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Feb. 23rd U.S. Global Change Seminar: "Abrupt Climate Changes Revisited: How Serious and How Likely?"

                       U.S. Global Change Research Program Seminar Series


What constitutes an abrupt climate change?  What does the paleo-climate
record say about how rapidly climate can change?  How much can climate
change during an abrupt climate shift?  Were there ecological or social
consequences associated with these abrupt changes?  What causes an abrupt
climate shift and how do changes evolve once set in motion?  Are there
climate thresholds which, when crossed, lead to rapid and dramatic,
non-linear climate shifts?  Are these climate threshholds known or
knowable?  What is the probability and likely outcome of provoking a rapid
climate shift as a result of a global warming due to the present and
projected build-up in the concentration of human-derived greenhouse gases?

                                               Public Invited

                           Monday, February 23, 1998, 3:15-4:45 PM
       New Location: Dirksen Senate Office Bldg., Room 562, Washington, DC
                                           Reception Following


Dr. Herman Zimmerman, Director of the Paleoclimate Program, National
Science Foundation, Arlington, VA


Dr. Richard B. Alley, Professor, Earth System Science Center and Department
of Geosciences,
Pennsylvania State University, University Park, PA

Dr. Peter B. deMenocal, Lamont-Doherty Earth Observatory, Columbia
University, Palisades, NY


Widespread climate changes in the distant past were larger and more rapid
than those experienced during more recent historical times.  For example,
the cooling of the climate leading into the last "ice age", the peak of
which occurred roughly 21,000 years ago, and the subsequent climate
transition to a warmer, more modern world were punctuated by abrupt climate
changes that were one-third to one-half as large as the change from an "ice
age" to a warm climate (i.e., the roughly 11-13 degrees F [6-7 degrees C]
transition from an "ice-age" to a warm climate, globally).  Paleoclimate
records further indicate that during these abrupt shifts many aspects of the
climate in many regions changed precipitously in the timespan of a few years
to as little as a single year.

Moreover, the current warm period since the peak of the last ice age
(21,000 years ago) was previously thought to be very stable with none of
the large climate shifts that so characterized "ice ages".  Contrary to
this once widely held notion, new evidence from deep-sea sediments and
ice-cores shows that this warm period was interrupted by a series of abrupt
cooling events, each lasting several hundred years.  One of the most
prominent of these events occurred roughly 12,800 years ago, after
Greenland had warmed to near present conditions.  Another smaller but
significant abrupt cooling event occurred roughly 8,200 years ago when
temperatures in Greenland were slightly above present-day temperatures.
These and other recent, abrupt cooling events have been detected from
Scandinavia to Africa, some of which occurred within a human lifetime.  One
such notable event 4,200 years ago (2,200 BC), is shown to be synchronous
with the collapse of the world's first human empire in Mesopotamia.

Thus, the paleoclimate record suggests that the climate system can respond
to various climate forcings in a non-linear manner.  In fact, these results
document significant and consequential climate shifts during the time of
human civilization, and highlight the characteristically abrupt aspects of
climate change, and their potential consequences.  This raises the
possibility that if humans alter the Earth's atmosphere rapidly enough,
resulting in a global warming, an abrupt climate shift might be induced,
with significant social and ecological consequences.

                         Abrupt Climate Changes and the "Younger Dryas" Event

Approximately 12,800 years ago, as the climate was warming following the
Earth's last glacial maximum ("ice age"), an abrupt transition to cold
conditions occurred, during which the surface temperature of the Northern
Hemisphere dropped precipitously (nearly 27 degrees F [15 degrees C] in
Greenland, for example) in a series of abrupt, decadal-scale jumps, some of
which involved temperature changes on the order of 5 degrees F (3 degrees C).
This abrupt climate cooling is known as the "Younger Dryas" event.  Once the
abrupt transition to a colder climate had occurred, the Northern Hemisphere,
especially Europe and Greenland, experienced considerably colder conditions
lasting about 1,300 years.  Other parts of the world were affected as well.
termination of this cold event around 11,500 years ago occurred as an even
more abrupt warming, most of which took place in a single, 5-year period.
The entire transition to a warmer, more modern climate took no more than 40
years.  During this transition snow accumulation in Greenland doubled in a
single three-year period, with 90% of that increase occurring in a single
year.  This abrupt transition to a warmer world led to a three-fold drop in
wind-blown sea salt, a seven-fold drop in wind-blown dust, and a climate
warming of 9-18 degrees F (5-10 degrees C) in Greenland, all in less than
a decade.

