Geoscience Reference
In-Depth Information
Marine and terrestrial paleoclimate records from the
Southern Hemisphere (SH) are sparse and lack sufficient
temporal resolution to characterize the timescales relevant
for high-frequency climate variability. Although the evi-
dence for abrupt climate change in the SH is not clear, a
one-to-one correlation has been established between the new
Eastern Droning Maud Land (EDML) Antarctic ice core
record and that from Greenland [EPICA Community Mem-
bers et al., 2006]. The EDML ice core was recovered from a
location facing the South Atlantic and is assumed to record
climate changes in the Atlantic. Paleoclimatic records from
the northern tropics and subtropics mainly show changes
concordant with those in the North Atlantic, while asynchro-
nous or even anticorrelated events are exhibited in records
from the southern tropics and high latitudes of the SH. For
example, the Indian and East Asian monsoon histories seem
to correlate with the North Atlantic climate, whereas South
American monsoon records are anticorrelated with the
Greenland records [Wang et al., 2006]. Furthermore, paleo-
climatic proxy records from the equatorial Pacific are char-
acterized by a complex pattern of abrupt climate change that
borrows elements from both the Northern Hemisphere and
Southern Hemisphere end members, suggesting that the
tropical Paci
1. Do paleoproxies suggest a one-to-one proxy-based re-
lationship between circum - North Atlantic freshwater pulses
and the strength of the MOC, as well as the determination of
meltwater sources?
2. Are kinematic and nutrient proxies for the strength of
the MOC (Pa/Th, Cd/Ca, and
13 C) congruent across abrupt
climate changes that occurred during the Younger Dryas
(YD) or Heinrich events? How do these proxies differ be-
tween periods of stable and transient climate?
3. Do current general circulation models capture abrupt
strengthening and gradual weakening of thermohaline circu-
lation, consistent with the rapid warming and gradual cooling
of D-O events? If not, what other factors must be considered?
4. Is there robust evidence for sea ice in the North Atlantic
during the last glacial cycle? How much has sea ice extent
δ
uctua-
tions influenced the surface salinity and thus water mass
strati
fluctuated on millennial timescales? How have the
cation?
5. With the exception of the tropical Atlantic, most tropical
paleorecords show a clear lack of D-O cooling events. Does
this indicate that various parts of the tropics respond differ-
ently to the North Atlantic freshwater forcing because of
local hydrological and temperature variability?
6. Given the dramatic changes in Arctic sea ice and circu-
lation, how did the Arctic freshwater budget affect the MOC
in the North Atlantic?
7. Why do Antarctic temperatures show more gradual and
less pronounced warmings and coolings compared to the
D-O events in Greenland? Does this suggest that the deep
ocean circulation is modulating the abrupt climate change?
8. In light of the current concern about instabilities of the
West Antarctic and Greenland ice sheets, how can paleocea-
nographic records be used to decipher past ice sheet
dynamics?
9. What is the link between sea ice extent and ice sheet
dynamics? How does ocean heat transport in
cant role in medi-
ating climate teleconnections between the hemispheres
[Clement et al., 2004; Li et al., 2010].
The Chapman Conference on Abrupt Climate Change
(ACC) held at the Ohio State University in June 2009
brought together a diverse group of researchers dealing with
various paleoclimatic proxy records (such as ice cores, cor-
als, marine sediments, lakes, and speleothems) and coupled
ocean-atmosphere climate models to discuss advances in
understanding the ACC history and mechanisms. Special
attention was given to the three most commonly invoked
ACC mechanisms: (1) freshwater forcing, in which meltwa-
ter from the circum
c may have played a signi
North Atlantic ice sheets may have
disrupted the meridional overturning circulation (MOC) by
preventing or slowing down the formation of NADW in the
Labrador and Nordic Seas; (2) changes in sea ice extent,
which affect the ocean-atmosphere heat exchange, moisture
supply, and salt content; and (3) tropical forcing that calls for
a combination of Earth
-
uence the ice
sheet margin? Might coastal ice shelves be slaves to ocean
currents?
10. Was there a relationship between the demise of past
civilizations and climatic deterioration? What are the climate
tipping points that have driven past civilizations to collapse?
One of the important points raised at the conference is that
a close examination of paleoclimatic data and modeling
results does not show adequate support for many of the
widely accepted explanations for abrupt climate change. For
example, it is almost taken for granted that fresh water
released from circum - North Atlantic ice sheets during Hein-
rich events perturbed the Atlantic meridional overturning
circulation (AMOC), which caused abrupt changes recorded
around the globe. However, with the exception of the Youn-
ger Dryas, there is no paleoproxy evidence from deep waters
-
Southern Oscillation, and sea surface temperature condi-
tions. Several contributions dealing with various aspects of
these topics are presented in this volume.
'
s orbital con
guration, El Ni
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o
2. KEY QUESTIONS FOR THE ACC CONFERENCE
Without providing an exhaustive list of topics raised at the
conference, we list a number of pressing scientific questions
that were discussed:
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