Ancient Climate Changes Suggest Earth On Global-Warming Fast-Track

http://www.sciencedaily.com/releases/2006/02/060220102603.htm
Source: University of California - Santa Cruz Posted: February 20, 2006

Studies Of Ancient Climates Suggest Earth Is Now On A Fast Track To
Global Warming

"The emissions that caused this past episode of global warming
probably lasted 10,000 years. By burning fossil fuels, we are likely
to emit the same amount over the next three centuries," said James
Zachos, professor of Earth sciences at the University of California,
Santa Cruz.

Zachos will present his findings this week at the annual meeting of
the American Association for the Advancement of Science (AAAS) in St.
Louis. He is a leading expert on the episode of global warming known
as the Paleocene-Eocene Thermal Maximum (PETM), when global
temperatures shot up by 5 degrees Celsius (9 degrees Fahrenheit).
This abrupt shift in the Earth's climate took place 55 million years
ago at the end of the Paleocene epoch as the result of a massive
release of carbon into the atmosphere in the form of two greenhouse
gases: methane and carbon dioxide.

Previous estimates put the amount of released carbon at 2 trillion
tons, but Zachos showed that more than twice that amount--about 4.5
trillion tons--entered the atmosphere over a period of 10,000 years
(Science, June 10, 2005). If present trends continue, this is the
same amount of carbon that industries and automobiles will emit
during the next 300 years, Zachos said.

Once the carbon is released into the atmosphere, it takes a long time
for natural mechanisms, such as ocean absorption and rock weathering,
to remove excess carbon from the air and store it in the soil and
marine sediments. Weathering of land rocks removes carbon dioxide
permanently from the air, but is a slow process requiring tens of
thousands of years. The ocean absorbs carbon dioxide much more
rapidly, but only to a point. The gas first dissolves in the thin
surface layer of the ocean, but this surface layer quickly becomes
saturated and its ability to absorb more carbon dioxide declines.

Only mixing with the deeper layers can help restore the ability of
the surface water to absorb additional carbon dioxide from the
atmosphere. But the natural processes that mix and circulate water
between the ocean surface and deeper ocean layers work very slowly. A
complete "mixing cycle" takes about 500 to 1,000 years, Zachos said.

The greenhouse emissions that triggered the PETM initially exceeded
the ocean's absorption capacity, allowing carbon to accumulate in the
atmosphere. Unfortunately, humans appear to be adding carbon dioxide
to the air at a much faster rate: about the same amount of carbon
(4.5 trillion tons), but within a few centuries instead of 10,000
years. What was emitted 55 million years ago over a period of about
20 ocean mixing cycles is now being emitted over a fraction of a
cycle.

"The rate at which the ocean is absorbing carbon will soon decrease,"
Zachos said.

Compounding this concern is the possibility that higher temperatures
could retard ocean mixing, further reducing the ocean's capacity to
absorb carbon dioxide. This could have the kind of "positive
feedback" effect that climate researchers worry about: reduced
absorption, leaving more carbon dioxide in the air, causing more
warming.

Higher ocean temperatures could also slowly release massive
quantities of methane that now lie frozen in marine deposits. A
greenhouse gas 20 times more potent than carbon dioxide, methane in
the atmosphere would accelerate global warming even further.

Such positive feedback or "threshold" effects probably drove global
warming during the PETM and a few other ancient climate extremes,
Zachos said, and they could happen again. It is possible that we
already are in the early stages of a similar climate shift, he said.

"Records of past climate change show that change starts slowly and
then accelerates," he said. "The system crosses some kind of
threshold."

Clues to what happened during the PETM lie buried deep inside the
sediment at the bottom of the sea, which Zachos and his colleagues
have probed during several cruises of the Ocean Drilling Program
(ODP). Composed mainly of clay and the carbonate shells of
microplankton, this sediment accumulates slowly, but steadily--up to
2 centimeters every millennium--and faithfully records changes in
ocean chemistry. The layer of sediment deposited during the PETM, now
buried hundreds of meters below the seafloor, tells a clear and
compelling story of sudden change and slow recovery, he said.

During the PETM, unknown factors released vast quantities of methane
that had been lying frozen in sediment deposits on the ocean floor.
After release, most of the methane reacted with dissolved oxygen to
form carbon dioxide, which made the seawater more acidic. Acidic
seawater corrodes the carbonate shells of microplankton, dissolving
them before they can reach the ocean floor and reducing the carbonate
content of marine sediment.

Zachos led an international team of scientists that analyzed sediment
cores recovered from several locations during an ODP cruise in the
southeastern Atlantic. Collected at depths ranging from 2.5 to 4.8
kilometers (1.6 to 3.0 miles), each sediment core bore a telltale
PETM imprint: a 10- to 30-centimeter layer of dark red carbonate-free
clay sandwiched between bright white carbonate-rich layers.