Within thirty years following this transition to a warmer climate,
atmospheric methane (another greenhouse gas) levels increased, as a result
of the creation of more wetlands globally.  Conversely, the climate cooling
associated with the onset of the "Younger Dryas" event resulted in a loss
of wetlands worldwide, and a drop in the concentration of atmospheric
methane.  Numerous climate records from other parts of the world confirm
these abrupt climate events recorded in the Greenland ice-cores, and extend
the signature of these events to other regions of the globe.

It appears that these abrupt climate shifts were caused and/or amplified by
fundamental changes in the mode of operation of the coupled Earth system -
the interactions among the atmosphere, ocean, ice, and life.  Changes in
the rate at which fresh water is delivered to the North Atlantic Ocean may
have played an especially important role in bringing about the changes.
Warm, salty, surface ocean currents presently moderate the European climate
by transporting heat from the tropics northward.  These warm surface
currents can be slowed or stopped if their salt content (density) becomes
sufficiently diluted (and less dense) because of excessive rain, the
melting of snow and ice, or large changes in river runoff into this region.
This appears to have been the mechanism which triggered the "Younger
Dryas" cooling event.

According to the 1995 IPCC (Intergovernmental Panel on Climate Change)
report, the human-induced warming resulting from the continued build-up of
greenhouse gases is projected to result in an increase in the melting of
glaciers and an increase in precipitation in the North Atlantic basin.
Just as
paleo-climate records suggest that sufficiently rapid increases in
precipitation or meltwater may trigger an abrupt reorganization of the ocean
circulation, the question arises about whether this might happen in the

Although climate models are not yet able to provide reliable estimates of
either the probability or the impact of such abrupt climate events, they do
provide some confirmation that similar changes could occur in the ocean
circulation, suggesting that the probability of abrupt climate changes in the
future, is not zero.  Proxy records of climate change also show that the
climate changes of the past altered ecosystems substantially, and that
considerably smaller and more recent, abrupt climate changes have
significantly affected human societies as well.

                            Abrupt Climate Shifts and Human Civilization

Deep ocean sediments are used to estimate past changes in ocean temperature
and circulation and climate changes on land based on analysis of their two
main components: Carbonate fossils (shells of organisms that once lived at
the sea surface) and mineral grains which come from land. The sediments
typically accumulate at rates of 2-8 inches each millennium (1,000 years).
In the North Atlantic, sediments accumulated since the end of the last ice
age nearly 12,000 years ago, show regular increases in the amount of coarse
sediment grains deposited from icebergs melting in the now open ocean,
indicating a series of 2-4 degrees F (1-2 degrees C) cooling events recurring
every 1500-years or so.  The most recent of these cooling events was the
Ice Age between 1500-1850 AD when European rivers and ports were choked
with ice, and glaciers overran alpine villages.

These same cooling events are detected in sediments accumulating off Africa
but the cooling events appear to be larger, ranging between 5-15 degrees F
degrees C).  The West African sediments additionally record the "African Humid
Period", an interval between 16,000 and 6,000 years ago when Africa was
much wetter due to a strengthening of the African monsoon by changes in
summer radiation resulting from long-term variations in the Earth's orbit
around the sun.  During this period the Saharan desert was dotted with
numerous lakes containing typical African lake crocodile and hippopotamus
fauna.  A curious discovery from the marine sediments is that the
transitions into and out of this wet period occurred within decades, not
millennia as previously thought.  While we understand how and why Africa
was wetter during this period we do not understand why the transitions are
so abrupt.  This adds to mounting evidence that Earth's climate seems to
reach certain thresholds, then switches abruptly (within a lifetime) from
one operating mode to another.

