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?
INTRODUCTION
Dr. Herman Zimmerman, Director of the Paleoclimate Program, National Science
Foundation, Arlington, VA
SPEAKERS
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
OVERVIEW
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. The
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 future.
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 abrupt 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 Little
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 (3-8
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.
Biographies:
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.
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.