warmth, such as the present. Past studies from high latitudes have
indicated that about 430,000 years ago—at a point known as the
Mid-Brunhes Event (MBE)—peak temperatures and levels of atmospheric
carbon dioxide in interglacial cycles were suddenly bumped up by about a
third. But no one has known whether this was also the case closer to
the equator.

By
studying the records from tropical stalagmites, Adkins and his team
found no evidence of such a bump. Instead, precipitation levels remained
the same across the glacial cycles, indicating that the tropics did not
experience a major shift in peak interglacial conditions following the
MBE. “The stalagmite records have glacial cycles in them, but the warm
times—the interglacials—don’t change in the same way as they do at high
latitudes,” Adkins says. “We don’t know what that tells us yet, but this
is the first time the difference has been recorded.”

   

At
the same time, some changes did appear in the climate records from both
the high latitudes and the tropics. The researchers found that extreme
drying in the tropics coincided with abrupt climate changes in the North
Atlantic, at the tail end of glacial periods. It is thought that these
rapid climate changes, known as Heinrich events, are triggered by large
ice sheets suddenly plunging into the ocean.

   

“In
the tropics, we see these events as very sharp periods of drying in the
stalagmite record,” Adkins says. “We think that these droughts indicate
that the tropics experienced a more El Niño–like climate at those
times, causing them to dry out.” During El Niño events, warm waters from
the tropics, near Borneo, shift toward the center of the Pacific Ocean,
often delivering heavier rainfall than usual to the western United
States while leaving Indonesia and its neighbors extremely dry and prone
to forest fires.

   

The
fact that the tropics responded to Heinrich events, but not to the
shift that affected the high latitudes following the MBE, suggests that
the climate system has two modes of responding to significant changes.
“It makes you wonder if maybe the climate system cares about what sort
of hammer you hit it with,” Adkins says. “If you nudge the system
consistently over long timescales, the tropics seem to be able to
continue independently of the high latitudes. But if you suddenly whack
the climate system with a big hammer, the impact spreads out and shows
up in the tropics.”

   

This
work raises questions about the future in light of recent increases in
atmospheric carbon dioxide: Is this increase more like a constant push?
Or is it a whack with a big hammer? A case could be made for either one
of these scenarios, says Adkins, but he adds that it would be easiest to
argue that the forcing is more like a sudden whack, since the amount of
carbon dioxide in the atmosphere has increased at such an unprecedented
rate.

In
addition to Adkins, Cobb, and Meckler, other coauthors on the paper,
“Interglacial hydroclimate in the tropical West Pacific through the late
Pleistocene,” are Matthew Clarkson of the University of Edinburgh and
Harald Sodemann of ETH Zürich. Cobb is also a former postdoctoral
scholar in Adkins’s group and has been collaborating on this project
since her time at Caltech. The work was supported by the National
Science Foundation, the Swiss National Science Foundation, the German
Research Foundation, and by an Edinburgh University Principal’s Career
Development PhD Scholarship.

Interglacial Hydroclimate in the Tropical West Pacific Through the Late Pleistocene

Source: Caltech

A slice through a stalagmite from a cave in northern Borneo reveals the gradual growth of the calcite structure. By measuring the ratio of oxygen isotopes in such samples, Adkins and his colleagues were able to reconstruct a history of the climate in the tropics throughout the late Pleistocene era. Credit: Adkins/Caltech