Abrupt Climate Change Revisited - Abrupt Climate Change: Mechanisms, Patterns, and Impacts - page 9

periods (MIS 2 3). However, as more new paleoclimatic

records emerge, such as palynological and paleolimnological
data from North America [Viau et al., 2006; Gajewski and
Viau, this volume] and tropical climate records reconstructed
from high-elevation ice cores [Thompson, this volume], it
appears that Holocene climate variability may have been
greater than has been assumed based on prior data sets.
In the early Holocene, large climatic fluctuations were
related to freshwater pulses from the disintegrating LIS into
the western North Atlantic [e.g., de Vernal et al., 2000].
Using palynological data (terrestrial palynomorphs and ma-
rine dinocysts) from the Newfoundland and northern Sco-
tian Shelf sediment cores, Levac et al. [this volume] assess
the impact of fresh water on the circulation over the Lab-
rador margin and related climate change during the

- conceptual framework for the 1500 year oscillation requires

more rigorous testing by more sophisticated, coupled ocean-
atmosphere models.
Gajewski and Viau [this volume] suggest that the Holo-
cene in North America can be divided into four general
periods with abrupt transitions at ~8, 6, and 3 ka, generally
consistent with the hypothesis of Ruzmaikin and Feynman
[this volume]. Gajewski and Viau further infer that the late
Holocene 5.2 ka, 4.2 ka, and Little Ice Age (circa 1300 1870

A.D.) cooling/drying events were part of a continual series of
millennial-scale climatic fluctuations.
Paliwal [this volume] documents abrupt climate events
during the Holocene from a series of lake records and
groundwater data from the western Indian peninsula. Some
of these abrupt climate events, in combination with neotec-
tonic activity, altered the drainage pattern that caused a
disappearance of the Vedic Sarasvati River and other
tributaries of the Indus River. Paliwal infers that the onset of
the arid climate in the Thar Desert around 3.5 ka may have
caused the decline of the Indus Valley Harappan and
Mohenjo-Daro civilizations. Interestingly, fossils of elephants
and bamboo curtains discovered in the Quaternary gypsum
deposits of Rajasthan suggest the existence of an even earlier
advanced civilization (>12.82 ka). This Vedic civilization,
which

-

nal
demise of the proglacial Lake Agassiz. The authors identify
a circa 8.7 ka detrital carbonate bed accompanied by two
meltwater pulses that lowered sea surface salinity (SSS) in
the coastal water of Newfoundland. In contrast, data from
the Scotian Shelf do not show changes in the SSS at this
time. The divergent impact of freshwater pulses at these
two sites is explained by the absence of a diluting effect of
the North Atlantic Current at the Newfoundland margin as
opposed to the warmer Scotian Shelf. Keigwin et al. [2005]
have documented a cooling event around 8.5 ka, likely
correlative to that identified by Levac et al., as far south
as Cape Hatteras, suggesting a large geographic impact of
the Lake Agassiz drainage. Surprisingly, such a pro-
nounced cooling event has not been found in the northern
Labrador Sea and Baf

flourished along the banks of the Sarasvati River, is
older than the Harappan and Mohenjo-Daro civilizations
(5 ka) of the Indus Valley [Weiss et al., 1993; Rashid et al.,
2011]. However, the lack of high-resolution radiocarbon dates
prevents correlation of these terrestrial records to Indian summer
monsoon records [Thompson et al., 1997; Rashid et al.,
2011], which is essential to evaluate the impact of climate
changes on the fate of these ancient civilizations.
From Sierra Nevada

n Bay, possibly because of a lack of
high-resolution records due to the stronger winnowing con-
ditions of the Labrador coastal current [Chapman, 2000;
Rashid et al., 2011].
Ruzmaikin and Feynman [this volume] investigate a qua-
siperiodic 1500 year climate oscillation during the Holocene
and its relation to the AMOC. They employed a simple
conceptual model to simulate the forcing mechanism for this
oscillation. The analysis of six paleoclimatic records from
the Atlantic Ocean and sunspot numbers using wavelet and
empirical mode decomposition methods allowed the authors
to overcome the leaks from one mode to another, a common
problem in conventional band-pass

s Coburn Lake, Wathen [this vol-
ume] reconstructed an 8500 year

'

fire history using charcoal
microparticles as a proxy. In addition to published data from
the Sierra Nevada and surroundings, the author correlates
his records with the long-distance

fire records from Lake
Francis of eastern Canada and soot records of Greenland ice
cores. Wathen suggests that severe res at Coburn Lake

occurred at the beginning of severe droughts, consistent
with the Greenland soot records. Furthermore, the 8.2 ka
and 5.2 ka climatic events identi

filtering. Ruzmaikin and
Feynman conclude that solar forcing does not drive the 1500
year climate cycle “ directly, ” contrary to the inference by
Bond et al. [2001]. Instead, they suggest a simple model of
excitation of this oscillation in a nonlinear dynamical system
with two equilibrium states. They reason that the transitions
between the two states are caused by the noise, ocean, and
solar variability, and the implication is that the beats between
the centennial ocean variability and ~90 year solar cycles
produce the 1500 year oscillation in the noisy system. This

ed in the Lake Coburn
microcharcoal record are inferred to be related to large-scale
shifts in the major precipitation belts: the Intertropical Con-
vergence Zone (ITCZ), the Subtropical Desert Zone, and
the Polar Front. Wathen hypothesizes that most instances of
severe fires and erosion at Coburn Lake occurred in re-
sponse to stresses on vegetation that were adapted to colder
and wetter conditions in response to abrupt climate events.
A similar mechanism is suggested for the Coburn Lake fire
history of the last 1800 years, although some major climatic