The
purpose of the CORC-ARCHES program is to describe, understand and assess the
likelihood of abrupt changes in the climate system, and to identify the
mechanisms involved.
RATIONALE:
Fast transitions from one climate regime to another have been documented in
the Earth's history by paleo-observations around the globe. The largest and
possibly swiftest changes of global climate are found during cold climates,
i.e. ice ages. However, even during the more stable climate of the last 10,000
years (the Holocene), rapid swings in temperature and/or
precipitation, at least on
regional scales, have occurred. None of these events have been fully described.
Exactly when did they happen? How large and rapid was the climate change? What
was the cause?
For the future, our best estimate of the
range of possibilities comes from climate model simulations under a
range of forcing scenarios.
However, it is unclear whether the most complex and state-of-the-art
of these models can produce abrupt climate changes such as those that
we know occurred in the past.
The CORC-ARCHES program combines modeling and observations of
the modern and past climate to develop and test hypotheses of the mechanisms and
forcings of abrupt climate change.
HISTORY:
In 1993 NOAA's Office of Global Programs initiated a partnership with Columbia University to strengthen their respective programs on Climate and Global Change.
After a period of defining the best way forward, a focus on abrupt climate
change was agreed upon in 1998. Columbia University's research portfolio had
the needed breadth of expertise that could generate paleoclimate data sets,
interpret the records in a dynamical consistent framework and evaluate what
additional observations would be needed to detect early warnings of a future
abrupt climate change. Columbia University enlarged the reach of the program by
working with researchers at other institutions, hosted a series of
internationally attended topical workshops and invited outside
expertise. In short,
this effort was instrumental in building the beginnings of an abrupt climate
change community. Both partners had always expected that this effort would soon
be transformed into a much more significant initiative on abrupt climate change
within NOAA. Over the last five years, NOAA has provided funding at the level of $2M
per year.
RESEARCH HIGHLIGHTS:
Atlantic Ocean Freshening.
Documentation of a systematic freshening through the western basins of the Atlantic Ocean
between the 1950s and the 1990s. In essence, the overflow system that
ventilates the deep Atlantic Ocean has freshened over a 40-year time
span. The results extend a growing body of evidence indicating that
shifts in the oceanic distribution of fresh and saline waters are
occurring worldwide in ways that suggest links to global warming and
possible changes in the hydrologic cycle of the Earth.
Change in Southern Ocean Bottom Water.
Implemented an ocean time series station in the North Western Weddell Sea to document
climate variability and change in one of the sources for Southern Ocean Bottom water.
Warming in Weddell Sea.
Documented an steady increase in subsurface temperatures in the Weddell Sea over the
last 20 years.
The Gulf Stream and North Atlantic Climate.
Assessed the relative importance of seasonal
storage and release of heat by the ocean, movement
of heat by ocean currents, and movement of heat
by the atmosphere for determining the climates around
the North Atlantic Ocean. Emphasized the role of atmospheric
flow, rather than ocean currents, in making Europe's winter warm and
Eastern North America cold. This allows a more
sure assessment of the climate impacts of a possible future
slowdown of the thermohaline circulation.
Changing ENSO.
Changes in ENSO over the last century and a half have
occurred abruptly and caused global climate impacts.
By many measures the 1976/77 transition was an abrupt climate change
that had serious consequences for climate around the world.
We have demonstrated that these decadal changes in ENSO
state are predictable, to a useful degree, years in
advance. We predict that the 1997/98 El Nino ended the
post 1976 warm state of the tropical Pacific ushering
in a cold state that will last a decade or more from now.
Droughts.
We have related decadal changes of ENSO to droughts and
wet conditions over North America. Significant droughts
in the Great Plains in the 1930s (the Dust Bowl) and
mid Nineteenth Century, and in the Southwest in the 1950s,
have been related by modeling to persistent La Nina
conditions. Similarly the rapid onset of drought in the
West since 1998 is related to the end of the post 1976 warm
state of the tropical Pacific and can be expected to last.
Global Teleconnections.
Determined
that snow-line lowerings during the Last Glacial Maximum, the Younger
Dryas, and the Little Ice Age are in phase in the mid-latitudes
of the northern and southern hemispheres, indicating that abrupt
climate change can occur globally, not just regionally.
Weakening SST Gradients.
Determined
that meridional and zonal gradients of sea surface temperature in the
equatorial Pacific Ocean were much less than today during the LGM,
implying that either thermal structure of the upper ocean or wind-driven
upwelling was substantially different from modern conditions
Paleo Evidence for Subantarctic Connection.
Found
substantial increase in biological productivity within the Subantarctic
zone of the Southern Ocean associated with Heinrich events, the first
clear manifestation of these events at high southern latitudes, likely
related to reorganization of meridional overturning ocean circulation