Historical social consequences of these abrupt climate changes can also be
assessed from the archeological record.  Archeologists had long known of a
large social disruption in Mesopotamia approximately 2,200 BC (4,200 years
ago) when the first known empire led by Sargon I of Akkad abruptly
collapsed and splintered after reigning from Turkey to the Persian Gulf for
several hundred years.  Noting that this event was contemporaneous with one
of the sharp cooling events detected in the North Atlantic and off Africa,
a second sediment core from the Persian Gulf was analyzed for evidence of
related changes in Mesopotamian climate during this time.  Analysis of this
evidence indicates that the supply of dust from the Mesopotamian region to
the adjacent oceans, at roughly 2,200 BC (4,200 years ago), was five times
the amount of dust supplied to the ocean in more modern or recent time.
Three hundred years later, around 1,900 BC (3,900 years ago), the supply of
dust that was being delivered to the oceans abruptly decreased, returning
to modern concentrations.  Geochemical analyses of a thin volcanic ash
layer found at both the archeological sites, and in the deep-sea sediment
core, indicate that this abrupt drying event coincided in time with the
collapse of certain historic civilizations.

While it has long been held that abrupt climate changes were limited to
glacial climates of the distant past, these and other results now document
the occurrence of abrupt shifts in climate during the present, modern warm
period, the interval encompassing the emergence of agriculture, the growth
and collapse of civilizations, and the current exponential expansion of
human population.  These results therefore, may have implications regarding
any human-induced, future climate warming.


Dr. Richard B. Alley is a Professor of Geosciences and an Associate at the
Earth System Science Center at Pennsylvania State University, University
Park, PA, where he teaches and conducts research on the paleo-climatic
records, on the dynamic behavior and sedimentary deposits of large ice
sheets, as a means of understanding the climate system and its history and
the potential for future changes in climate and sea.  Dr. Alley has spent
three field seasons in Antarctica and five in Greenland.  He has been
awarded a Packard Fellowship, a Presidential Young Investigator award, the
Horton Award of the American Geophysical Union Hydrology Section, and the
Wilson Teaching Award of the College of Earth and Mineral Sciences at
Pennsylvania State University.  He also serves, or has served, on many
advisory panels and steering committees, including the Polar Research Board
of the National Research Council, the Antarctic External Review Panel (the
"Augustine Commission"), and the board of directors of the Arctic
Consortium of the United States.

Dr. Alley received his Ph.D. in Geology, with a minor in Materials Science,
from the University of Wisconsin-Madison in 1987, and earned an MS degree
(1983) and BS degree (1980) in Geology from Ohio State University in
Columbus, OH.

Dr. Peter B. deMenocal is an Associate Research Scientist at the
Lamont-Doherty Earth Observatory of Columbia University.  Dr.  deMenocal's
research focuses on analyzing the geochemistry and composition of marine
sediments in order to reconstruct past changes in ocean circulation and
terrestrial climate, with a primary focus on the signatures and causes of
tropical-extratropical climate linkages over various timescales.  His most
current research focuses on African climate change and its link to early
human evolution over the last several million years, signatures of ocean
and climate variability over the last 12,000 years of the present warm
climate period, and paleoclimate applications of climate model simulations.
Dr. deMenocal has a Ph.D. (1991) and a Master of Philosophy degree (1989)
in Geology from Columbia University, NY, an M.S. degree (1987) in
Oceanography from the University of Rhode Island, and a B.S. degree (1982)
in Geology from St. Lawrence University, NY.

                 The Next Seminar is scheduled for Monday, March 23, 1998

    Planned Topic: The Health and Well-Being of the Amazon and Its
  The Intersection of Resource Use, Resource Management, Economics, Human
Practices, and El Nino.

For more information please contact:

Anthony D. Socci, Ph.D., U.S. Global Change Research Program Office, 400
Virginia Ave. SW, Suite 750, Washington, DC 20024; Telephone: (202)
314-2235; Fax: (202) 488-8681 E-Mail: TSOCCI@USGCRP.GOV.

Additional information on the U.S. Global Change Research Program (USGCRP)
and this Seminar Series is available on the USGCRP Home Page at:
http://www.usgcrp.gov. Normally these seminars are held on the second
Monday of each month.