Geoscience Reference
In-Depth Information
settings, such as the position of the ITCZ, were different in
the late Holocene.
Accumulation of past climate records over the last decade
shows that during the past 5000 years when climatic condi-
tions were similar to recent (prior to the rise in anthropogenic
greenhouse gases), Earth experienced at least two abrupt
global-scale climate events (5.2 ka and 4.2 ka). Thompson
[this volume] provides a detailed synthesis of these events
using ice core records from the world
around the 4.2 ka event. Like the 5.2 ka event, it is also
believed to have lasted at least a few centuries. If such a rapid
and sustained drought as during the 4.2 ka event were to
occur today, with Earth
s population rapidly approaching
7 billion, the impact would be very troubling.
'
4. RECOMMENDATIONS
s highest mountains
combined with other published paleoclimate histories. These
events, which persisted for a few centuries, were related to
profound changes in the hydrological cycle in the tropics and
midlatitudes [Magny and Haas, 2004] and appear to corre-
late with the decline of ancient civilizations [Weiss et al.,
1993; MacDonald, 2011; Rashid et al., 2011; Thompson, this
volume]. However, the forcing mechanisms for these abrupt
changes remain elusive thus far due in large measure to
insuf
'
Participants recommended several major areas in which
improved approaches to understanding abrupt climate
change are needed. These include the following: (1) collect-
ing more high-accumulation-rate and high-resolution data
and improving coordination of paleoclimate proxy and
modeling approaches; (2) concentrating on a few key time
intervals (such as 5.2 ka and 4.2 ka events) but using
multiple proxies; (3) developing novel paleoclimatic proxies
such as clumped isotopes, a promising tool for an indepen-
dent paleotemperature record, and biomarker proxies for sea
ice and temperature; (4) reconstructing a history of sea ice
distribution that provides a fast and powerful feedback in
polar and subpolar regions such as the North Atlantic and
sub-Antarctic; and (5) facilitating deep drilling in the Indian
Ocean, an under investigated ocean region that is critical to
understanding the history of the Indian monsoon as well as
the erosion and uplift of the Himalayas, with the recent
recovery of a nearly a million yearlong climate record from
the EPICA Dome C ice core raising the scientific necessity
of attaining an equally long record from the southern Indian
Ocean.
Many meeting participants emphasized that more paleocli-
matic records with improved temporal resolution and near-
zero uncertainty dating are required for state-of-the-art
model to data comparison studies. Plans are urgently needed
for the paleoclimatic community to expand the number and
spatial distribution of longer high-quality proxy data sets to
complement those that already exist for the last glacial cycle.
Data sets that include multiple evolutions of glacial and
interglacial conditions are essential for current efforts to
project climate change under a warmer Earth scenario as is
currently underway by the Intergovernmental Panel on Cli-
mate Change.
To contribute to climate change prediction efforts, it is
especially important to understand the mechanisms driving
late Holocene abrupt climate events such as those at circa 5.2
and 4.2 ka. These events are found mainly in low-latitude
climate archives related to the hydrological cycle history and
appear to be contemporaneous with collapses of civilizations
in the Middle East, Indian subcontinent, and Mesoamerica.
As more than half of the humanity lives in the tropical belt,
the significance of any change in regional hydrological cy-
cles cannot be overemphasized.
cient spatial coverage.
The 5.2 ka event is preserved in diverse paleoclimate
archives that include ice core oxygen isotope records from
Kilimanjaro in Tanzania and Huascar
in in northern Peru,
methane records from Antarctica and Greenland, marine
sediment records from the Bay of Bengal, and deposition of
Saharan dust in the South American Andes. Other ice core
proxies including dust and soluble chemical species such as
Cl and SO 2 also show drastic changes associated with the
5.2 ka event. Terrestrial records such as trees preserved in a
standing position hundreds of feet below the surface of Lake
Tahoe [Lindström, 1990], plants emerging from the retreat-
ing margins of an Andean glacier [Thompson et al., 2006],
and the
á
in the Eastern Alps are
other notable observations concomitant with the 5.2 ka
event. There are not many deep-sea records that contain the
5.2 ka event at present. However, all of these paleoclimatic
records point to a near-global low- to mid-low-latitude
abrupt climate event that appears to have impacted civiliza-
tions on three continents.
The other abrupt climate event during the late Holocene is
dated between 4.0 and 4.5 ka and seems to be very wide-
spread as it is found in many tropical and extratropical
paleoclimate archives. It is associated with prominent dust
peaks in the Huascar
Tyrolean ice man
or
Özti
in and Kilimanjaro ice cores and is thus
interpreted as a major drought interval during the late Holo-
cene [Davis and Thompson, 2006]. Sediment cores from the
Gulf of Oman and Bay of Bengal, as well as lake records
from the Gharwal Himalayas and northern Africa also indi-
cate dry climatic conditions around 4.2 ka. These events are
also expressed in archeological data from the Euphrates and
Tigris drainage basins. Thompson [this volume] further
synthesizes many North African, Middle Eastern, South
Paci
á
c, and South Asian paleoclimate records centered
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