By relating the thickness of the clay layer to the rate of
accumulation of marine sediment, Zachos estimated that it took
100,000 years after the PETM for carbon dioxide levels in the air and
water to return to normal. This finding is consistent with what
geochemists have predicted using models of how the global carbon
cycle will respond to carbon dioxide emissions from the burning of
fossil fuels.

"We set out to test the hypotheses put forward by a small group of
geochemists who model the global carbon cycle, and our findings
support their predictions," Zachos said. "It will take tens of
thousands of years before atmospheric carbon dioxide comes down to
preindustrial levels. Even after humans stop burning fossil fuels,
the effects will be long lasting."
----

Source: Penn State

Posted: February 19, 2006

Phytoplankton Bounce Back From Abrupt Climate Change

The majority of tiny marine plants weathered the abrupt climate
changes that occurred in Earth's past and bounced back, according to
a Penn State geoscientist.

"Populations of plankton are pretty resilient," says Dr. Timothy J.
Bralower, head and professor of geoscience.

Bralower looked at cores of marine sediments related to thousands of
years of deposition, to locate populations of these plankton during
three periods of abrupt climate change. These abrupt changes were
caused either by Oceanic Anoxic Events during the middle Jurassic to
late Cretaceous when the oceans became uniformly depleted of oxygen
or by a warming event in the early Paleocene around 55 million years
ago.

Marine sediment cores contain calcareous plankton -- single-celled
organisms with a coating or shell of calcium carbonate -- as fossils.
These tiny photosynthesizing plants float in the ocean and move with
the currents. They are around 10 micrometers in size, about half the
width of a human hair. Anything bigger than phytoplankton eat them.
Eventually, their calcium carbonate shell falls to the ocean floor to
become part of the sediment.

The factors that were altered in the upper marine environment during
the abrupt climate change events included increases in temperature
and changes in thermal structure, changes in salinity and alkalinity,
and changes in nutrient patterns and trace elements.

"In every case, changes in surface habitats resulted in transient
plankton communities," Bralower told attendees at the 2006 annual
meeting of the American Association for the Advancement of Science.
"Although we have a poor understanding of ancient plankton ecology,
it appears that extinctions were selective and targeted more
specialized and often deeper-dwelling species."

For example, about 55 million years ago there was a warming event
that geologists call the Paleocene/Eocene thermal maximum. During
that time, there were mass extinctions of organisms living on the
ocean floor, but surface phytoplankton populations dipped and for the
most part came back. During this event one genus of phytoplankton -
Fasciculithus -- which had about five species went extinct.

"We do not have anything like Fasciculitus in the oceans today," says
Bralower. "But, these organisms were probably highly specialized and
existed in a very narrow ecological niche. The other thing is that,
as soon as some group disappears, another species comes in to occupy
that niche." About 120 million years ago, during an episode of oxygen
depletion another genus inhabiting surface waters -- Nannococus --
which also had about five species, went extinct. Otherwise only a few
species here and there were unable to survive these abrupt changes.
However, on the ocean floor during these same times, mass extinctions
occurred.

Other extinctions, such as that at the Cetaceous Tertiary boundary
(K/T) that caused the demise of the dinosaurs, are thought to be
caused by other than abrupt climate changes. The K/T event had mass
extinctions on land and in the upper portions of the oceans, but not
on the ocean floors.

During the abrupt climate changes that Bralower investigated, the
temperature of the oceans changed about 11 degrees Fahrenheit over
the course of 1,000 years.

"This rate of change in ocean temperature is probably slower than
what is happening today in the oceans," the Penn State researcher
adds. "We are not yet seeing the same effect in today's
phytoplankton."

Besides being a major food source, phytoplankton are also important
in the balance of carbon dioxide in the atmosphere as opposed to the
carbon that is sequestered in the ocean sediment.

Photosynthesizing organisms use carbon dioxide to create energy and
so remove carbon dioxide from the atmosphere. Some of the carbon that
phytoplankton take out of the air as carbon dioxide is used to make
their calcium carbonate coatings. Because these coatings eventually
make it into the sediment, they do not immediately return to the
atmosphere. It is not until chalk or limestone beds are exposed to
the elements that weathering returns the carbon to the atmosphere.

"Today, we are sort of in the middle of a mass experiment," says
Bralower. "With the oceans warming, we do not really know what the
end result will be, but we can look to the fossil record to see how
they were affected in the past. It appears that abrupt climate change
affects plankton with selectivity and most of the organisms bounce
right back after the change."
###

The National Science Foundation's Integrated Ocean Drilling Program
supported this research.